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MCPServer - A more ergonomic interface for MCP servers.

Extension

Base class for an opt-in MCP extension. Override only the methods you need.

Subclass and set identifier, then override the contribution methods that apply. Every method has a default, so a minimal extension overrides nothing but identifier and one of tools/resources/methods. identifier is enforced at subclass-definition time.

Source code in src/mcp/server/extension.py 
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class Extension:
    """Base class for an opt-in MCP extension. Override only the methods you need.

    Subclass and set `identifier`, then override the contribution methods that
    apply. Every method has a default, so a minimal extension overrides nothing
    but `identifier` and one of `tools`/`resources`/`methods`. `identifier` is
    enforced at subclass-definition time.
    """

    #: Reverse-DNS extension identifier, advertised under `ServerCapabilities.extensions`.
    identifier: str

    def __init_subclass__(cls, **kwargs: Any) -> None:
        super().__init_subclass__(**kwargs)
        # Validate a class-level `identifier` at definition time. A subclass may
        # instead assign `identifier` in `__init__` (per-instance ids); that case
        # is validated when the extension is applied, since no class attribute
        # exists to inspect here.
        identifier = cls.__dict__.get("identifier")
        if identifier is not None:
            validate_extension_identifier(identifier, owner=cls.__name__)

    def settings(self) -> dict[str, Any]:
        """Per-extension settings advertised at `capabilities.extensions[identifier]`.

        An empty dict (the default) advertises the extension with no settings.
        """
        return {}

    def tools(self) -> Sequence[ToolBinding]:
        """Tools this extension contributes (additive)."""
        return ()

    def resources(self) -> Sequence[ResourceBinding]:
        """Resources this extension contributes (additive)."""
        return ()

    def methods(self) -> Sequence[MethodBinding]:
        """New request methods this extension serves (additive)."""
        return ()

    async def intercept_tool_call(
        self,
        params: CallToolRequestParams,
        ctx: ServerRequestContext[Any, Any],
        call_next: CallNext,
    ) -> HandlerResult:
        """Wrap `tools/call`. Default: pass through unchanged.

        Override to short-circuit (return a result without calling `call_next`)
        or to observe the call. `params` is the validated `tools/call` params;
        `call_next(ctx)` runs the rest of the chain and the real handler.
        """
        return await call_next(ctx)

settings 

settings() -> dict[str, Any]

Per-extension settings advertised at capabilities.extensions[identifier].

An empty dict (the default) advertises the extension with no settings.

Source code in src/mcp/server/extension.py 
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def settings(self) -> dict[str, Any]:
    """Per-extension settings advertised at `capabilities.extensions[identifier]`.

    An empty dict (the default) advertises the extension with no settings.
    """
    return {}

tools 

tools() -> Sequence[ToolBinding]

Tools this extension contributes (additive).

Source code in src/mcp/server/extension.py 
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def tools(self) -> Sequence[ToolBinding]:
    """Tools this extension contributes (additive)."""
    return ()

resources 

resources() -> Sequence[ResourceBinding]

Resources this extension contributes (additive).

Source code in src/mcp/server/extension.py 
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def resources(self) -> Sequence[ResourceBinding]:
    """Resources this extension contributes (additive)."""
    return ()

methods 

methods() -> Sequence[MethodBinding]

New request methods this extension serves (additive).

Source code in src/mcp/server/extension.py 
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def methods(self) -> Sequence[MethodBinding]:
    """New request methods this extension serves (additive)."""
    return ()

intercept_tool_call async 

intercept_tool_call(
    params: CallToolRequestParams,
    ctx: ServerRequestContext[Any, Any],
    call_next: CallNext,
) -> HandlerResult

Wrap tools/call. Default: pass through unchanged.

Override to short-circuit (return a result without calling call_next) or to observe the call. params is the validated tools/call params; call_next(ctx) runs the rest of the chain and the real handler.

Source code in src/mcp/server/extension.py 
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async def intercept_tool_call(
    self,
    params: CallToolRequestParams,
    ctx: ServerRequestContext[Any, Any],
    call_next: CallNext,
) -> HandlerResult:
    """Wrap `tools/call`. Default: pass through unchanged.

    Override to short-circuit (return a result without calling `call_next`)
    or to observe the call. `params` is the validated `tools/call` params;
    `call_next(ctx)` runs the rest of the chain and the real handler.
    """
    return await call_next(ctx)

MethodBinding dataclass

A new request method an extension serves, e.g. tasks/get.

params_type validates incoming params before handler runs; it should subclass RequestParams so _meta parses uniformly. protocol_versions, when set, restricts the method to those wire versions - a request for the method at any other version is rejected as METHOD_NOT_FOUND, mirroring the spec's (method, version) boundary table. None (the default) admits the method at every version.

Extension methods are additive: method must not name a spec-defined request method (tools/list, completion/complete, ...) — those handlers belong to the server, and an extension binding one would silently shadow or be shadowed by it. Both constraints are enforced at construction. To re-provide a spec method the 2026 revision removed (e.g. logging/setLevel for legacy clients), use the lowlevel Server.add_request_handler API instead — the runner's per-version surface gate would never route such a method to an extension handler anyway.

Source code in src/mcp/server/extension.py 
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@dataclass(frozen=True)
class MethodBinding:
    """A new request method an extension serves, e.g. `tasks/get`.

    `params_type` validates incoming params before `handler` runs; it should
    subclass `RequestParams` so `_meta` parses uniformly. `protocol_versions`,
    when set, restricts the method to those wire versions - a request for the
    method at any other version is rejected as `METHOD_NOT_FOUND`, mirroring the
    spec's `(method, version)` boundary table. `None` (the default) admits the
    method at every version.

    Extension methods are additive: `method` must not name a spec-defined
    request method (`tools/list`, `completion/complete`, ...) — those handlers
    belong to the server, and an extension binding one would silently shadow or
    be shadowed by it. Both constraints are enforced at construction. To
    re-provide a spec method the 2026 revision removed (e.g. `logging/setLevel`
    for legacy clients), use the lowlevel `Server.add_request_handler` API
    instead — the runner's per-version surface gate would never route such a
    method to an extension handler anyway.
    """

    method: str
    params_type: type[BaseModel]
    handler: RequestHandler
    protocol_versions: frozenset[str] | None = None

    def __post_init__(self) -> None:
        if self.method in SPEC_CLIENT_METHODS:
            raise ValueError(
                f"MethodBinding cannot bind spec method {self.method!r}; extension methods are "
                "additive — use Extension.intercept_tool_call or Server.middleware to wrap core behaviour"
            )
        if self.protocol_versions is not None and not self.protocol_versions:
            raise ValueError(
                f"MethodBinding for {self.method!r} has an empty protocol_versions set, so it could "
                "never be served; use None to admit every version"
            )

ResourceBinding dataclass

A pre-built resource an extension contributes.

Source code in src/mcp/server/extension.py 
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@dataclass(frozen=True)
class ResourceBinding:
    """A pre-built resource an extension contributes."""

    resource: Resource

ToolBinding dataclass

A tool an extension contributes, plus the _meta to stamp on it.

Source code in src/mcp/server/extension.py 
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@dataclass(frozen=True)
class ToolBinding:
    """A tool an extension contributes, plus the `_meta` to stamp on it."""

    fn: Callable[..., Any]
    meta: dict[str, Any] | None = None
    kwargs: dict[str, Any] = field(default_factory=lambda: {})

AESGCMRequestStateCodec

Built-in codec: AES-256-GCM under key(s) derived with HKDF-SHA256.

Tokens are encrypted, not merely signed, so clients cannot read the state. keys[0] seals; all keys unseal (rotation, see RequestStateSecurity). Each token carries a 4-byte non-secret key fingerprint for an O(1) ring lookup, and the "v1." prefix and fingerprint are bound into the GCM associated data, so a token cannot be replayed into another format version or ring slot. Key bytes are copied at construction.

Source code in src/mcp/server/request_state.py 
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class AESGCMRequestStateCodec:
    """Built-in codec: AES-256-GCM under key(s) derived with HKDF-SHA256.

    Tokens are encrypted, not merely signed, so clients cannot read the state.
    `keys[0]` seals; all keys unseal (rotation, see `RequestStateSecurity`).
    Each token carries a 4-byte non-secret key fingerprint for an O(1) ring
    lookup, and the "v1." prefix and fingerprint are bound into the GCM
    associated data, so a token cannot be replayed into another format version
    or ring slot. Key bytes are copied at construction.
    """

    def __init__(self, keys: Sequence[bytes | bytearray | str]) -> None:
        for i, key in enumerate(cast("Sequence[object]", keys)):
            if not isinstance(key, bytes | bytearray | str):
                # Never coerce: bytes(32) would silently build an all-zero key.
                raise TypeError(
                    f"request-state keys must be bytes, bytearray, or str; keys[{i}] is {type(key).__name__}"
                )
        material = [k.encode() if isinstance(k, str) else bytes(k) for k in keys]
        if not material:
            raise ValueError("AESGCMRequestStateCodec requires at least one key")
        for i, k in enumerate(material):
            if len(k) < 32:
                raise ValueError(
                    f"request-state keys must be at least 32 bytes of secret randomness; "
                    f"keys[{i}] is {len(k)} bytes. "
                    'Generate one with: python -c "import secrets; print(secrets.token_hex(32))"'
                )
        self._ring: dict[bytes, AESGCM] = {}
        self._mint_kid = b""
        for i, secret in enumerate(material):
            key = _derive_key(secret)
            kid = hashlib.sha256(_KID_INFO + key).digest()[:_KID_LEN]
            if kid in self._ring:
                raise ValueError(f"keys[{i}] duplicates an earlier ring key")
            self._ring[kid] = AESGCM(key)
            if i == 0:
                self._mint_kid = kid

    def seal(self, payload: bytes) -> str:
        kid = self._mint_kid
        nonce = os.urandom(_NONCE_LEN)
        sealed = self._ring[kid].encrypt(nonce, payload, _TOKEN_PREFIX.encode() + kid)
        return _TOKEN_PREFIX + _b64u(kid + nonce + sealed)

    def unseal(self, token: str) -> bytes:
        if not token.startswith(_TOKEN_PREFIX):
            raise InvalidRequestState("malformed")
        try:
            raw = _b64u_decode(token[len(_TOKEN_PREFIX) :])
        except ValueError as exc:
            raise InvalidRequestState("malformed") from exc
        if len(raw) < _KID_LEN + _NONCE_LEN + 16:
            raise InvalidRequestState("malformed")
        kid, nonce, sealed = raw[:_KID_LEN], raw[_KID_LEN : _KID_LEN + _NONCE_LEN], raw[_KID_LEN + _NONCE_LEN :]
        aead = self._ring.get(kid)
        if aead is None:
            raise InvalidRequestState("unknown key")
        try:
            return aead.decrypt(nonce, sealed, _TOKEN_PREFIX.encode() + kid)
        except InvalidTag:
            raise InvalidRequestState("seal") from None

InvalidRequestState

Bases: Exception

A sealed requestState token failed verification.

The message is a log-only reason code; the boundary never puts it on the wire.

Source code in src/mcp/server/request_state.py 
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class InvalidRequestState(Exception):
    """A sealed `requestState` token failed verification.

    The message is a log-only reason code; the boundary never puts it on the wire.
    """

RequestStateBoundary

Server middleware sealing/unsealing requestState at the wire boundary.

Acts only on the multi-round-trip carriers (tools/call, prompts/get, resources/read); every other method passes through untouched.

Inbound state is verified (codec unseal plus claims check) and replaced with the plaintext the server minted before any interceptor or handler runs; failure answers -32602 with the frozen message "Invalid or expired requestState", the real reason going to the server log only. Outbound, an input_required result carrying requestState is sealed in a fresh claims envelope; handlers and resolvers never call the codec.

default_audience seeds the audience claim when the policy sets none, and must be stated explicitly: it is the service identity that stops state minted by another service sharing the same keys. MCPServer installs this middleware with its server name by default (under an ephemeral policy unless request_state_security= supplies one); lowlevel Server users append one to server.middleware, passing their server's name (or None to deliberately leave tokens audience-free).

Source code in src/mcp/server/request_state.py 
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class RequestStateBoundary:
    """Server middleware sealing/unsealing `requestState` at the wire boundary.

    Acts only on the multi-round-trip carriers (tools/call, prompts/get,
    resources/read); every other method passes through untouched.

    Inbound state is verified (codec unseal plus claims check) and replaced
    with the plaintext the server minted before any interceptor or handler
    runs; failure answers -32602 with the frozen message "Invalid or expired
    requestState", the real reason going to the server log only. Outbound, an
    `input_required` result carrying `requestState` is sealed in a fresh
    claims envelope; handlers and resolvers never call the codec.

    `default_audience` seeds the audience claim when the policy sets none, and
    must be stated explicitly: it is the service identity that stops state
    minted by another service sharing the same keys. `MCPServer` installs this
    middleware with its server name by default (under an ephemeral policy
    unless `request_state_security=` supplies one); lowlevel `Server` users
    append one to `server.middleware`, passing their server's name (or `None`
    to deliberately leave tokens audience-free).
    """

    def __init__(self, security: RequestStateSecurity, *, default_audience: str | None) -> None:
        self._security = security
        self._audience = security.audience if security.audience is not None else default_audience

    async def __call__(self, ctx: ServerRequestContext[Any, Any], call_next: CallNext) -> HandlerResult:
        if ctx.method not in _MRTR_METHODS:
            return await call_next(ctx)
        binding: _RoundBinding | None = None
        if ctx.params is not None and ctx.params.get("requestState") is not None:
            # An explicit JSON null counts as absent: stripping the field is already in any client's power.
            plaintext, binding = self._unseal(ctx)
            ctx = replace(ctx, params={**ctx.params, "requestState": plaintext})
        result = await call_next(ctx)
        return self._seal_result(ctx, result, binding)

    def _unseal(self, ctx: ServerRequestContext[Any, Any]) -> tuple[str, _RoundBinding]:
        assert ctx.params is not None
        wire = ctx.params["requestState"]
        if not isinstance(wire, str):
            _reject(ctx.method, "non-string requestState")
        security = self._security
        try:
            payload = security.codec.unseal(wire)
        except InvalidRequestState as exc:
            _reject(ctx.method, str(exc))
        except Exception:  # deny-on-error: a buggy custom codec must fail closed
            logger.exception("requestState codec raised during unseal on %s", ctx.method)
            _reject(ctx.method, "codec error")
        try:
            claims = json.loads(payload)
            version, iat, exp, inner = claims["v"], claims["iat"], claims["exp"], claims["s"]
        except (ValueError, KeyError, TypeError):
            _reject(ctx.method, "malformed")
        if version != _ENVELOPE_VERSION or not isinstance(inner, str):
            _reject(ctx.method, "malformed")
        now = time.time()
        # Accept-conditions are stated positively so a NaN claim fails the comparison and rejects.
        if not isinstance(iat, int | float) or not (iat <= now + _FUTURE_SKEW):
            _reject(ctx.method, "minted in the future")
        if not isinstance(exp, int | float) or not (now < exp):
            _reject(ctx.method, "expired")
        target, args_digest = _request_identity(ctx.method, ctx.params)
        if claims.get("m") != ctx.method or claims.get("t") != target or claims.get("a") != args_digest:
            _reject(ctx.method, "request binding")
        if claims.get("aud") != self._audience:
            _reject(ctx.method, "audience")

        def fail_verify(reason: str) -> NoReturn:
            _reject(ctx.method, reason)

        principal = _bound_principal(security, ctx, fail_verify)
        claim = claims.get("p")
        if (claim is None) != (principal is None):
            _reject(ctx.method, "principal drift")
        if claim is not None and principal is not None:
            if not isinstance(claim, str) or not _principal_matches(claim, principal):
                _reject(ctx.method, "principal")
        return inner, (target, args_digest, principal)

    def _seal_result(
        self, ctx: ServerRequestContext[Any, Any], result: HandlerResult, binding: _RoundBinding | None
    ) -> HandlerResult:
        # Spec-path results arrive as wire mappings; a short-circuiting middleware may return a model.
        if not is_input_required(result):
            return result
        state = result.get("requestState") if isinstance(result, Mapping) else result.request_state
        if state is None:
            return result
        if isinstance(result, Mapping):
            if not isinstance(state, str):
                # Only a short-circuiting middleware can put a non-string here; nothing to seal.
                return result
            return {**result, "requestState": self._seal(ctx, state, binding)}
        return result.model_copy(update={"request_state": self._seal(ctx, state, binding)})

    def _seal(self, ctx: ServerRequestContext[Any, Any], state: str, binding: _RoundBinding | None = None) -> str:
        security = self._security
        if binding is None:

            def fail_seal(reason: str) -> NoReturn:
                logger.error("refusing to seal requestState on %s: %s", ctx.method, reason)
                raise MCPError(code=INTERNAL_ERROR, message="Internal error")

            target, args_digest = _request_identity(ctx.method, ctx.params)
            binding = (target, args_digest, _bound_principal(security, ctx, fail_seal))
        target, args_digest, principal = binding
        now = time.time()
        claims: dict[str, Any] = {
            "v": _ENVELOPE_VERSION,
            "iat": now,
            "exp": now + security.ttl,
            "m": ctx.method,
            "t": target,
            "a": args_digest,
            "s": state,
        }
        if self._audience is not None:
            claims["aud"] = self._audience
        if principal is not None:
            claims["p"] = _principal_claim(principal)
        payload = compact_json(claims).encode()
        try:
            return security.codec.seal(payload)
        except Exception:  # deny-on-error: a raising custom codec must not leak its failure
            logger.exception("requestState codec raised during seal on %s", ctx.method)
            raise MCPError(code=INTERNAL_ERROR, message="Internal error") from None

RequestStateCodec

Bases: Protocol

Authenticated crypto over the framework's request-state envelope.

The framework stamps and re-verifies every envelope claim (expiry, request binding, principal); a codec only provides integrity and, ideally, confidentiality (a sign-only codec leaves the payload client-readable).

Requirements: unseal(seal(payload)) round-trips, and unseal raises InvalidRequestState for any token it did not mint unmodified; tokens never name their algorithm (version with a format prefix bound under the authentication tag, RFC 8725); comparisons are constant-time. Both methods are synchronous, so cache key material rather than calling a KMS per token.

Source code in src/mcp/server/request_state.py 
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class RequestStateCodec(Protocol):
    """Authenticated crypto over the framework's request-state envelope.

    The framework stamps and re-verifies every envelope claim (expiry, request
    binding, principal); a codec only provides integrity and, ideally,
    confidentiality (a sign-only codec leaves the payload client-readable).

    Requirements: `unseal(seal(payload))` round-trips, and `unseal` raises
    `InvalidRequestState` for any token it did not mint unmodified; tokens
    never name their algorithm (version with a format prefix bound under the
    authentication tag, RFC 8725); comparisons are constant-time. Both methods
    are synchronous, so cache key material rather than calling a KMS per token.
    """

    def seal(self, payload: bytes) -> str:
        """Return an opaque URL-safe token protecting `payload`."""
        ...

    def unseal(self, token: str) -> bytes:
        """Reverse `seal`.

        Raises:
            InvalidRequestState: Malformed, unauthentic, or unknown-key token.
        """
        ...

seal 

seal(payload: bytes) -> str

Return an opaque URL-safe token protecting payload.

Source code in src/mcp/server/request_state.py 
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def seal(self, payload: bytes) -> str:
    """Return an opaque URL-safe token protecting `payload`."""
    ...

unseal 

unseal(token: str) -> bytes

Reverse seal.

Raises:

Type Description
InvalidRequestState

Malformed, unauthentic, or unknown-key token.
Source code in src/mcp/server/request_state.py 
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def unseal(self, token: str) -> bytes:
    """Reverse `seal`.

    Raises:
        InvalidRequestState: Malformed, unauthentic, or unknown-key token.
    """
    ...

RequestStateSecurity

Policy for protecting requestState: codec, TTL, principal, audience.

Exactly one of keys or codec:

RequestStateSecurity(keys=[secret])      # built-in AES-256-GCM
RequestStateSecurity(codec=MyKmsCodec()) # bring your own crypto
RequestStateSecurity.ephemeral()         # process-local key

keys is the rotation ring: keys[0] seals, every key unseals. Zero-downtime rotation, each phase fully rolled out before the next: keys=[old, new], then keys=[new, old], then keys=[new] after one TTL.

The boundary enforces expiry, request binding, audience, and principal for every codec, fail-closed in both directions. audience=None defers to the boundary's default_audience (MCPServer passes its server name).

Source code in src/mcp/server/request_state.py 
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class RequestStateSecurity:
    """Policy for protecting `requestState`: codec, TTL, principal, audience.

    Exactly one of `keys` or `codec`:

        RequestStateSecurity(keys=[secret])      # built-in AES-256-GCM
        RequestStateSecurity(codec=MyKmsCodec()) # bring your own crypto
        RequestStateSecurity.ephemeral()         # process-local key

    `keys` is the rotation ring: `keys[0]` seals, every key unseals.
    Zero-downtime rotation, each phase fully rolled out before the next:
    `keys=[old, new]`, then `keys=[new, old]`, then `keys=[new]` after one TTL.

    The boundary enforces expiry, request binding, audience, and principal for
    every codec, fail-closed in both directions. `audience=None` defers to the
    boundary's `default_audience` (`MCPServer` passes its server name).
    """

    codec: RequestStateCodec
    ttl: float
    bind_principal: Callable[[ServerRequestContext[Any, Any]], str | None] | None
    audience: str | None

    def __init__(
        self,
        *,
        keys: Sequence[bytes | bytearray | str] | None = None,
        codec: RequestStateCodec | None = None,
        ttl: float = 600.0,
        bind_principal: Callable[[ServerRequestContext[Any, Any]], str | None] | None = authenticated_principal,
        audience: str | None = None,
    ) -> None:
        if (keys is None) == (codec is None):
            raise ValueError("RequestStateSecurity takes exactly one of keys= or codec=")
        if not (math.isfinite(ttl) and ttl > 0):
            raise ValueError(f"request-state ttl must be a positive finite number, got {ttl!r}")
        if keys is not None:
            self.codec = AESGCMRequestStateCodec(keys)
        else:
            assert codec is not None
            self.codec = codec
        self.ttl = ttl
        self.bind_principal = bind_principal
        self.audience = audience

    @classmethod
    def ephemeral(cls, *, ttl: float = 600.0, audience: str | None = None) -> RequestStateSecurity:
        """Protection under a key generated now and held only by this process.

        This is the policy `MCPServer` installs when `request_state_security=`
        is omitted; call it yourself on the lowlevel tier or to set `ttl`/
        `audience`. Suits single-process deployments (stdio, one HTTP worker):
        state minted before a restart or by another worker is rejected.
        Multi-instance deployments must share a key via `keys=[...]`.
        """
        return cls(keys=[os.urandom(32)], ttl=ttl, audience=audience)

ephemeral classmethod 

ephemeral(
    *, ttl: float = 600.0, audience: str | None = None
) -> RequestStateSecurity

Protection under a key generated now and held only by this process.

This is the policy MCPServer installs when request_state_security= is omitted; call it yourself on the lowlevel tier or to set ttl/ audience. Suits single-process deployments (stdio, one HTTP worker): state minted before a restart or by another worker is rejected. Multi-instance deployments must share a key via keys=[...].

Source code in src/mcp/server/request_state.py 
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@classmethod
def ephemeral(cls, *, ttl: float = 600.0, audience: str | None = None) -> RequestStateSecurity:
    """Protection under a key generated now and held only by this process.

    This is the policy `MCPServer` installs when `request_state_security=`
    is omitted; call it yourself on the lowlevel tier or to set `ttl`/
    `audience`. Suits single-process deployments (stdio, one HTTP worker):
    state minted before a restart or by another worker is rejected.
    Multi-instance deployments must share a key via `keys=[...]`.
    """
    return cls(keys=[os.urandom(32)], ttl=ttl, audience=audience)

authenticated_principal 

authenticated_principal(
    ctx: ServerRequestContext[Any, Any],
) -> str | None

Default principal binding: the authenticated (client, issuer, subject) identity.

Uses the same components session ownership uses, so two users of one OAuth client are distinct principals whenever the token verifier supplies a subject, and the binding degrades to the client identity when it does not. Returns None (state not principal-bound) on unauthenticated transports.

Source code in src/mcp/server/request_state.py 
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def authenticated_principal(ctx: ServerRequestContext[Any, Any]) -> str | None:
    """Default principal binding: the authenticated (client, issuer, subject) identity.

    Uses the same components session ownership uses, so two users of one OAuth
    client are distinct principals whenever the token verifier supplies a
    subject, and the binding degrades to the client identity when it does not.
    Returns `None` (state not principal-bound) on unauthenticated transports.
    """
    token = get_access_token()
    if token is None:
        return None
    return compact_json(principal_components(token))

Context

Bases: BaseModel, Generic[LifespanContextT, RequestT]

Context object providing access to MCP capabilities.

This provides a cleaner interface to MCP's RequestContext functionality. It gets injected into tool and resource functions that request it via type hints.

To use context in a tool function, add a parameter with the Context type annotation:

@server.tool()
async def my_tool(x: int, ctx: Context) -> str:
    # Log messages to the client
    await ctx.info(f"Processing {x}")
    await ctx.debug("Debug info")
    await ctx.warning("Warning message")
    await ctx.error("Error message")

    # Report progress
    await ctx.report_progress(50, 100)

    # Access resources
    data = await ctx.read_resource("resource://data")

    # Get request info
    request_id = ctx.request_id
    client_id = ctx.client_id

    return str(x)

The context parameter name can be anything as long as it's annotated with Context. The context is optional - tools that don't need it can omit the parameter.

Source code in src/mcp/server/mcpserver/context.py 
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class Context(BaseModel, Generic[LifespanContextT, RequestT]):
    """Context object providing access to MCP capabilities.

    This provides a cleaner interface to MCP's RequestContext functionality.
    It gets injected into tool and resource functions that request it via type hints.

    To use context in a tool function, add a parameter with the Context type annotation:

    ```python
    @server.tool()
    async def my_tool(x: int, ctx: Context) -> str:
        # Log messages to the client
        await ctx.info(f"Processing {x}")
        await ctx.debug("Debug info")
        await ctx.warning("Warning message")
        await ctx.error("Error message")

        # Report progress
        await ctx.report_progress(50, 100)

        # Access resources
        data = await ctx.read_resource("resource://data")

        # Get request info
        request_id = ctx.request_id
        client_id = ctx.client_id

        return str(x)
    ```

    The context parameter name can be anything as long as it's annotated with Context.
    The context is optional - tools that don't need it can omit the parameter.
    """

    _request_context: ServerRequestContext[LifespanContextT, RequestT] | None
    _mcp_server: MCPServer | None
    _input_params: InputResponseRequestParams | None
    _subscriptions: SubscriptionBus | None

    # TODO(maxisbey): Consider making request_context/mcp_server required, or refactor Context entirely.
    def __init__(
        self,
        *,
        request_context: ServerRequestContext[LifespanContextT, RequestT] | None = None,
        mcp_server: MCPServer | None = None,
        input_params: InputResponseRequestParams | None = None,
        subscriptions: SubscriptionBus | None = None,
        # TODO(Marcelo): We should drop this kwargs parameter.
        **kwargs: Any,
    ):
        super().__init__(**kwargs)
        self._request_context = request_context
        self._mcp_server = mcp_server
        self._input_params = input_params
        self._subscriptions = subscriptions

    @property
    def mcp_server(self) -> MCPServer:
        """Access to the MCPServer instance."""
        if self._mcp_server is None:
            raise ValueError("Context is not available outside of a request")
        return self._mcp_server

    @property
    def request_context(self) -> ServerRequestContext[LifespanContextT, RequestT]:
        """Access to the underlying request context."""
        if self._request_context is None:
            raise ValueError("Context is not available outside of a request")
        return self._request_context

    def _nested_invocation(self) -> Context[LifespanContextT, RequestT]:
        """A Context for invoking another handler's function from inside this request.

        Shares the request infrastructure (session, request metadata, lifespan) but
        carries no `input_responses`/`request_state`: those are addressed to the wire
        request's own target — their keys are ones that handler minted — so a nested
        invocation always starts on round one.
        """
        return Context(
            request_context=self._request_context, mcp_server=self._mcp_server, subscriptions=self._subscriptions
        )

    async def report_progress(self, progress: float, total: float | None = None, message: str | None = None) -> None:
        """Report progress for the current operation.

        Args:
            progress: Current progress value (e.g., 24)
            total: Optional total value (e.g., 100)
            message: Optional message (e.g., "Starting render...")
        """
        await self.request_context.session.report_progress(progress, total, message)

    @property
    def _bus(self) -> SubscriptionBus:
        if self._subscriptions is None:
            raise ValueError("Context is not available outside of a request")
        return self._subscriptions

    async def notify_tools_changed(self) -> None:
        """Publish a tools list-changed event to `subscriptions/listen` subscribers."""
        await self._bus.publish(ToolsListChanged())

    async def notify_prompts_changed(self) -> None:
        """Publish a prompts list-changed event to `subscriptions/listen` subscribers."""
        await self._bus.publish(PromptsListChanged())

    async def notify_resources_changed(self) -> None:
        """Publish a resources list-changed event to `subscriptions/listen` subscribers."""
        await self._bus.publish(ResourcesListChanged())

    async def notify_resource_updated(self, uri: str | AnyUrl) -> None:
        """Publish a resource-updated event for `uri` to `subscriptions/listen` subscribers.

        The URI is matched as an exact string against each stream's filter.
        Reaches `subscriptions/listen` streams only; clients on earlier
        protocol versions that used `resources/subscribe` are notified via
        `ctx.session.send_resource_updated(uri)` instead.
        """
        await self._bus.publish(ResourceUpdated(uri=str(uri)))

    async def read_resource(self, uri: str | AnyUrl) -> Iterable[ReadResourceContents]:
        """Read a resource by URI.

        This is a content reader: an `InputRequiredResult` returned by a
        resource template function (the 2026-07-28 multi-round-trip flow)
        raises here, and the nested template never sees this request's
        `input_responses`/`request_state` — those answer the outer handler's
        own questions, so the template always behaves as round one. A handler
        that wants to receive and forward an `InputRequiredResult` as its own
        result calls `MCPServer.read_resource(uri, context)` instead — but
        not from a tool whose dependencies elicit via `Resolve(...)`: the
        resolver owns that tool's `request_state` channel, and a forwarded
        result's state would clobber it.

        Args:
            uri: Resource URI to read

        Returns:
            The resource content as either text or bytes

        Raises:
            ResourceNotFoundError: If no resource or template matches the URI.
            ResourceError: If template creation or resource reading fails.
            RuntimeError: If the resource returned an `InputRequiredResult`.
        """
        assert self._mcp_server is not None, "Context is not available outside of a request"
        result = await self._mcp_server.read_resource(uri, self._nested_invocation())
        if isinstance(result, InputRequiredResult):
            raise RuntimeError(
                "Resource returned InputRequiredResult; ctx.read_resource() only returns "
                "content — use MCPServer.read_resource(uri, context) to receive and forward it."
            )
        return result

    async def elicit(
        self,
        message: str,
        schema: type[ElicitSchemaModelT],
    ) -> ElicitationResult[ElicitSchemaModelT]:
        """Elicit information from the client/user.

        This method can be used to interactively ask for additional information from the
        client within a tool's execution. The client might display the message to the
        user and collect a response according to the provided schema. If the client
        is an agent, it might decide how to handle the elicitation -- either by asking
        the user or automatically generating a response.

        Args:
            message: Message to present to the user
            schema: A Pydantic model class defining the expected response structure.
                    According to the specification, only primitive types are allowed.

        Returns:
            An ElicitationResult containing the action taken and the data if accepted

        Note:
            Check the result.action to determine if the user accepted, declined, or cancelled.
            The result.data will only be populated if action is "accept" and validation succeeded.
        """

        return await elicit_with_validation(
            session=self.request_context.session,
            message=message,
            schema=schema,
            related_request_id=self.request_id,
        )

    async def elicit_url(
        self,
        message: str,
        url: str,
        elicitation_id: str,
    ) -> UrlElicitationResult:
        """Request URL mode elicitation from the client.

        This directs the user to an external URL for out-of-band interactions
        that must not pass through the MCP client. Use this for:
        - Collecting sensitive credentials (API keys, passwords)
        - OAuth authorization flows with third-party services
        - Payment and subscription flows
        - Any interaction where data should not pass through the LLM context

        The response indicates whether the user consented to navigate to the URL.
        The actual interaction happens out-of-band. When the elicitation completes,
        call `ctx.session.send_elicit_complete(elicitation_id)` to notify the client.

        Args:
            message: Human-readable explanation of why the interaction is needed
            url: The URL the user should navigate to
            elicitation_id: Unique identifier for tracking this elicitation

        Returns:
            UrlElicitationResult indicating accept, decline, or cancel
        """
        return await elicit_url(
            session=self.request_context.session,
            message=message,
            url=url,
            elicitation_id=elicitation_id,
            related_request_id=self.request_id,
        )

    @deprecated("The logging capability is deprecated as of 2026-07-28 (SEP-2577).", category=MCPDeprecationWarning)
    async def log(
        self,
        level: LoggingLevel,
        data: Any,
        *,
        logger_name: str | None = None,
    ) -> None:
        """Send a log message to the client.

        Args:
            level: Log level (debug, info, notice, warning, error, critical,
                alert, emergency)
            data: The data to be logged. Any JSON serializable type is allowed
                (string, dict, list, number, bool, etc.) per the MCP specification.
            logger_name: Optional logger name
        """
        await self.request_context.session.send_log_message(  # pyright: ignore[reportDeprecated]
            level=level,
            data=data,
            logger=logger_name,
            related_request_id=self.request_id,
        )

    # TODO(maxisbey): see if this is needed otherwise remove
    @property
    def client_id(self) -> str | None:
        """Get the client ID if available.

        Note: this reads from the MCP request's `_meta` params, not the OAuth
        bearer token. For that, use `get_access_token().client_id`.
        """
        return self.request_context.meta.get("client_id") if self.request_context.meta else None  # pragma: no cover

    @property
    def headers(self) -> Mapping[str, str] | None:
        """Request headers carried by this message, when the transport has them.

        Populated by HTTP-based transports; `None` on stdio or when the
        transport's request object carries no headers. Headers are
        client-supplied input - never treat one as an identity assertion.
        """
        return cast("Mapping[str, str] | None", getattr(self.request_context.request, "headers", None))

    @property
    def request_id(self) -> str:
        """Get the unique ID for this request."""
        return str(self.request_context.request_id)

    @property
    def protocol_version(self) -> str | None:
        """The negotiated protocol version, or `None` outside of an active request."""
        return self._request_context.protocol_version if self._request_context is not None else None

    @property
    def input_responses(self) -> InputResponses | None:
        """Client responses to a prior `InputRequiredResult.input_requests`.

        `None` on the initial round, or when the client retried without
        responses.
        """
        return self._input_params.input_responses if self._input_params else None

    @property
    def request_state(self) -> str | None:
        """Opaque state echoed from a prior `InputRequiredResult.request_state`.

        `None` on the initial round.
        """
        return self._input_params.request_state if self._input_params else None

    @property
    def client_capabilities(self) -> ClientCapabilities | None:
        """The client's declared capabilities for this connection.

        `None` when the client supplied no client info (e.g. an anonymous
        stateless request without the reserved `_meta` keys).
        """
        client_params = self.request_context.session.client_params
        return client_params.capabilities if client_params else None

    @property
    def session(self):
        """Access to the underlying session for advanced usage."""
        return self.request_context.session

    async def close_sse_stream(self) -> None:
        """Close the SSE stream to trigger client reconnection.

        This method closes the HTTP connection for the current request, triggering
        client reconnection. Events continue to be stored in the event store and will
        be replayed when the client reconnects with Last-Event-ID.

        Use this to implement polling behavior during long-running operations -
        the client will reconnect after the retry interval specified in the priming event.

        Note:
            This is a no-op if not using StreamableHTTP transport with event_store.
            The callback is only available when event_store is configured.
        """
        if self._request_context and self._request_context.close_sse_stream:  # pragma: no branch
            await self._request_context.close_sse_stream()

    async def close_standalone_sse_stream(self) -> None:
        """Close the standalone GET SSE stream to trigger client reconnection.

        This method closes the HTTP connection for the standalone GET stream used
        for unsolicited server-to-client notifications. The client SHOULD reconnect
        with Last-Event-ID to resume receiving notifications.

        Note:
            This is a no-op if not using StreamableHTTP transport with event_store.
            Currently, client reconnection for standalone GET streams is NOT
            implemented - this is a known gap.
        """
        if self._request_context and self._request_context.close_standalone_sse_stream:  # pragma: no cover
            await self._request_context.close_standalone_sse_stream()

    # Convenience methods for common log levels
    @deprecated("The logging capability is deprecated as of 2026-07-28 (SEP-2577).", category=MCPDeprecationWarning)
    async def debug(self, data: Any, *, logger_name: str | None = None) -> None:
        """Send a debug log message."""
        await self.log("debug", data, logger_name=logger_name)  # pyright: ignore[reportDeprecated]

    @deprecated("The logging capability is deprecated as of 2026-07-28 (SEP-2577).", category=MCPDeprecationWarning)
    async def info(self, data: Any, *, logger_name: str | None = None) -> None:
        """Send an info log message."""
        await self.log("info", data, logger_name=logger_name)  # pyright: ignore[reportDeprecated]

    @deprecated("The logging capability is deprecated as of 2026-07-28 (SEP-2577).", category=MCPDeprecationWarning)
    async def warning(self, data: Any, *, logger_name: str | None = None) -> None:
        """Send a warning log message."""
        await self.log("warning", data, logger_name=logger_name)  # pyright: ignore[reportDeprecated]

    @deprecated("The logging capability is deprecated as of 2026-07-28 (SEP-2577).", category=MCPDeprecationWarning)
    async def error(self, data: Any, *, logger_name: str | None = None) -> None:
        """Send an error log message."""
        await self.log("error", data, logger_name=logger_name)  # pyright: ignore[reportDeprecated]

mcp_server property 

mcp_server: MCPServer

Access to the MCPServer instance.

request_context property 

request_context: ServerRequestContext[
    LifespanContextT, RequestT
]

Access to the underlying request context.

report_progress async 

report_progress(
    progress: float,
    total: float | None = None,
    message: str | None = None,
) -> None

Report progress for the current operation.

Parameters:

Name Type Description Default
progress float

Current progress value (e.g., 24)

 required
total float | None

Optional total value (e.g., 100)

 None
message str | None

Optional message (e.g., "Starting render...")

 None
Source code in src/mcp/server/mcpserver/context.py 
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async def report_progress(self, progress: float, total: float | None = None, message: str | None = None) -> None:
    """Report progress for the current operation.

    Args:
        progress: Current progress value (e.g., 24)
        total: Optional total value (e.g., 100)
        message: Optional message (e.g., "Starting render...")
    """
    await self.request_context.session.report_progress(progress, total, message)

notify_tools_changed async 

notify_tools_changed() -> None

Publish a tools list-changed event to subscriptions/listen subscribers.

Source code in src/mcp/server/mcpserver/context.py 
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async def notify_tools_changed(self) -> None:
    """Publish a tools list-changed event to `subscriptions/listen` subscribers."""
    await self._bus.publish(ToolsListChanged())

notify_prompts_changed async 

notify_prompts_changed() -> None

Publish a prompts list-changed event to subscriptions/listen subscribers.

Source code in src/mcp/server/mcpserver/context.py 
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async def notify_prompts_changed(self) -> None:
    """Publish a prompts list-changed event to `subscriptions/listen` subscribers."""
    await self._bus.publish(PromptsListChanged())

notify_resources_changed async 

notify_resources_changed() -> None

Publish a resources list-changed event to subscriptions/listen subscribers.

Source code in src/mcp/server/mcpserver/context.py 
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async def notify_resources_changed(self) -> None:
    """Publish a resources list-changed event to `subscriptions/listen` subscribers."""
    await self._bus.publish(ResourcesListChanged())

notify_resource_updated async 

notify_resource_updated(uri: str | AnyUrl) -> None

Publish a resource-updated event for uri to subscriptions/listen subscribers.

The URI is matched as an exact string against each stream's filter. Reaches subscriptions/listen streams only; clients on earlier protocol versions that used resources/subscribe are notified via ctx.session.send_resource_updated(uri) instead.

Source code in src/mcp/server/mcpserver/context.py 
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async def notify_resource_updated(self, uri: str | AnyUrl) -> None:
    """Publish a resource-updated event for `uri` to `subscriptions/listen` subscribers.

    The URI is matched as an exact string against each stream's filter.
    Reaches `subscriptions/listen` streams only; clients on earlier
    protocol versions that used `resources/subscribe` are notified via
    `ctx.session.send_resource_updated(uri)` instead.
    """
    await self._bus.publish(ResourceUpdated(uri=str(uri)))

read_resource async 

read_resource(
    uri: str | AnyUrl,
) -> Iterable[ReadResourceContents]

Read a resource by URI.

This is a content reader: an InputRequiredResult returned by a resource template function (the 2026-07-28 multi-round-trip flow) raises here, and the nested template never sees this request's input_responses/request_state — those answer the outer handler's own questions, so the template always behaves as round one. A handler that wants to receive and forward an InputRequiredResult as its own result calls MCPServer.read_resource(uri, context) instead — but not from a tool whose dependencies elicit via Resolve(...): the resolver owns that tool's request_state channel, and a forwarded result's state would clobber it.

Parameters:

Name Type Description Default
uri str | AnyUrl

Resource URI to read

 required

Returns:

Type Description
Iterable[ReadResourceContents]

The resource content as either text or bytes

Raises:

Type Description
ResourceNotFoundError

If no resource or template matches the URI.
ResourceError

If template creation or resource reading fails.
RuntimeError

If the resource returned an InputRequiredResult.
Source code in src/mcp/server/mcpserver/context.py 
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async def read_resource(self, uri: str | AnyUrl) -> Iterable[ReadResourceContents]:
    """Read a resource by URI.

    This is a content reader: an `InputRequiredResult` returned by a
    resource template function (the 2026-07-28 multi-round-trip flow)
    raises here, and the nested template never sees this request's
    `input_responses`/`request_state` — those answer the outer handler's
    own questions, so the template always behaves as round one. A handler
    that wants to receive and forward an `InputRequiredResult` as its own
    result calls `MCPServer.read_resource(uri, context)` instead — but
    not from a tool whose dependencies elicit via `Resolve(...)`: the
    resolver owns that tool's `request_state` channel, and a forwarded
    result's state would clobber it.

    Args:
        uri: Resource URI to read

    Returns:
        The resource content as either text or bytes

    Raises:
        ResourceNotFoundError: If no resource or template matches the URI.
        ResourceError: If template creation or resource reading fails.
        RuntimeError: If the resource returned an `InputRequiredResult`.
    """
    assert self._mcp_server is not None, "Context is not available outside of a request"
    result = await self._mcp_server.read_resource(uri, self._nested_invocation())
    if isinstance(result, InputRequiredResult):
        raise RuntimeError(
            "Resource returned InputRequiredResult; ctx.read_resource() only returns "
            "content — use MCPServer.read_resource(uri, context) to receive and forward it."
        )
    return result

elicit async 

elicit(
    message: str, schema: type[ElicitSchemaModelT]
) -> ElicitationResult[ElicitSchemaModelT]

Elicit information from the client/user.

This method can be used to interactively ask for additional information from the client within a tool's execution. The client might display the message to the user and collect a response according to the provided schema. If the client is an agent, it might decide how to handle the elicitation -- either by asking the user or automatically generating a response.

Parameters:

Name Type Description Default
message str

Message to present to the user

 required
schema type[ElicitSchemaModelT]

A Pydantic model class defining the expected response structure. According to the specification, only primitive types are allowed.

 required

Returns:

Type Description
ElicitationResult[ElicitSchemaModelT]

An ElicitationResult containing the action taken and the data if accepted
Note

Check the result.action to determine if the user accepted, declined, or cancelled. The result.data will only be populated if action is "accept" and validation succeeded.

Source code in src/mcp/server/mcpserver/context.py 
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async def elicit(
    self,
    message: str,
    schema: type[ElicitSchemaModelT],
) -> ElicitationResult[ElicitSchemaModelT]:
    """Elicit information from the client/user.

    This method can be used to interactively ask for additional information from the
    client within a tool's execution. The client might display the message to the
    user and collect a response according to the provided schema. If the client
    is an agent, it might decide how to handle the elicitation -- either by asking
    the user or automatically generating a response.

    Args:
        message: Message to present to the user
        schema: A Pydantic model class defining the expected response structure.
                According to the specification, only primitive types are allowed.

    Returns:
        An ElicitationResult containing the action taken and the data if accepted

    Note:
        Check the result.action to determine if the user accepted, declined, or cancelled.
        The result.data will only be populated if action is "accept" and validation succeeded.
    """

    return await elicit_with_validation(
        session=self.request_context.session,
        message=message,
        schema=schema,
        related_request_id=self.request_id,
    )

elicit_url async 

elicit_url(
    message: str, url: str, elicitation_id: str
) -> UrlElicitationResult

Request URL mode elicitation from the client.

This directs the user to an external URL for out-of-band interactions that must not pass through the MCP client. Use this for: - Collecting sensitive credentials (API keys, passwords) - OAuth authorization flows with third-party services - Payment and subscription flows - Any interaction where data should not pass through the LLM context

The response indicates whether the user consented to navigate to the URL. The actual interaction happens out-of-band. When the elicitation completes, call ctx.session.send_elicit_complete(elicitation_id) to notify the client.

Parameters:

Name Type Description Default
message str

Human-readable explanation of why the interaction is needed

 required
url str

The URL the user should navigate to

 required
elicitation_id str

Unique identifier for tracking this elicitation

 required

Returns:

Type Description
UrlElicitationResult

UrlElicitationResult indicating accept, decline, or cancel
Source code in src/mcp/server/mcpserver/context.py 
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async def elicit_url(
    self,
    message: str,
    url: str,
    elicitation_id: str,
) -> UrlElicitationResult:
    """Request URL mode elicitation from the client.

    This directs the user to an external URL for out-of-band interactions
    that must not pass through the MCP client. Use this for:
    - Collecting sensitive credentials (API keys, passwords)
    - OAuth authorization flows with third-party services
    - Payment and subscription flows
    - Any interaction where data should not pass through the LLM context

    The response indicates whether the user consented to navigate to the URL.
    The actual interaction happens out-of-band. When the elicitation completes,
    call `ctx.session.send_elicit_complete(elicitation_id)` to notify the client.

    Args:
        message: Human-readable explanation of why the interaction is needed
        url: The URL the user should navigate to
        elicitation_id: Unique identifier for tracking this elicitation

    Returns:
        UrlElicitationResult indicating accept, decline, or cancel
    """
    return await elicit_url(
        session=self.request_context.session,
        message=message,
        url=url,
        elicitation_id=elicitation_id,
        related_request_id=self.request_id,
    )

log async 

log(
    level: LoggingLevel,
    data: Any,
    *,
    logger_name: str | None = None
) -> None

Send a log message to the client.

Parameters:

Name Type Description Default
level LoggingLevel

Log level (debug, info, notice, warning, error, critical, alert, emergency)

 required
data Any

The data to be logged. Any JSON serializable type is allowed (string, dict, list, number, bool, etc.) per the MCP specification.

 required
logger_name str | None

Optional logger name

 None
Source code in src/mcp/server/mcpserver/context.py 
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@deprecated("The logging capability is deprecated as of 2026-07-28 (SEP-2577).", category=MCPDeprecationWarning)
async def log(
    self,
    level: LoggingLevel,
    data: Any,
    *,
    logger_name: str | None = None,
) -> None:
    """Send a log message to the client.

    Args:
        level: Log level (debug, info, notice, warning, error, critical,
            alert, emergency)
        data: The data to be logged. Any JSON serializable type is allowed
            (string, dict, list, number, bool, etc.) per the MCP specification.
        logger_name: Optional logger name
    """
    await self.request_context.session.send_log_message(  # pyright: ignore[reportDeprecated]
        level=level,
        data=data,
        logger=logger_name,
        related_request_id=self.request_id,
    )

client_id property 

client_id: str | None

Get the client ID if available.

Note: this reads from the MCP request's _meta params, not the OAuth bearer token. For that, use get_access_token().client_id.

headers property 

headers: Mapping[str, str] | None

Request headers carried by this message, when the transport has them.

Populated by HTTP-based transports; None on stdio or when the transport's request object carries no headers. Headers are client-supplied input - never treat one as an identity assertion.

request_id property 

request_id: str

Get the unique ID for this request.

protocol_version property 

protocol_version: str | None

The negotiated protocol version, or None outside of an active request.

input_responses property 

input_responses: InputResponses | None

Client responses to a prior InputRequiredResult.input_requests.

None on the initial round, or when the client retried without responses.

request_state property 

request_state: str | None

Opaque state echoed from a prior InputRequiredResult.request_state.

None on the initial round.

client_capabilities property 

client_capabilities: ClientCapabilities | None

The client's declared capabilities for this connection.

None when the client supplied no client info (e.g. an anonymous stateless request without the reserved _meta keys).

session property 

session

Access to the underlying session for advanced usage.

close_sse_stream async 

close_sse_stream() -> None

Close the SSE stream to trigger client reconnection.

This method closes the HTTP connection for the current request, triggering client reconnection. Events continue to be stored in the event store and will be replayed when the client reconnects with Last-Event-ID.

Use this to implement polling behavior during long-running operations - the client will reconnect after the retry interval specified in the priming event.

Note

This is a no-op if not using StreamableHTTP transport with event_store. The callback is only available when event_store is configured.

Source code in src/mcp/server/mcpserver/context.py 
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async def close_sse_stream(self) -> None:
    """Close the SSE stream to trigger client reconnection.

    This method closes the HTTP connection for the current request, triggering
    client reconnection. Events continue to be stored in the event store and will
    be replayed when the client reconnects with Last-Event-ID.

    Use this to implement polling behavior during long-running operations -
    the client will reconnect after the retry interval specified in the priming event.

    Note:
        This is a no-op if not using StreamableHTTP transport with event_store.
        The callback is only available when event_store is configured.
    """
    if self._request_context and self._request_context.close_sse_stream:  # pragma: no branch
        await self._request_context.close_sse_stream()

close_standalone_sse_stream async 

close_standalone_sse_stream() -> None

Close the standalone GET SSE stream to trigger client reconnection.

This method closes the HTTP connection for the standalone GET stream used for unsolicited server-to-client notifications. The client SHOULD reconnect with Last-Event-ID to resume receiving notifications.

Note

This is a no-op if not using StreamableHTTP transport with event_store. Currently, client reconnection for standalone GET streams is NOT implemented - this is a known gap.

Source code in src/mcp/server/mcpserver/context.py 
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async def close_standalone_sse_stream(self) -> None:
    """Close the standalone GET SSE stream to trigger client reconnection.

    This method closes the HTTP connection for the standalone GET stream used
    for unsolicited server-to-client notifications. The client SHOULD reconnect
    with Last-Event-ID to resume receiving notifications.

    Note:
        This is a no-op if not using StreamableHTTP transport with event_store.
        Currently, client reconnection for standalone GET streams is NOT
        implemented - this is a known gap.
    """
    if self._request_context and self._request_context.close_standalone_sse_stream:  # pragma: no cover
        await self._request_context.close_standalone_sse_stream()

debug async 

debug(data: Any, *, logger_name: str | None = None) -> None

Send a debug log message.

Source code in src/mcp/server/mcpserver/context.py 
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@deprecated("The logging capability is deprecated as of 2026-07-28 (SEP-2577).", category=MCPDeprecationWarning)
async def debug(self, data: Any, *, logger_name: str | None = None) -> None:
    """Send a debug log message."""
    await self.log("debug", data, logger_name=logger_name)  # pyright: ignore[reportDeprecated]

info async 

info(data: Any, *, logger_name: str | None = None) -> None

Send an info log message.

Source code in src/mcp/server/mcpserver/context.py 
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@deprecated("The logging capability is deprecated as of 2026-07-28 (SEP-2577).", category=MCPDeprecationWarning)
async def info(self, data: Any, *, logger_name: str | None = None) -> None:
    """Send an info log message."""
    await self.log("info", data, logger_name=logger_name)  # pyright: ignore[reportDeprecated]

warning async 

warning(
    data: Any, *, logger_name: str | None = None
) -> None

Send a warning log message.

Source code in src/mcp/server/mcpserver/context.py 
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@deprecated("The logging capability is deprecated as of 2026-07-28 (SEP-2577).", category=MCPDeprecationWarning)
async def warning(self, data: Any, *, logger_name: str | None = None) -> None:
    """Send a warning log message."""
    await self.log("warning", data, logger_name=logger_name)  # pyright: ignore[reportDeprecated]

error async 

error(data: Any, *, logger_name: str | None = None) -> None

Send an error log message.

Source code in src/mcp/server/mcpserver/context.py 
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@deprecated("The logging capability is deprecated as of 2026-07-28 (SEP-2577).", category=MCPDeprecationWarning)
async def error(self, data: Any, *, logger_name: str | None = None) -> None:
    """Send an error log message."""
    await self.log("error", data, logger_name=logger_name)  # pyright: ignore[reportDeprecated]

AcceptedElicitation

Bases: BaseModel, Generic[ElicitSchemaModelT]

Result when user accepts the elicitation.

Source code in src/mcp/server/elicitation.py 
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class AcceptedElicitation(BaseModel, Generic[ElicitSchemaModelT]):
    """Result when user accepts the elicitation."""

    action: Literal["accept"] = "accept"
    data: ElicitSchemaModelT

CancelledElicitation

Bases: BaseModel

Result when user cancels the elicitation.

Source code in src/mcp/server/elicitation.py 
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class CancelledElicitation(BaseModel):
    """Result when user cancels the elicitation."""

    action: Literal["cancel"] = "cancel"

DeclinedElicitation

Bases: BaseModel

Result when user declines the elicitation.

Source code in src/mcp/server/elicitation.py 
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class DeclinedElicitation(BaseModel):
    """Result when user declines the elicitation."""

    action: Literal["decline"] = "decline"

Elicit

Bases: Generic[T]

A resolver's request to ask the client.

Returned from a resolver to signal that the value must be elicited. The framework runs ctx.elicit(message, schema) and injects the outcome.

Source code in src/mcp/server/mcpserver/resolve.py 
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class Elicit(Generic[T]):
    """A resolver's request to ask the client.

    Returned from a resolver to signal that the value must be elicited. The
    framework runs `ctx.elicit(message, schema)` and injects the outcome.
    """

    def __init__(self, message: str, schema: type[T]) -> None:
        self.message = message
        self.schema = schema

Resolve

Marker for Annotated[T, Resolve(fn)]: fill the parameter by running fn.

Source code in src/mcp/server/mcpserver/resolve.py 
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class Resolve:
    """Marker for `Annotated[T, Resolve(fn)]`: fill the parameter by running `fn`."""

    def __init__(self, fn: Callable[..., Any]) -> None:
        self.fn = fn

DEFAULT_RESOURCE_SECURITY module-attribute 

DEFAULT_RESOURCE_SECURITY = ResourceSecurity()

Secure-by-default policy: traversal, absolute paths, and null bytes rejected.

ResourceSecurity dataclass

Security policy applied to extracted resource template parameters.

These checks run after :meth:~mcp.shared.uri_template.UriTemplate.match has extracted and decoded parameter values. They catch path-traversal and absolute-path injection regardless of how the value was encoded in the URI (literal, %2F, %5C, %2E%2E).

Example::

# Opt out for a parameter that legitimately contains ..
@mcp.resource(
    "git://diff/{+range}",
    security=ResourceSecurity(exempt_params={"range"}),
)
def git_diff(range: str) -> str: ...
Source code in src/mcp/server/mcpserver/resources/templates.py 
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@dataclass(frozen=True)
class ResourceSecurity:
    """Security policy applied to extracted resource template parameters.

    These checks run after :meth:`~mcp.shared.uri_template.UriTemplate.match`
    has extracted and decoded parameter values. They catch path-traversal
    and absolute-path injection regardless of how the value was encoded in
    the URI (literal, ``%2F``, ``%5C``, ``%2E%2E``).

    Example::

        # Opt out for a parameter that legitimately contains ..
        @mcp.resource(
            "git://diff/{+range}",
            security=ResourceSecurity(exempt_params={"range"}),
        )
        def git_diff(range: str) -> str: ...
    """

    reject_path_traversal: bool = True
    """Reject values containing ``..`` as a path component."""

    reject_absolute_paths: bool = True
    """Reject values that look like absolute filesystem paths."""

    reject_null_bytes: bool = True
    """Reject values containing NUL (``\\x00``). Null bytes defeat string
    comparisons (``"..\\x00" != ".."``) and can cause truncation in C
    extensions or subprocess calls."""

    exempt_params: Set[str] = field(default_factory=frozenset[str])
    """Parameter names to skip all checks for."""

    def validate(self, params: Mapping[str, str | list[str]]) -> str | None:
        """Check all parameter values against the configured policy.

        Args:
            params: Extracted template parameters. List values (from
                explode variables) are checked element-wise.

        Returns:
            The name of the first parameter that fails, or ``None`` if
            all values pass.
        """
        for name, value in params.items():
            if name in self.exempt_params:
                continue
            values = value if isinstance(value, list) else [value]
            for v in values:
                if self.reject_null_bytes and "\0" in v:
                    return name
                if self.reject_path_traversal and contains_path_traversal(v):
                    return name
                if self.reject_absolute_paths and is_absolute_path(v):
                    return name
        return None

reject_path_traversal class-attribute instance-attribute 

reject_path_traversal: bool = True

Reject values containing .. as a path component.

reject_absolute_paths class-attribute instance-attribute 

reject_absolute_paths: bool = True

Reject values that look like absolute filesystem paths.

reject_null_bytes class-attribute instance-attribute 

reject_null_bytes: bool = True

Reject values containing NUL (\x00). Null bytes defeat string comparisons ("..\x00" != "..") and can cause truncation in C extensions or subprocess calls.

exempt_params class-attribute instance-attribute 

exempt_params: Set[str] = field(
    default_factory=frozenset[str]
)

Parameter names to skip all checks for.

validate 

validate(
    params: Mapping[str, str | list[str]],
) -> str | None

Check all parameter values against the configured policy.

Parameters:

Name Type Description Default
params Mapping[str, str | list[str]]

Extracted template parameters. List values (from explode variables) are checked element-wise.

 required

Returns:

Type Description
str | None

The name of the first parameter that fails, or None if
str | None

all values pass.
Source code in src/mcp/server/mcpserver/resources/templates.py 
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def validate(self, params: Mapping[str, str | list[str]]) -> str | None:
    """Check all parameter values against the configured policy.

    Args:
        params: Extracted template parameters. List values (from
            explode variables) are checked element-wise.

    Returns:
        The name of the first parameter that fails, or ``None`` if
        all values pass.
    """
    for name, value in params.items():
        if name in self.exempt_params:
            continue
        values = value if isinstance(value, list) else [value]
        for v in values:
            if self.reject_null_bytes and "\0" in v:
                return name
            if self.reject_path_traversal and contains_path_traversal(v):
                return name
            if self.reject_absolute_paths and is_absolute_path(v):
                return name
    return None

MCPServer

Bases: Generic[LifespanResultT]

Source code in src/mcp/server/mcpserver/server.py 
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class MCPServer(Generic[LifespanResultT]):
    def __init__(
        self,
        name: str | None = None,
        title: str | None = None,
        description: str | None = None,
        instructions: str | None = None,
        website_url: str | None = None,
        icons: list[Icon] | None = None,
        version: str | None = None,
        auth_server_provider: OAuthAuthorizationServerProvider[Any, Any, Any] | None = None,
        token_verifier: TokenVerifier | None = None,
        *,
        tools: list[Tool] | None = None,
        resources: list[Resource] | None = None,
        extensions: Sequence[Extension] | None = None,
        debug: bool = False,
        log_level: Literal["DEBUG", "INFO", "WARNING", "ERROR", "CRITICAL"] = "INFO",
        warn_on_duplicate_resources: bool = True,
        warn_on_duplicate_tools: bool = True,
        warn_on_duplicate_prompts: bool = True,
        dependencies: list[str] | None = None,
        lifespan: Callable[[MCPServer[LifespanResultT]], AbstractAsyncContextManager[LifespanResultT]] | None = None,
        auth: AuthSettings | None = None,
        resource_security: ResourceSecurity = DEFAULT_RESOURCE_SECURITY,
        request_state_security: RequestStateSecurity | None = None,
        cache_hints: Mapping[CacheableMethod, CacheHint] | None = None,
        subscriptions: SubscriptionBus | None = None,
    ):
        self._resource_security = resource_security
        self.settings = Settings(
            debug=debug,
            log_level=log_level,
            warn_on_duplicate_resources=warn_on_duplicate_resources,
            warn_on_duplicate_tools=warn_on_duplicate_tools,
            warn_on_duplicate_prompts=warn_on_duplicate_prompts,
            dependencies=dependencies or [],
            lifespan=lifespan,
            auth=auth,
        )
        self.dependencies = self.settings.dependencies

        self._tool_manager = ToolManager(tools=tools, warn_on_duplicate_tools=self.settings.warn_on_duplicate_tools)
        self._resource_manager = ResourceManager(
            resources=resources, warn_on_duplicate_resources=self.settings.warn_on_duplicate_resources
        )
        self._prompt_manager = PromptManager(warn_on_duplicate_prompts=self.settings.warn_on_duplicate_prompts)
        # The subscriptions/listen fan-out seam (2026-07-28). The default bus is
        # in-process; pass an `SubscriptionBus` implementation over an external pub/sub
        # backend to fan events out across replicas.
        self._subscriptions: SubscriptionBus = subscriptions if subscriptions is not None else InMemorySubscriptionBus()
        self._lowlevel_server = Server(
            name=name or "mcp-server",
            title=title,
            description=description,
            instructions=instructions,
            website_url=website_url,
            icons=icons,
            version=version,
            cache_hints=cache_hints,
            on_list_tools=self._handle_list_tools,
            on_call_tool=self._handle_call_tool,
            on_list_resources=self._handle_list_resources,
            on_read_resource=self._handle_read_resource,
            on_list_resource_templates=self._handle_list_resource_templates,
            on_list_prompts=self._handle_list_prompts,
            on_get_prompt=self._handle_get_prompt,
            on_subscriptions_listen=ListenHandler(self._subscriptions),
            # TODO(Marcelo): It seems there's a type mismatch between the lifespan type from an MCPServer and Server.
            # We need to create a Lifespan type that is a generic on the server type, like Starlette does.
            lifespan=(lifespan_wrapper(self, self.settings.lifespan) if self.settings.lifespan else default_lifespan),  # type: ignore
        )
        # Ordering: inside OpenTelemetry (spans record the sealed wire form),
        # outside extension interceptors (extensions see plaintext).
        if request_state_security is None:
            security = RequestStateSecurity.ephemeral()
        else:
            # A supplied policy usually means shared keys, where the audience claim is
            # what separates services; an unnamed server would stamp the placeholder.
            if not name and request_state_security.audience is None:
                raise ValueError(_MISSING_AUDIENCE)
            security = request_state_security
        self._lowlevel_server.middleware.append(RequestStateBoundary(security, default_audience=self.name))
        # Validate auth configuration
        if self.settings.auth is not None:
            if auth_server_provider and token_verifier:  # pragma: no cover
                raise ValueError("Cannot specify both auth_server_provider and token_verifier")
            if not auth_server_provider and not token_verifier:  # pragma: no cover
                raise ValueError("Must specify either auth_server_provider or token_verifier when auth is enabled")
        elif auth_server_provider or token_verifier:
            raise ValueError("Cannot specify auth_server_provider or token_verifier without auth settings")

        self._auth_server_provider = auth_server_provider
        self._token_verifier = token_verifier

        # Create token verifier from provider if needed (backwards compatibility)
        if auth_server_provider and not token_verifier:
            self._token_verifier = ProviderTokenVerifier(auth_server_provider)
        self._custom_starlette_routes: list[Route] = []

        # Configure logging
        configure_logging(self.settings.log_level)

        self._extensions: list[Extension] = []
        for extension in extensions or ():
            self._apply_extension(extension)
        self._install_extension_interceptor()

    @property
    def name(self) -> str:
        return self._lowlevel_server.name

    @property
    def title(self) -> str | None:
        return self._lowlevel_server.title

    @property
    def description(self) -> str | None:
        return self._lowlevel_server.description

    @property
    def instructions(self) -> str | None:
        return self._lowlevel_server.instructions

    @property
    def website_url(self) -> str | None:
        return self._lowlevel_server.website_url

    @property
    def icons(self) -> list[Icon] | None:
        return self._lowlevel_server.icons

    @property
    def version(self) -> str | None:
        return self._lowlevel_server.version

    @property
    def session_manager(self) -> StreamableHTTPSessionManager:
        """Get the StreamableHTTP session manager.

        This is exposed to enable advanced use cases like mounting multiple
        MCPServer instances in a single FastAPI application.

        Raises:
            RuntimeError: If called before streamable_http_app() has been called.
        """
        return self._lowlevel_server.session_manager

    def _apply_extension(self, extension: Extension) -> None:
        """Apply one opt-in extension's contributions through the public surface.

        Registers its tools/resources/methods and advertises its settings under
        `ServerCapabilities.extensions[extension.identifier]`. Extensions are fixed
        at construction, so this is private; the `tools/call` interceptor is
        composed once afterwards by `_install_extension_interceptor`.
        """
        identifier = getattr(extension, "identifier", None)
        validate_extension_identifier(identifier, owner=type(extension).__name__)
        if any(e.identifier == identifier for e in self._extensions):
            raise ValueError(f"Extension {identifier!r} is already registered")
        self._extensions.append(extension)

        for tool in extension.tools():
            self.add_tool(tool.fn, meta=tool.meta, **tool.kwargs)
        for resource in extension.resources():
            self.add_resource(resource.resource)
        for method in extension.methods():
            if self._lowlevel_server.get_request_handler(method.method) is not None:
                raise ValueError(
                    f"Extension {identifier!r} binds method {method.method!r}, which is already "
                    "registered; extension methods are additive and cannot replace another handler"
                )
            handler = _version_gated(method) if method.protocol_versions is not None else method.handler
            self._lowlevel_server.add_request_handler(method.method, method.params_type, handler)

        self._lowlevel_server.extensions[extension.identifier] = extension.settings()

    def _install_extension_interceptor(self) -> None:
        """Compose every extension's `tools/call` interceptor into one middleware.

        Installed only when at least one extension overrides `intercept_tool_call`,
        so a server with purely additive extensions adds no middleware.
        """
        if any(type(e).intercept_tool_call is not Extension.intercept_tool_call for e in self._extensions):
            self._lowlevel_server.middleware.append(compose_tool_call_interceptor(self._extensions))

    @overload
    def run(self, transport: Literal["stdio"] = ...) -> None: ...

    @overload
    def run(
        self,
        transport: Literal["sse"],
        *,
        host: str = ...,
        port: int = ...,
        sse_path: str = ...,
        message_path: str = ...,
        transport_security: TransportSecuritySettings | None = ...,
    ) -> None: ...

    @overload
    def run(
        self,
        transport: Literal["streamable-http"],
        *,
        host: str = ...,
        port: int = ...,
        streamable_http_path: str = ...,
        json_response: bool = ...,
        stateless_http: bool = ...,
        event_store: EventStore | None = ...,
        retry_interval: int | None = ...,
        transport_security: TransportSecuritySettings | None = ...,
    ) -> None: ...

    def run(
        self,
        transport: Literal["stdio", "sse", "streamable-http"] = "stdio",
        **kwargs: Any,
    ) -> None:
        """Run the MCP server. Note this is a synchronous function.

        Args:
            transport: Transport protocol to use ("stdio", "sse", or "streamable-http")
            **kwargs: Transport-specific options (see overloads for details)
        """
        TRANSPORTS = Literal["stdio", "sse", "streamable-http"]
        if transport not in TRANSPORTS.__args__:  # type: ignore  # pragma: no cover
            raise ValueError(f"Unknown transport: {transport}")

        match transport:
            case "stdio":
                anyio.run(self.run_stdio_async)
            case "sse":  # pragma: no cover
                anyio.run(lambda: self.run_sse_async(**kwargs))
            case "streamable-http":  # pragma: no cover
                anyio.run(lambda: self.run_streamable_http_async(**kwargs))

    async def _handle_list_tools(
        self, ctx: ServerRequestContext[LifespanResultT], params: PaginatedRequestParams | None
    ) -> ListToolsResult:
        return ListToolsResult(tools=await self.list_tools())

    async def _handle_call_tool(
        self, ctx: ServerRequestContext[LifespanResultT], params: CallToolRequestParams
    ) -> CallToolResult | InputRequiredResult:
        context = Context(request_context=ctx, mcp_server=self, input_params=params, subscriptions=self._subscriptions)
        try:
            return await self.call_tool(params.name, params.arguments or {}, context)
        except MCPError:
            raise
        except Exception as e:
            return CallToolResult(content=[TextContent(type="text", text=str(e))], is_error=True)

    async def _handle_list_resources(
        self, ctx: ServerRequestContext[LifespanResultT], params: PaginatedRequestParams | None
    ) -> ListResourcesResult:
        return ListResourcesResult(resources=await self.list_resources())

    async def _handle_read_resource(
        self, ctx: ServerRequestContext[LifespanResultT], params: ReadResourceRequestParams
    ) -> ReadResourceResult | InputRequiredResult:
        context = Context(request_context=ctx, mcp_server=self, input_params=params, subscriptions=self._subscriptions)
        try:
            results = await self.read_resource(params.uri, context)
        except ResourceNotFoundError as err:
            raise MCPError(code=INVALID_PARAMS, message=str(err), data={"uri": str(params.uri)})
        except ResourceError as err:
            raise MCPError(code=INTERNAL_ERROR, message=str(err), data={"uri": str(params.uri)})
        if isinstance(results, InputRequiredResult):
            return results
        contents: list[TextResourceContents | BlobResourceContents] = []
        for item in results:
            if isinstance(item.content, bytes):
                contents.append(
                    BlobResourceContents(
                        uri=params.uri,
                        blob=base64.b64encode(item.content).decode(),
                        mime_type=item.mime_type or "application/octet-stream",
                        _meta=item.meta,
                    )
                )
            else:
                contents.append(
                    TextResourceContents(
                        uri=params.uri,
                        text=item.content,
                        mime_type=item.mime_type or "text/plain",
                        _meta=item.meta,
                    )
                )
        return ReadResourceResult(contents=contents)

    async def _handle_list_resource_templates(
        self, ctx: ServerRequestContext[LifespanResultT], params: PaginatedRequestParams | None
    ) -> ListResourceTemplatesResult:
        return ListResourceTemplatesResult(resource_templates=await self.list_resource_templates())

    async def _handle_list_prompts(
        self, ctx: ServerRequestContext[LifespanResultT], params: PaginatedRequestParams | None
    ) -> ListPromptsResult:
        return ListPromptsResult(prompts=await self.list_prompts())

    async def _handle_get_prompt(
        self, ctx: ServerRequestContext[LifespanResultT], params: GetPromptRequestParams
    ) -> GetPromptResult | InputRequiredResult:
        context = Context(request_context=ctx, mcp_server=self, input_params=params, subscriptions=self._subscriptions)
        return await self.get_prompt(params.name, params.arguments, context)

    async def list_tools(self) -> list[MCPTool]:
        """List all available tools."""
        tools = self._tool_manager.list_tools()
        return [
            MCPTool(
                name=info.name,
                title=info.title,
                description=info.description,
                input_schema=info.parameters,
                output_schema=info.output_schema,
                annotations=info.annotations,
                icons=info.icons,
                _meta=info.meta,
            )
            for info in tools
        ]

    async def call_tool(
        self, name: str, arguments: dict[str, Any], context: Context[LifespanResultT, Any] | None = None
    ) -> CallToolResult | InputRequiredResult:
        """Call a tool by name with arguments."""
        if context is None:
            context = Context(mcp_server=self, subscriptions=self._subscriptions)
        return await self._tool_manager.call_tool(name, arguments, context, convert_result=True)

    async def list_resources(self) -> list[MCPResource]:
        """List all available resources."""

        resources = self._resource_manager.list_resources()
        return [
            MCPResource(
                uri=resource.uri,
                name=resource.name or "",
                title=resource.title,
                description=resource.description,
                mime_type=resource.mime_type,
                icons=resource.icons,
                annotations=resource.annotations,
                _meta=resource.meta,
            )
            for resource in resources
        ]

    async def list_resource_templates(self) -> list[MCPResourceTemplate]:
        templates = self._resource_manager.list_templates()
        return [
            MCPResourceTemplate(
                uri_template=template.uri_template,
                name=template.name,
                title=template.title,
                description=template.description,
                mime_type=template.mime_type,
                icons=template.icons,
                annotations=template.annotations,
                _meta=template.meta,
            )
            for template in templates
        ]

    async def read_resource(
        self, uri: AnyUrl | str, context: Context[LifespanResultT, Any] | None = None
    ) -> Iterable[ReadResourceContents] | InputRequiredResult:
        """Read a resource by URI.

        An `InputRequiredResult` returned by a resource template function is
        passed through unchanged (the 2026-07-28 multi-round-trip flow); the
        retry's answers arrive on `ctx.input_responses`, with
        `ctx.request_state` carrying the echoed opaque state.

        Raises:
            ResourceNotFoundError: If no resource or template matches the URI.
            ResourceError: If template creation or resource reading fails.
        """
        if context is None:
            context = Context(mcp_server=self, subscriptions=self._subscriptions)
        resource = await self._resource_manager.get_resource(uri, context)
        if isinstance(resource, InputRequiredResult):
            return resource

        try:
            content = await resource.read()
            return [ReadResourceContents(content=content, mime_type=resource.mime_type, meta=resource.meta)]
        except MCPError:
            raise
        except Exception as exc:
            logger.exception(f"Error getting resource {uri}")
            # If an exception happens when reading the resource, we should not leak the exception to the client.
            raise ResourceError(f"Error reading resource {uri}") from exc

    def add_tool(
        self,
        fn: Callable[..., Any],
        name: str | None = None,
        title: str | None = None,
        description: str | None = None,
        annotations: ToolAnnotations | None = None,
        icons: list[Icon] | None = None,
        meta: dict[str, Any] | None = None,
        structured_output: bool | None = None,
    ) -> None:
        """Add a tool to the server.

        The tool function can optionally request a Context object by adding a parameter
        with the Context type annotation. See the @tool decorator for examples.

        Args:
            fn: The function to register as a tool
            name: Optional name for the tool (defaults to function name)
            title: Optional human-readable title for the tool
            description: Optional description of what the tool does
            annotations: Optional ToolAnnotations providing additional tool information
            icons: Optional list of icons for the tool
            meta: Optional metadata dictionary for the tool
            structured_output: Controls whether the tool's output is structured or unstructured
                - If None, auto-detects based on the function's return type annotation
                - If True, creates a structured tool (return type annotation permitting)
                - If False, unconditionally creates an unstructured tool
        """
        self._tool_manager.add_tool(
            fn,
            name=name,
            title=title,
            description=description,
            annotations=annotations,
            icons=icons,
            meta=meta,
            structured_output=structured_output,
        )

    def remove_tool(self, name: str) -> None:
        """Remove a tool from the server by name.

        Args:
            name: The name of the tool to remove

        Raises:
            ToolError: If the tool does not exist
        """
        self._tool_manager.remove_tool(name)

    def tool(
        self,
        name: str | None = None,
        title: str | None = None,
        description: str | None = None,
        annotations: ToolAnnotations | None = None,
        icons: list[Icon] | None = None,
        meta: dict[str, Any] | None = None,
        structured_output: bool | None = None,
    ) -> Callable[[_CallableT], _CallableT]:
        """Decorator to register a tool.

        Tools can optionally request a Context object by adding a parameter with the
        Context type annotation. The context provides access to MCP capabilities like
        logging, progress reporting, and resource access.

        Args:
            name: Optional name for the tool (defaults to function name)
            title: Optional human-readable title for the tool
            description: Optional description of what the tool does
            annotations: Optional ToolAnnotations providing additional tool information
            icons: Optional list of icons for the tool
            meta: Optional metadata dictionary for the tool
            structured_output: Controls whether the tool's output is structured or unstructured
                - If None, auto-detects based on the function's return type annotation
                - If True, creates a structured tool (return type annotation permitting)
                - If False, unconditionally creates an unstructured tool

        Example:
            ```python
            @server.tool()
            def my_tool(x: int) -> str:
                return str(x)
            ```

            ```python
            @server.tool()
            async def tool_with_context(x: int, ctx: Context) -> str:
                await ctx.info(f"Processing {x}")
                return str(x)
            ```

            ```python
            @server.tool()
            async def async_tool(x: int, context: Context) -> str:
                await context.report_progress(50, 100)
                return str(x)
            ```
        """
        # Check if user passed function directly instead of calling decorator
        if callable(name):
            raise TypeError(
                "The @tool decorator was used incorrectly. Did you forget to call it? Use @tool() instead of @tool"
            )

        def decorator(fn: _CallableT) -> _CallableT:
            self.add_tool(
                fn,
                name=name,
                title=title,
                description=description,
                annotations=annotations,
                icons=icons,
                meta=meta,
                structured_output=structured_output,
            )
            return fn

        return decorator

    def completion(self):
        """Decorator to register a completion handler.

        The completion handler receives:
        - ref: PromptReference or ResourceTemplateReference
        - argument: CompletionArgument with name and partial value
        - context: Optional CompletionContext with previously resolved arguments

        Example:
            ```python
            @mcp.completion()
            async def handle_completion(ref, argument, context):
                if isinstance(ref, ResourceTemplateReference):
                    # Return completions based on ref, argument, and context
                    return Completion(values=["option1", "option2"])
                return None
            ```
        """

        def decorator(func: _CallableT) -> _CallableT:
            async def handler(
                ctx: ServerRequestContext[LifespanResultT], params: CompleteRequestParams
            ) -> CompleteResult:
                result = await func(params.ref, params.argument, params.context)
                return CompleteResult(
                    completion=result if result is not None else Completion(values=[], total=None, has_more=None),
                )

            self._lowlevel_server.add_request_handler("completion/complete", CompleteRequestParams, handler)
            return func

        return decorator

    def add_resource(self, resource: Resource) -> None:
        """Add a resource to the server.

        Args:
            resource: A Resource instance to add
        """
        self._resource_manager.add_resource(resource)

    def resource(
        self,
        uri: str,
        *,
        name: str | None = None,
        title: str | None = None,
        description: str | None = None,
        mime_type: str | None = None,
        icons: list[Icon] | None = None,
        annotations: Annotations | None = None,
        meta: dict[str, Any] | None = None,
        security: ResourceSecurity | None = None,
    ) -> Callable[[_CallableT], _CallableT]:
        """Decorator to register a function as a resource.

        The function will be called when the resource is read to generate its content.
        The function can return:
        - str for text content
        - bytes for binary content
        - an InputRequiredResult (template resources only; passed through
          unchanged for the 2026-07-28 multi-round-trip flow — read
          `ctx.input_responses` on the retry)
        - other types will be converted to JSON

        If the URI contains parameters (e.g. "resource://{param}"), it is
        registered as a template resource. Otherwise it is registered as a
        static resource; function parameters on a static URI raise an error.

        Args:
            uri: URI for the resource (e.g. "resource://my-resource" or "resource://{param}")
            name: Optional name for the resource
            title: Optional human-readable title for the resource
            description: Optional description of the resource
            mime_type: Optional MIME type for the resource
            icons: Optional list of icons for the resource
            annotations: Optional annotations for the resource
            meta: Optional metadata dictionary for the resource
            security: Path-safety policy for extracted template parameters.
                Defaults to the server's ``resource_security`` setting.
                Only applies to template resources.

        Example:
            ```python
            @server.resource("resource://my-resource")
            def get_data() -> str:
                return "Hello, world!"

            @server.resource("resource://my-resource")
            async def get_data() -> str:
                data = await fetch_data()
                return f"Hello, world! {data}"

            @server.resource("resource://{city}/weather")
            def get_weather(city: str) -> str:
                return f"Weather for {city}"

            @server.resource("resource://{city}/weather")
            async def get_weather(city: str) -> str:
                data = await fetch_weather(city)
                return f"Weather for {city}: {data}"
            ```

        Raises:
            InvalidUriTemplate: If ``uri`` is not a valid RFC 6570 template.
            ValueError: If URI template parameters don't match the
                function's parameters, or if a parameter bound to a
                ``{?...}``/``{&...}`` query variable has no default
                (the client may omit it).
            TypeError: If the decorator is applied without being called
                (``@resource`` instead of ``@resource("uri")``).
        """
        # Check if user passed function directly instead of calling decorator
        if callable(uri):
            raise TypeError(
                "The @resource decorator was used incorrectly. "
                "Did you forget to call it? Use @resource('uri') instead of @resource"
            )

        # Parse once, early — surfaces malformed-template errors at
        # decoration time with a clear position, and gives us correct
        # variable names for all RFC 6570 operators.
        parsed = UriTemplate.parse(uri)
        uri_params = set(parsed.variable_names)

        def decorator(fn: _CallableT) -> _CallableT:
            sig = inspect.signature(fn)
            context_param = find_context_parameter(fn)
            func_params = {p for p in sig.parameters.keys() if p != context_param}

            # Template/static is decided purely by the URI: variables
            # present means template, none means static.
            if uri_params:
                if uri_params != func_params:
                    raise ValueError(
                        f"Mismatch between URI parameters {uri_params} and function parameters {func_params}"
                    )

                # A {?...}/{&...} query variable is optional on the wire:
                # match() omits it from the extracted parameters when the
                # client leaves it out of the URI. The handler parameter
                # bound to it must therefore have a Python default; without
                # one, the author only finds out on the first request that
                # omits it, as an opaque internal error.
                missing_defaults = sorted(
                    name
                    for name in parsed.query_variable_names
                    if sig.parameters[name].default is inspect.Parameter.empty
                )
                if missing_defaults:
                    raise ValueError(
                        f"Resource {uri!r}: query parameter(s) {missing_defaults} have no "
                        f"default value. A client may omit a {{?...}}/{{&...}} query "
                        f"parameter, so the matching handler parameter must declare a "
                        f"default."
                    )

                # Register as template
                self._resource_manager.add_template(
                    fn=fn,
                    uri_template=uri,
                    name=name,
                    title=title,
                    description=description,
                    mime_type=mime_type,
                    icons=icons,
                    annotations=annotations,
                    security=security if security is not None else self._resource_security,
                    meta=meta,
                )
            else:
                if func_params:
                    raise ValueError(
                        f"Resource {uri!r} has no URI template variables, but the "
                        f"handler declares parameters {func_params}. Add matching "
                        f"{{...}} variables to the URI or remove the parameters."
                    )
                if context_param is not None:
                    raise ValueError(
                        f"Resource {uri!r} has no URI template variables, but the "
                        f"handler declares a Context parameter. Context injection "
                        f"for static resources is not supported. "
                        f"Add a template variable to the URI or remove the "
                        f"Context parameter."
                    )
                # Register as regular resource
                resource = FunctionResource.from_function(
                    fn=fn,
                    uri=uri,
                    name=name,
                    title=title,
                    description=description,
                    mime_type=mime_type,
                    icons=icons,
                    annotations=annotations,
                    meta=meta,
                )
                self.add_resource(resource)
            return fn

        return decorator

    def add_prompt(self, prompt: Prompt) -> None:
        """Add a prompt to the server.

        Args:
            prompt: A Prompt instance to add
        """
        self._prompt_manager.add_prompt(prompt)

    def remove_prompt(self, name: str) -> None:
        """Remove a prompt from the server by name.

        Args:
            name: The name of the prompt to remove

        Raises:
            ValueError: If the prompt does not exist
        """
        self._prompt_manager.remove_prompt(name)

    def prompt(
        self,
        name: str | None = None,
        title: str | None = None,
        description: str | None = None,
        icons: list[Icon] | None = None,
    ) -> Callable[[_CallableT], _CallableT]:
        """Decorator to register a prompt.

        The function returns the prompt messages (a string, `Message`, dict,
        or a sequence of these), or an `InputRequiredResult` to request
        client input first (the 2026-07-28 multi-round-trip flow — read
        `ctx.input_responses` on the retry).

        Args:
            name: Optional name for the prompt (defaults to function name)
            title: Optional human-readable title for the prompt
            description: Optional description of what the prompt does
            icons: Optional list of icons for the prompt

        Example:
            ```python
            @server.prompt()
            def analyze_table(table_name: str) -> list[Message]:
                schema = read_table_schema(table_name)
                return [
                    {
                        "role": "user",
                        "content": f"Analyze this schema:\n{schema}"
                    }
                ]

            @server.prompt()
            async def analyze_file(path: str) -> list[Message]:
                content = await read_file(path)
                return [
                    {
                        "role": "user",
                        "content": {
                            "type": "resource",
                            "resource": {
                                "uri": f"file://{path}",
                                "text": content
                            }
                        }
                    }
                ]
            ```
        """
        # Check if user passed function directly instead of calling decorator
        if callable(name):
            raise TypeError(
                "The @prompt decorator was used incorrectly. "
                "Did you forget to call it? Use @prompt() instead of @prompt"
            )

        def decorator(func: _CallableT) -> _CallableT:
            prompt = Prompt.from_function(func, name=name, title=title, description=description, icons=icons)
            self.add_prompt(prompt)
            return func

        return decorator

    def custom_route(
        self,
        path: str,
        methods: list[str],
        name: str | None = None,
        include_in_schema: bool = True,
    ):
        """Decorator to register a custom HTTP route on the MCP server.

        Allows adding arbitrary HTTP endpoints outside the standard MCP protocol,
        which can be useful for OAuth callbacks, health checks, or admin APIs.
        The handler function must be an async function that accepts a Starlette
        Request and returns a Response.

        Routes using this decorator will not require authorization. It is intended
        for uses that are either a part of authorization flows or intended to be
        public such as health check endpoints.

        Args:
            path: URL path for the route (e.g., "/oauth/callback")
            methods: List of HTTP methods to support (e.g., ["GET", "POST"])
            name: Optional name for the route (to reference this route with
                  Starlette's reverse URL lookup feature)
            include_in_schema: Whether to include in OpenAPI schema, defaults to True

        Example:
            ```python
            @server.custom_route("/health", methods=["GET"])
            async def health_check(request: Request) -> Response:
                return JSONResponse({"status": "ok"})
            ```
        """

        def decorator(
            func: Callable[[Request], Awaitable[Response]],
        ) -> Callable[[Request], Awaitable[Response]]:
            self._custom_starlette_routes.append(
                Route(path, endpoint=func, methods=methods, name=name, include_in_schema=include_in_schema)
            )
            return func

        return decorator

    async def run_stdio_async(self) -> None:
        """Run the server using stdio transport."""
        async with stdio_server() as (read_stream, write_stream):
            await self._lowlevel_server.run(
                read_stream,
                write_stream,
                self._lowlevel_server.create_initialization_options(),
            )

    async def run_sse_async(  # pragma: no cover
        self,
        *,
        host: str = "127.0.0.1",
        port: int = 8000,
        sse_path: str = "/sse",
        message_path: str = "/messages/",
        transport_security: TransportSecuritySettings | None = None,
    ) -> None:
        """Run the server using SSE transport."""
        import uvicorn

        starlette_app = self.sse_app(
            sse_path=sse_path,
            message_path=message_path,
            transport_security=transport_security,
            host=host,
        )

        config = uvicorn.Config(
            starlette_app,
            host=host,
            port=port,
            log_level=self.settings.log_level.lower(),
        )
        server = uvicorn.Server(config)
        await server.serve()

    async def run_streamable_http_async(  # pragma: no cover
        self,
        *,
        host: str = "127.0.0.1",
        port: int = 8000,
        streamable_http_path: str = "/mcp",
        json_response: bool = False,
        stateless_http: bool = False,
        event_store: EventStore | None = None,
        retry_interval: int | None = None,
        transport_security: TransportSecuritySettings | None = None,
    ) -> None:
        """Run the server using StreamableHTTP transport."""
        import uvicorn

        starlette_app = self.streamable_http_app(
            streamable_http_path=streamable_http_path,
            json_response=json_response,
            stateless_http=stateless_http,
            event_store=event_store,
            retry_interval=retry_interval,
            transport_security=transport_security,
            host=host,
        )

        config = uvicorn.Config(
            starlette_app,
            host=host,
            port=port,
            log_level=self.settings.log_level.lower(),
        )
        server = uvicorn.Server(config)
        await server.serve()

    def sse_app(
        self,
        *,
        sse_path: str = "/sse",
        message_path: str = "/messages/",
        transport_security: TransportSecuritySettings | None = None,
        host: str = "127.0.0.1",
    ) -> Starlette:
        """Return an instance of the SSE server app."""
        # Auto-enable DNS rebinding protection for localhost (IPv4 and IPv6)
        if transport_security is None and host in ("127.0.0.1", "localhost", "::1"):
            transport_security = TransportSecuritySettings(
                enable_dns_rebinding_protection=True,
                allowed_hosts=["127.0.0.1:*", "localhost:*", "[::1]:*"],
                allowed_origins=["http://127.0.0.1:*", "http://localhost:*", "http://[::1]:*"],
            )

        sse = SseServerTransport(message_path, security_settings=transport_security)

        async def handle_sse(scope: Scope, receive: Receive, send: Send):  # pragma: no cover
            # Add client ID from auth context into request context if available

            async with sse.connect_sse(scope, receive, send) as streams:
                await self._lowlevel_server.run(
                    streams[0], streams[1], self._lowlevel_server.create_initialization_options()
                )
            return Response()

        # Create routes
        routes: list[Route | Mount] = []
        middleware: list[Middleware] = []
        required_scopes: list[str] = []

        # Set up auth if configured
        if self.settings.auth:  # pragma: no cover
            required_scopes = self.settings.auth.required_scopes or []

            # Add auth middleware if token verifier is available
            if self._token_verifier:
                middleware = [
                    # extract auth info from request (but do not require it)
                    Middleware(
                        AuthenticationMiddleware,
                        backend=BearerAuthBackend(self._token_verifier),
                    ),
                    # Add the auth context middleware to store
                    # authenticated user in a contextvar
                    Middleware(AuthContextMiddleware),
                ]

            # Add auth endpoints if auth server provider is configured
            if self._auth_server_provider:
                from mcp.server.auth.routes import create_auth_routes

                routes.extend(
                    create_auth_routes(
                        provider=self._auth_server_provider,
                        issuer_url=self.settings.auth.issuer_url,
                        service_documentation_url=self.settings.auth.service_documentation_url,
                        client_registration_options=self.settings.auth.client_registration_options,
                        revocation_options=self.settings.auth.revocation_options,
                        identity_assertion_enabled=self.settings.auth.identity_assertion_enabled,
                    )
                )

        # When auth is configured, require authentication
        if self._token_verifier:  # pragma: no cover
            # Determine resource metadata URL
            resource_metadata_url = None
            if self.settings.auth and self.settings.auth.resource_server_url:
                from mcp.server.auth.routes import build_resource_metadata_url

                # Build compliant metadata URL for WWW-Authenticate header
                resource_metadata_url = build_resource_metadata_url(self.settings.auth.resource_server_url)

            # Auth is enabled, wrap the endpoints with RequireAuthMiddleware
            routes.append(
                Route(
                    sse_path,
                    endpoint=RequireAuthMiddleware(handle_sse, required_scopes, resource_metadata_url),
                    methods=["GET"],
                )
            )
            routes.append(
                Mount(
                    message_path,
                    app=RequireAuthMiddleware(sse.handle_post_message, required_scopes, resource_metadata_url),
                )
            )
        else:
            # Auth is disabled, no need for RequireAuthMiddleware
            # Since handle_sse is an ASGI app, we need to create a compatible endpoint
            async def sse_endpoint(request: Request) -> Response:  # pragma: no cover
                # Convert the Starlette request to ASGI parameters
                return await handle_sse(request.scope, request.receive, request._send)  # type: ignore[reportPrivateUsage]

            routes.append(
                Route(
                    sse_path,
                    endpoint=sse_endpoint,
                    methods=["GET"],
                )
            )
            routes.append(
                Mount(
                    message_path,
                    app=sse.handle_post_message,
                )
            )
        # Add protected resource metadata endpoint if configured as RS
        if self.settings.auth and self.settings.auth.resource_server_url:  # pragma: no cover
            from mcp.server.auth.routes import create_protected_resource_routes

            routes.extend(
                create_protected_resource_routes(
                    resource_url=self.settings.auth.resource_server_url,
                    authorization_servers=[self.settings.auth.issuer_url],
                    scopes_supported=self.settings.auth.required_scopes,
                )
            )

        # mount these routes last, so they have the lowest route matching precedence
        routes.extend(self._custom_starlette_routes)

        # Create Starlette app with routes and middleware
        return Starlette(debug=self.settings.debug, routes=routes, middleware=middleware)

    def streamable_http_app(
        self,
        *,
        streamable_http_path: str = "/mcp",
        json_response: bool = False,
        stateless_http: bool = False,
        event_store: EventStore | None = None,
        retry_interval: int | None = None,
        transport_security: TransportSecuritySettings | None = None,
        host: str = "127.0.0.1",
    ) -> Starlette:
        """Return an instance of the StreamableHTTP server app."""
        return self._lowlevel_server.streamable_http_app(
            streamable_http_path=streamable_http_path,
            json_response=json_response,
            stateless_http=stateless_http,
            event_store=event_store,
            retry_interval=retry_interval,
            transport_security=transport_security,
            host=host,
            auth=self.settings.auth,
            token_verifier=self._token_verifier,
            auth_server_provider=self._auth_server_provider,
            custom_starlette_routes=self._custom_starlette_routes,
            debug=self.settings.debug,
        )

    async def list_prompts(self) -> list[MCPPrompt]:
        """List all available prompts."""
        prompts = self._prompt_manager.list_prompts()
        return [
            MCPPrompt(
                name=prompt.name,
                title=prompt.title,
                description=prompt.description,
                arguments=[
                    MCPPromptArgument(
                        name=arg.name,
                        description=arg.description,
                        required=arg.required,
                    )
                    for arg in (prompt.arguments or [])
                ],
                icons=prompt.icons,
            )
            for prompt in prompts
        ]

    async def get_prompt(
        self, name: str, arguments: dict[str, Any] | None = None, context: Context[LifespanResultT, Any] | None = None
    ) -> GetPromptResult | InputRequiredResult:
        """Get a prompt by name with arguments.

        An `InputRequiredResult` returned by the prompt function is passed
        through unchanged (the 2026-07-28 multi-round-trip flow); the retry's
        answers arrive on `ctx.input_responses`, with `ctx.request_state`
        carrying the echoed opaque state.
        """
        if context is None:
            context = Context(mcp_server=self, subscriptions=self._subscriptions)
        try:
            prompt = self._prompt_manager.get_prompt(name)
            if not prompt:
                raise ValueError(f"Unknown prompt: {name}")

            rendered = await prompt.render(arguments, context)
            if isinstance(rendered, InputRequiredResult):
                return rendered

            return GetPromptResult(
                description=prompt.description,
                messages=pydantic_core.to_jsonable_python(rendered),
            )
        except MCPError:
            raise
        except Exception as e:
            logger.exception(f"Error getting prompt {name}")
            raise ValueError(str(e)) from e

session_manager property 

session_manager: StreamableHTTPSessionManager

Get the StreamableHTTP session manager.

This is exposed to enable advanced use cases like mounting multiple MCPServer instances in a single FastAPI application.

Raises:

Type Description
RuntimeError

If called before streamable_http_app() has been called.

run 

run(transport: Literal['stdio'] = ...) -> None

run(
    transport: Literal["sse"],
    *,
    host: str = ...,
    port: int = ...,
    sse_path: str = ...,
    message_path: str = ...,
    transport_security: (
        TransportSecuritySettings | None
    ) = ...
) -> None

run(
    transport: Literal["streamable-http"],
    *,
    host: str = ...,
    port: int = ...,
    streamable_http_path: str = ...,
    json_response: bool = ...,
    stateless_http: bool = ...,
    event_store: EventStore | None = ...,
    retry_interval: int | None = ...,
    transport_security: (
        TransportSecuritySettings | None
    ) = ...
) -> None

run(
    transport: Literal[
        "stdio", "sse", "streamable-http"
    ] = "stdio",
    **kwargs: Any
) -> None

Run the MCP server. Note this is a synchronous function.

Parameters:

Name Type Description Default
transport Literal['stdio', 'sse', 'streamable-http']

Transport protocol to use ("stdio", "sse", or "streamable-http")

 'stdio'
**kwargs Any

Transport-specific options (see overloads for details)

 {}
Source code in src/mcp/server/mcpserver/server.py 
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def run(
    self,
    transport: Literal["stdio", "sse", "streamable-http"] = "stdio",
    **kwargs: Any,
) -> None:
    """Run the MCP server. Note this is a synchronous function.

    Args:
        transport: Transport protocol to use ("stdio", "sse", or "streamable-http")
        **kwargs: Transport-specific options (see overloads for details)
    """
    TRANSPORTS = Literal["stdio", "sse", "streamable-http"]
    if transport not in TRANSPORTS.__args__:  # type: ignore  # pragma: no cover
        raise ValueError(f"Unknown transport: {transport}")

    match transport:
        case "stdio":
            anyio.run(self.run_stdio_async)
        case "sse":  # pragma: no cover
            anyio.run(lambda: self.run_sse_async(**kwargs))
        case "streamable-http":  # pragma: no cover
            anyio.run(lambda: self.run_streamable_http_async(**kwargs))

list_tools async 

list_tools() -> list[Tool]

List all available tools.

Source code in src/mcp/server/mcpserver/server.py 
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async def list_tools(self) -> list[MCPTool]:
    """List all available tools."""
    tools = self._tool_manager.list_tools()
    return [
        MCPTool(
            name=info.name,
            title=info.title,
            description=info.description,
            input_schema=info.parameters,
            output_schema=info.output_schema,
            annotations=info.annotations,
            icons=info.icons,
            _meta=info.meta,
        )
        for info in tools
    ]

call_tool async 

call_tool(
    name: str,
    arguments: dict[str, Any],
    context: Context[LifespanResultT, Any] | None = None,
) -> CallToolResult | InputRequiredResult

Call a tool by name with arguments.

Source code in src/mcp/server/mcpserver/server.py 
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async def call_tool(
    self, name: str, arguments: dict[str, Any], context: Context[LifespanResultT, Any] | None = None
) -> CallToolResult | InputRequiredResult:
    """Call a tool by name with arguments."""
    if context is None:
        context = Context(mcp_server=self, subscriptions=self._subscriptions)
    return await self._tool_manager.call_tool(name, arguments, context, convert_result=True)

list_resources async 

list_resources() -> list[Resource]

List all available resources.

Source code in src/mcp/server/mcpserver/server.py 
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async def list_resources(self) -> list[MCPResource]:
    """List all available resources."""

    resources = self._resource_manager.list_resources()
    return [
        MCPResource(
            uri=resource.uri,
            name=resource.name or "",
            title=resource.title,
            description=resource.description,
            mime_type=resource.mime_type,
            icons=resource.icons,
            annotations=resource.annotations,
            _meta=resource.meta,
        )
        for resource in resources
    ]

read_resource async 

read_resource(
    uri: AnyUrl | str,
    context: Context[LifespanResultT, Any] | None = None,
) -> Iterable[ReadResourceContents] | InputRequiredResult

Read a resource by URI.

An InputRequiredResult returned by a resource template function is passed through unchanged (the 2026-07-28 multi-round-trip flow); the retry's answers arrive on ctx.input_responses, with ctx.request_state carrying the echoed opaque state.

Raises:

Type Description
ResourceNotFoundError

If no resource or template matches the URI.
ResourceError

If template creation or resource reading fails.
Source code in src/mcp/server/mcpserver/server.py 
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async def read_resource(
    self, uri: AnyUrl | str, context: Context[LifespanResultT, Any] | None = None
) -> Iterable[ReadResourceContents] | InputRequiredResult:
    """Read a resource by URI.

    An `InputRequiredResult` returned by a resource template function is
    passed through unchanged (the 2026-07-28 multi-round-trip flow); the
    retry's answers arrive on `ctx.input_responses`, with
    `ctx.request_state` carrying the echoed opaque state.

    Raises:
        ResourceNotFoundError: If no resource or template matches the URI.
        ResourceError: If template creation or resource reading fails.
    """
    if context is None:
        context = Context(mcp_server=self, subscriptions=self._subscriptions)
    resource = await self._resource_manager.get_resource(uri, context)
    if isinstance(resource, InputRequiredResult):
        return resource

    try:
        content = await resource.read()
        return [ReadResourceContents(content=content, mime_type=resource.mime_type, meta=resource.meta)]
    except MCPError:
        raise
    except Exception as exc:
        logger.exception(f"Error getting resource {uri}")
        # If an exception happens when reading the resource, we should not leak the exception to the client.
        raise ResourceError(f"Error reading resource {uri}") from exc

add_tool 

add_tool(
    fn: Callable[..., Any],
    name: str | None = None,
    title: str | None = None,
    description: str | None = None,
    annotations: ToolAnnotations | None = None,
    icons: list[Icon] | None = None,
    meta: dict[str, Any] | None = None,
    structured_output: bool | None = None,
) -> None

Add a tool to the server.

The tool function can optionally request a Context object by adding a parameter with the Context type annotation. See the @tool decorator for examples.

Parameters:

Name Type Description Default
fn Callable[..., Any]

The function to register as a tool

 required
name str | None

Optional name for the tool (defaults to function name)

 None
title str | None

Optional human-readable title for the tool

 None
description str | None

Optional description of what the tool does

 None
annotations ToolAnnotations | None

Optional ToolAnnotations providing additional tool information

 None
icons list[Icon] | None

Optional list of icons for the tool

 None
meta dict[str, Any] | None

Optional metadata dictionary for the tool

 None
structured_output bool | None

Controls whether the tool's output is structured or unstructured - If None, auto-detects based on the function's return type annotation - If True, creates a structured tool (return type annotation permitting) - If False, unconditionally creates an unstructured tool

 None
Source code in src/mcp/server/mcpserver/server.py 
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def add_tool(
    self,
    fn: Callable[..., Any],
    name: str | None = None,
    title: str | None = None,
    description: str | None = None,
    annotations: ToolAnnotations | None = None,
    icons: list[Icon] | None = None,
    meta: dict[str, Any] | None = None,
    structured_output: bool | None = None,
) -> None:
    """Add a tool to the server.

    The tool function can optionally request a Context object by adding a parameter
    with the Context type annotation. See the @tool decorator for examples.

    Args:
        fn: The function to register as a tool
        name: Optional name for the tool (defaults to function name)
        title: Optional human-readable title for the tool
        description: Optional description of what the tool does
        annotations: Optional ToolAnnotations providing additional tool information
        icons: Optional list of icons for the tool
        meta: Optional metadata dictionary for the tool
        structured_output: Controls whether the tool's output is structured or unstructured
            - If None, auto-detects based on the function's return type annotation
            - If True, creates a structured tool (return type annotation permitting)
            - If False, unconditionally creates an unstructured tool
    """
    self._tool_manager.add_tool(
        fn,
        name=name,
        title=title,
        description=description,
        annotations=annotations,
        icons=icons,
        meta=meta,
        structured_output=structured_output,
    )

remove_tool 

remove_tool(name: str) -> None

Remove a tool from the server by name.

Parameters:

Name Type Description Default
name str

The name of the tool to remove

 required

Raises:

Type Description
ToolError

If the tool does not exist
Source code in src/mcp/server/mcpserver/server.py 
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def remove_tool(self, name: str) -> None:
    """Remove a tool from the server by name.

    Args:
        name: The name of the tool to remove

    Raises:
        ToolError: If the tool does not exist
    """
    self._tool_manager.remove_tool(name)

tool 

tool(
    name: str | None = None,
    title: str | None = None,
    description: str | None = None,
    annotations: ToolAnnotations | None = None,
    icons: list[Icon] | None = None,
    meta: dict[str, Any] | None = None,
    structured_output: bool | None = None,
) -> Callable[[_CallableT], _CallableT]

Decorator to register a tool.

Tools can optionally request a Context object by adding a parameter with the Context type annotation. The context provides access to MCP capabilities like logging, progress reporting, and resource access.

Parameters:

Name Type Description Default
name str | None

Optional name for the tool (defaults to function name)

 None
title str | None

Optional human-readable title for the tool

 None
description str | None

Optional description of what the tool does

 None
annotations ToolAnnotations | None

Optional ToolAnnotations providing additional tool information

 None
icons list[Icon] | None

Optional list of icons for the tool

 None
meta dict[str, Any] | None

Optional metadata dictionary for the tool

 None
structured_output bool | None

Controls whether the tool's output is structured or unstructured - If None, auto-detects based on the function's return type annotation - If True, creates a structured tool (return type annotation permitting) - If False, unconditionally creates an unstructured tool

 None
Example 
@server.tool()
def my_tool(x: int) -> str:
    return str(x)

@server.tool()
async def tool_with_context(x: int, ctx: Context) -> str:
    await ctx.info(f"Processing {x}")
    return str(x)

@server.tool()
async def async_tool(x: int, context: Context) -> str:
    await context.report_progress(50, 100)
    return str(x)
Source code in src/mcp/server/mcpserver/server.py 
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def tool(
    self,
    name: str | None = None,
    title: str | None = None,
    description: str | None = None,
    annotations: ToolAnnotations | None = None,
    icons: list[Icon] | None = None,
    meta: dict[str, Any] | None = None,
    structured_output: bool | None = None,
) -> Callable[[_CallableT], _CallableT]:
    """Decorator to register a tool.

    Tools can optionally request a Context object by adding a parameter with the
    Context type annotation. The context provides access to MCP capabilities like
    logging, progress reporting, and resource access.

    Args:
        name: Optional name for the tool (defaults to function name)
        title: Optional human-readable title for the tool
        description: Optional description of what the tool does
        annotations: Optional ToolAnnotations providing additional tool information
        icons: Optional list of icons for the tool
        meta: Optional metadata dictionary for the tool
        structured_output: Controls whether the tool's output is structured or unstructured
            - If None, auto-detects based on the function's return type annotation
            - If True, creates a structured tool (return type annotation permitting)
            - If False, unconditionally creates an unstructured tool

    Example:
        ```python
        @server.tool()
        def my_tool(x: int) -> str:
            return str(x)
        ```

        ```python
        @server.tool()
        async def tool_with_context(x: int, ctx: Context) -> str:
            await ctx.info(f"Processing {x}")
            return str(x)
        ```

        ```python
        @server.tool()
        async def async_tool(x: int, context: Context) -> str:
            await context.report_progress(50, 100)
            return str(x)
        ```
    """
    # Check if user passed function directly instead of calling decorator
    if callable(name):
        raise TypeError(
            "The @tool decorator was used incorrectly. Did you forget to call it? Use @tool() instead of @tool"
        )

    def decorator(fn: _CallableT) -> _CallableT:
        self.add_tool(
            fn,
            name=name,
            title=title,
            description=description,
            annotations=annotations,
            icons=icons,
            meta=meta,
            structured_output=structured_output,
        )
        return fn

    return decorator

completion 

completion()

Decorator to register a completion handler.

The completion handler receives: - ref: PromptReference or ResourceTemplateReference - argument: CompletionArgument with name and partial value - context: Optional CompletionContext with previously resolved arguments

Example 
@mcp.completion()
async def handle_completion(ref, argument, context):
    if isinstance(ref, ResourceTemplateReference):
        # Return completions based on ref, argument, and context
        return Completion(values=["option1", "option2"])
    return None
Source code in src/mcp/server/mcpserver/server.py 
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def completion(self):
    """Decorator to register a completion handler.

    The completion handler receives:
    - ref: PromptReference or ResourceTemplateReference
    - argument: CompletionArgument with name and partial value
    - context: Optional CompletionContext with previously resolved arguments

    Example:
        ```python
        @mcp.completion()
        async def handle_completion(ref, argument, context):
            if isinstance(ref, ResourceTemplateReference):
                # Return completions based on ref, argument, and context
                return Completion(values=["option1", "option2"])
            return None
        ```
    """

    def decorator(func: _CallableT) -> _CallableT:
        async def handler(
            ctx: ServerRequestContext[LifespanResultT], params: CompleteRequestParams
        ) -> CompleteResult:
            result = await func(params.ref, params.argument, params.context)
            return CompleteResult(
                completion=result if result is not None else Completion(values=[], total=None, has_more=None),
            )

        self._lowlevel_server.add_request_handler("completion/complete", CompleteRequestParams, handler)
        return func

    return decorator

add_resource 

add_resource(resource: Resource) -> None

Add a resource to the server.

Parameters:

Name Type Description Default
resource Resource

A Resource instance to add

 required
Source code in src/mcp/server/mcpserver/server.py 
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def add_resource(self, resource: Resource) -> None:
    """Add a resource to the server.

    Args:
        resource: A Resource instance to add
    """
    self._resource_manager.add_resource(resource)

resource 

resource(
    uri: str,
    *,
    name: str | None = None,
    title: str | None = None,
    description: str | None = None,
    mime_type: str | None = None,
    icons: list[Icon] | None = None,
    annotations: Annotations | None = None,
    meta: dict[str, Any] | None = None,
    security: ResourceSecurity | None = None
) -> Callable[[_CallableT], _CallableT]

Decorator to register a function as a resource.

The function will be called when the resource is read to generate its content. The function can return: - str for text content - bytes for binary content - an InputRequiredResult (template resources only; passed through unchanged for the 2026-07-28 multi-round-trip flow — read ctx.input_responses on the retry) - other types will be converted to JSON

If the URI contains parameters (e.g. "resource://{param}"), it is registered as a template resource. Otherwise it is registered as a static resource; function parameters on a static URI raise an error.

Parameters:

Name Type Description Default
uri str

URI for the resource (e.g. "resource://my-resource" or "resource://{param}")

 required
name str | None

Optional name for the resource

 None
title str | None

Optional human-readable title for the resource

 None
description str | None

Optional description of the resource

 None
mime_type str | None

Optional MIME type for the resource

 None
icons list[Icon] | None

Optional list of icons for the resource

 None
annotations Annotations | None

Optional annotations for the resource

 None
meta dict[str, Any] | None

Optional metadata dictionary for the resource

 None
security ResourceSecurity | None

Path-safety policy for extracted template parameters. Defaults to the server's resource_security setting. Only applies to template resources.

 None
Example 
@server.resource("resource://my-resource")
def get_data() -> str:
    return "Hello, world!"

@server.resource("resource://my-resource")
async def get_data() -> str:
    data = await fetch_data()
    return f"Hello, world! {data}"

@server.resource("resource://{city}/weather")
def get_weather(city: str) -> str:
    return f"Weather for {city}"

@server.resource("resource://{city}/weather")
async def get_weather(city: str) -> str:
    data = await fetch_weather(city)
    return f"Weather for {city}: {data}"

Raises:

Type Description
InvalidUriTemplate

If uri is not a valid RFC 6570 template.
ValueError

If URI template parameters don't match the function's parameters, or if a parameter bound to a {?...}/{&...} query variable has no default (the client may omit it).
TypeError

If the decorator is applied without being called (@resource instead of @resource("uri")).
Source code in src/mcp/server/mcpserver/server.py 
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def resource(
    self,
    uri: str,
    *,
    name: str | None = None,
    title: str | None = None,
    description: str | None = None,
    mime_type: str | None = None,
    icons: list[Icon] | None = None,
    annotations: Annotations | None = None,
    meta: dict[str, Any] | None = None,
    security: ResourceSecurity | None = None,
) -> Callable[[_CallableT], _CallableT]:
    """Decorator to register a function as a resource.

    The function will be called when the resource is read to generate its content.
    The function can return:
    - str for text content
    - bytes for binary content
    - an InputRequiredResult (template resources only; passed through
      unchanged for the 2026-07-28 multi-round-trip flow — read
      `ctx.input_responses` on the retry)
    - other types will be converted to JSON

    If the URI contains parameters (e.g. "resource://{param}"), it is
    registered as a template resource. Otherwise it is registered as a
    static resource; function parameters on a static URI raise an error.

    Args:
        uri: URI for the resource (e.g. "resource://my-resource" or "resource://{param}")
        name: Optional name for the resource
        title: Optional human-readable title for the resource
        description: Optional description of the resource
        mime_type: Optional MIME type for the resource
        icons: Optional list of icons for the resource
        annotations: Optional annotations for the resource
        meta: Optional metadata dictionary for the resource
        security: Path-safety policy for extracted template parameters.
            Defaults to the server's ``resource_security`` setting.
            Only applies to template resources.

    Example:
        ```python
        @server.resource("resource://my-resource")
        def get_data() -> str:
            return "Hello, world!"

        @server.resource("resource://my-resource")
        async def get_data() -> str:
            data = await fetch_data()
            return f"Hello, world! {data}"

        @server.resource("resource://{city}/weather")
        def get_weather(city: str) -> str:
            return f"Weather for {city}"

        @server.resource("resource://{city}/weather")
        async def get_weather(city: str) -> str:
            data = await fetch_weather(city)
            return f"Weather for {city}: {data}"
        ```

    Raises:
        InvalidUriTemplate: If ``uri`` is not a valid RFC 6570 template.
        ValueError: If URI template parameters don't match the
            function's parameters, or if a parameter bound to a
            ``{?...}``/``{&...}`` query variable has no default
            (the client may omit it).
        TypeError: If the decorator is applied without being called
            (``@resource`` instead of ``@resource("uri")``).
    """
    # Check if user passed function directly instead of calling decorator
    if callable(uri):
        raise TypeError(
            "The @resource decorator was used incorrectly. "
            "Did you forget to call it? Use @resource('uri') instead of @resource"
        )

    # Parse once, early — surfaces malformed-template errors at
    # decoration time with a clear position, and gives us correct
    # variable names for all RFC 6570 operators.
    parsed = UriTemplate.parse(uri)
    uri_params = set(parsed.variable_names)

    def decorator(fn: _CallableT) -> _CallableT:
        sig = inspect.signature(fn)
        context_param = find_context_parameter(fn)
        func_params = {p for p in sig.parameters.keys() if p != context_param}

        # Template/static is decided purely by the URI: variables
        # present means template, none means static.
        if uri_params:
            if uri_params != func_params:
                raise ValueError(
                    f"Mismatch between URI parameters {uri_params} and function parameters {func_params}"
                )

            # A {?...}/{&...} query variable is optional on the wire:
            # match() omits it from the extracted parameters when the
            # client leaves it out of the URI. The handler parameter
            # bound to it must therefore have a Python default; without
            # one, the author only finds out on the first request that
            # omits it, as an opaque internal error.
            missing_defaults = sorted(
                name
                for name in parsed.query_variable_names
                if sig.parameters[name].default is inspect.Parameter.empty
            )
            if missing_defaults:
                raise ValueError(
                    f"Resource {uri!r}: query parameter(s) {missing_defaults} have no "
                    f"default value. A client may omit a {{?...}}/{{&...}} query "
                    f"parameter, so the matching handler parameter must declare a "
                    f"default."
                )

            # Register as template
            self._resource_manager.add_template(
                fn=fn,
                uri_template=uri,
                name=name,
                title=title,
                description=description,
                mime_type=mime_type,
                icons=icons,
                annotations=annotations,
                security=security if security is not None else self._resource_security,
                meta=meta,
            )
        else:
            if func_params:
                raise ValueError(
                    f"Resource {uri!r} has no URI template variables, but the "
                    f"handler declares parameters {func_params}. Add matching "
                    f"{{...}} variables to the URI or remove the parameters."
                )
            if context_param is not None:
                raise ValueError(
                    f"Resource {uri!r} has no URI template variables, but the "
                    f"handler declares a Context parameter. Context injection "
                    f"for static resources is not supported. "
                    f"Add a template variable to the URI or remove the "
                    f"Context parameter."
                )
            # Register as regular resource
            resource = FunctionResource.from_function(
                fn=fn,
                uri=uri,
                name=name,
                title=title,
                description=description,
                mime_type=mime_type,
                icons=icons,
                annotations=annotations,
                meta=meta,
            )
            self.add_resource(resource)
        return fn

    return decorator

add_prompt 

add_prompt(prompt: Prompt) -> None

Add a prompt to the server.

Parameters:

Name Type Description Default
prompt Prompt

A Prompt instance to add

 required
Source code in src/mcp/server/mcpserver/server.py 
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def add_prompt(self, prompt: Prompt) -> None:
    """Add a prompt to the server.

    Args:
        prompt: A Prompt instance to add
    """
    self._prompt_manager.add_prompt(prompt)

remove_prompt 

remove_prompt(name: str) -> None

Remove a prompt from the server by name.

Parameters:

Name Type Description Default
name str

The name of the prompt to remove

 required

Raises:

Type Description
ValueError

If the prompt does not exist
Source code in src/mcp/server/mcpserver/server.py 
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def remove_prompt(self, name: str) -> None:
    """Remove a prompt from the server by name.

    Args:
        name: The name of the prompt to remove

    Raises:
        ValueError: If the prompt does not exist
    """
    self._prompt_manager.remove_prompt(name)

prompt 

prompt(
    name: str | None = None,
    title: str | None = None,
    description: str | None = None,
    icons: list[Icon] | None = None,
) -> Callable[[_CallableT], _CallableT]

Decorator to register a prompt.

    The function returns the prompt messages (a string, `Message`, dict,
    or a sequence of these), or an `InputRequiredResult` to request
    client input first (the 2026-07-28 multi-round-trip flow — read
    `ctx.input_responses` on the retry).

    Args:
        name: Optional name for the prompt (defaults to function name)
        title: Optional human-readable title for the prompt
        description: Optional description of what the prompt does
        icons: Optional list of icons for the prompt

    Example:
        ```python
        @server.prompt()
        def analyze_table(table_name: str) -> list[Message]:
            schema = read_table_schema(table_name)
            return [
                {
                    "role": "user",
                    "content": f"Analyze this schema:

{schema}" } ]

        @server.prompt()
        async def analyze_file(path: str) -> list[Message]:
            content = await read_file(path)
            return [
                {
                    "role": "user",
                    "content": {
                        "type": "resource",
                        "resource": {
                            "uri": f"file://{path}",
                            "text": content
                        }
                    }
                }
            ]
        ```
Source code in src/mcp/server/mcpserver/server.py 
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def prompt(
    self,
    name: str | None = None,
    title: str | None = None,
    description: str | None = None,
    icons: list[Icon] | None = None,
) -> Callable[[_CallableT], _CallableT]:
    """Decorator to register a prompt.

    The function returns the prompt messages (a string, `Message`, dict,
    or a sequence of these), or an `InputRequiredResult` to request
    client input first (the 2026-07-28 multi-round-trip flow — read
    `ctx.input_responses` on the retry).

    Args:
        name: Optional name for the prompt (defaults to function name)
        title: Optional human-readable title for the prompt
        description: Optional description of what the prompt does
        icons: Optional list of icons for the prompt

    Example:
        ```python
        @server.prompt()
        def analyze_table(table_name: str) -> list[Message]:
            schema = read_table_schema(table_name)
            return [
                {
                    "role": "user",
                    "content": f"Analyze this schema:\n{schema}"
                }
            ]

        @server.prompt()
        async def analyze_file(path: str) -> list[Message]:
            content = await read_file(path)
            return [
                {
                    "role": "user",
                    "content": {
                        "type": "resource",
                        "resource": {
                            "uri": f"file://{path}",
                            "text": content
                        }
                    }
                }
            ]
        ```
    """
    # Check if user passed function directly instead of calling decorator
    if callable(name):
        raise TypeError(
            "The @prompt decorator was used incorrectly. "
            "Did you forget to call it? Use @prompt() instead of @prompt"
        )

    def decorator(func: _CallableT) -> _CallableT:
        prompt = Prompt.from_function(func, name=name, title=title, description=description, icons=icons)
        self.add_prompt(prompt)
        return func

    return decorator

custom_route 

custom_route(
    path: str,
    methods: list[str],
    name: str | None = None,
    include_in_schema: bool = True,
)

Decorator to register a custom HTTP route on the MCP server.

Allows adding arbitrary HTTP endpoints outside the standard MCP protocol, which can be useful for OAuth callbacks, health checks, or admin APIs. The handler function must be an async function that accepts a Starlette Request and returns a Response.

Routes using this decorator will not require authorization. It is intended for uses that are either a part of authorization flows or intended to be public such as health check endpoints.

Parameters:

Name Type Description Default
path str

URL path for the route (e.g., "/oauth/callback")

 required
methods list[str]

List of HTTP methods to support (e.g., ["GET", "POST"])

 required
name str | None

Optional name for the route (to reference this route with Starlette's reverse URL lookup feature)

 None
include_in_schema bool

Whether to include in OpenAPI schema, defaults to True

 True
Example 
@server.custom_route("/health", methods=["GET"])
async def health_check(request: Request) -> Response:
    return JSONResponse({"status": "ok"})
Source code in src/mcp/server/mcpserver/server.py 
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def custom_route(
    self,
    path: str,
    methods: list[str],
    name: str | None = None,
    include_in_schema: bool = True,
):
    """Decorator to register a custom HTTP route on the MCP server.

    Allows adding arbitrary HTTP endpoints outside the standard MCP protocol,
    which can be useful for OAuth callbacks, health checks, or admin APIs.
    The handler function must be an async function that accepts a Starlette
    Request and returns a Response.

    Routes using this decorator will not require authorization. It is intended
    for uses that are either a part of authorization flows or intended to be
    public such as health check endpoints.

    Args:
        path: URL path for the route (e.g., "/oauth/callback")
        methods: List of HTTP methods to support (e.g., ["GET", "POST"])
        name: Optional name for the route (to reference this route with
              Starlette's reverse URL lookup feature)
        include_in_schema: Whether to include in OpenAPI schema, defaults to True

    Example:
        ```python
        @server.custom_route("/health", methods=["GET"])
        async def health_check(request: Request) -> Response:
            return JSONResponse({"status": "ok"})
        ```
    """

    def decorator(
        func: Callable[[Request], Awaitable[Response]],
    ) -> Callable[[Request], Awaitable[Response]]:
        self._custom_starlette_routes.append(
            Route(path, endpoint=func, methods=methods, name=name, include_in_schema=include_in_schema)
        )
        return func

    return decorator

run_stdio_async async 

run_stdio_async() -> None

Run the server using stdio transport.

Source code in src/mcp/server/mcpserver/server.py 
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async def run_stdio_async(self) -> None:
    """Run the server using stdio transport."""
    async with stdio_server() as (read_stream, write_stream):
        await self._lowlevel_server.run(
            read_stream,
            write_stream,
            self._lowlevel_server.create_initialization_options(),
        )

run_sse_async async 

run_sse_async(
    *,
    host: str = "127.0.0.1",
    port: int = 8000,
    sse_path: str = "/sse",
    message_path: str = "/messages/",
    transport_security: (
        TransportSecuritySettings | None
    ) = None
) -> None

Run the server using SSE transport.

Source code in src/mcp/server/mcpserver/server.py 
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async def run_sse_async(  # pragma: no cover
    self,
    *,
    host: str = "127.0.0.1",
    port: int = 8000,
    sse_path: str = "/sse",
    message_path: str = "/messages/",
    transport_security: TransportSecuritySettings | None = None,
) -> None:
    """Run the server using SSE transport."""
    import uvicorn

    starlette_app = self.sse_app(
        sse_path=sse_path,
        message_path=message_path,
        transport_security=transport_security,
        host=host,
    )

    config = uvicorn.Config(
        starlette_app,
        host=host,
        port=port,
        log_level=self.settings.log_level.lower(),
    )
    server = uvicorn.Server(config)
    await server.serve()

run_streamable_http_async async 

run_streamable_http_async(
    *,
    host: str = "127.0.0.1",
    port: int = 8000,
    streamable_http_path: str = "/mcp",
    json_response: bool = False,
    stateless_http: bool = False,
    event_store: EventStore | None = None,
    retry_interval: int | None = None,
    transport_security: (
        TransportSecuritySettings | None
    ) = None
) -> None

Run the server using StreamableHTTP transport.

Source code in src/mcp/server/mcpserver/server.py 
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async def run_streamable_http_async(  # pragma: no cover
    self,
    *,
    host: str = "127.0.0.1",
    port: int = 8000,
    streamable_http_path: str = "/mcp",
    json_response: bool = False,
    stateless_http: bool = False,
    event_store: EventStore | None = None,
    retry_interval: int | None = None,
    transport_security: TransportSecuritySettings | None = None,
) -> None:
    """Run the server using StreamableHTTP transport."""
    import uvicorn

    starlette_app = self.streamable_http_app(
        streamable_http_path=streamable_http_path,
        json_response=json_response,
        stateless_http=stateless_http,
        event_store=event_store,
        retry_interval=retry_interval,
        transport_security=transport_security,
        host=host,
    )

    config = uvicorn.Config(
        starlette_app,
        host=host,
        port=port,
        log_level=self.settings.log_level.lower(),
    )
    server = uvicorn.Server(config)
    await server.serve()

sse_app 

sse_app(
    *,
    sse_path: str = "/sse",
    message_path: str = "/messages/",
    transport_security: (
        TransportSecuritySettings | None
    ) = None,
    host: str = "127.0.0.1"
) -> Starlette

Return an instance of the SSE server app.

Source code in src/mcp/server/mcpserver/server.py 
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def sse_app(
    self,
    *,
    sse_path: str = "/sse",
    message_path: str = "/messages/",
    transport_security: TransportSecuritySettings | None = None,
    host: str = "127.0.0.1",
) -> Starlette:
    """Return an instance of the SSE server app."""
    # Auto-enable DNS rebinding protection for localhost (IPv4 and IPv6)
    if transport_security is None and host in ("127.0.0.1", "localhost", "::1"):
        transport_security = TransportSecuritySettings(
            enable_dns_rebinding_protection=True,
            allowed_hosts=["127.0.0.1:*", "localhost:*", "[::1]:*"],
            allowed_origins=["http://127.0.0.1:*", "http://localhost:*", "http://[::1]:*"],
        )

    sse = SseServerTransport(message_path, security_settings=transport_security)

    async def handle_sse(scope: Scope, receive: Receive, send: Send):  # pragma: no cover
        # Add client ID from auth context into request context if available

        async with sse.connect_sse(scope, receive, send) as streams:
            await self._lowlevel_server.run(
                streams[0], streams[1], self._lowlevel_server.create_initialization_options()
            )
        return Response()

    # Create routes
    routes: list[Route | Mount] = []
    middleware: list[Middleware] = []
    required_scopes: list[str] = []

    # Set up auth if configured
    if self.settings.auth:  # pragma: no cover
        required_scopes = self.settings.auth.required_scopes or []

        # Add auth middleware if token verifier is available
        if self._token_verifier:
            middleware = [
                # extract auth info from request (but do not require it)
                Middleware(
                    AuthenticationMiddleware,
                    backend=BearerAuthBackend(self._token_verifier),
                ),
                # Add the auth context middleware to store
                # authenticated user in a contextvar
                Middleware(AuthContextMiddleware),
            ]

        # Add auth endpoints if auth server provider is configured
        if self._auth_server_provider:
            from mcp.server.auth.routes import create_auth_routes

            routes.extend(
                create_auth_routes(
                    provider=self._auth_server_provider,
                    issuer_url=self.settings.auth.issuer_url,
                    service_documentation_url=self.settings.auth.service_documentation_url,
                    client_registration_options=self.settings.auth.client_registration_options,
                    revocation_options=self.settings.auth.revocation_options,
                    identity_assertion_enabled=self.settings.auth.identity_assertion_enabled,
                )
            )

    # When auth is configured, require authentication
    if self._token_verifier:  # pragma: no cover
        # Determine resource metadata URL
        resource_metadata_url = None
        if self.settings.auth and self.settings.auth.resource_server_url:
            from mcp.server.auth.routes import build_resource_metadata_url

            # Build compliant metadata URL for WWW-Authenticate header
            resource_metadata_url = build_resource_metadata_url(self.settings.auth.resource_server_url)

        # Auth is enabled, wrap the endpoints with RequireAuthMiddleware
        routes.append(
            Route(
                sse_path,
                endpoint=RequireAuthMiddleware(handle_sse, required_scopes, resource_metadata_url),
                methods=["GET"],
            )
        )
        routes.append(
            Mount(
                message_path,
                app=RequireAuthMiddleware(sse.handle_post_message, required_scopes, resource_metadata_url),
            )
        )
    else:
        # Auth is disabled, no need for RequireAuthMiddleware
        # Since handle_sse is an ASGI app, we need to create a compatible endpoint
        async def sse_endpoint(request: Request) -> Response:  # pragma: no cover
            # Convert the Starlette request to ASGI parameters
            return await handle_sse(request.scope, request.receive, request._send)  # type: ignore[reportPrivateUsage]

        routes.append(
            Route(
                sse_path,
                endpoint=sse_endpoint,
                methods=["GET"],
            )
        )
        routes.append(
            Mount(
                message_path,
                app=sse.handle_post_message,
            )
        )
    # Add protected resource metadata endpoint if configured as RS
    if self.settings.auth and self.settings.auth.resource_server_url:  # pragma: no cover
        from mcp.server.auth.routes import create_protected_resource_routes

        routes.extend(
            create_protected_resource_routes(
                resource_url=self.settings.auth.resource_server_url,
                authorization_servers=[self.settings.auth.issuer_url],
                scopes_supported=self.settings.auth.required_scopes,
            )
        )

    # mount these routes last, so they have the lowest route matching precedence
    routes.extend(self._custom_starlette_routes)

    # Create Starlette app with routes and middleware
    return Starlette(debug=self.settings.debug, routes=routes, middleware=middleware)

streamable_http_app 

streamable_http_app(
    *,
    streamable_http_path: str = "/mcp",
    json_response: bool = False,
    stateless_http: bool = False,
    event_store: EventStore | None = None,
    retry_interval: int | None = None,
    transport_security: (
        TransportSecuritySettings | None
    ) = None,
    host: str = "127.0.0.1"
) -> Starlette

Return an instance of the StreamableHTTP server app.

Source code in src/mcp/server/mcpserver/server.py 
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def streamable_http_app(
    self,
    *,
    streamable_http_path: str = "/mcp",
    json_response: bool = False,
    stateless_http: bool = False,
    event_store: EventStore | None = None,
    retry_interval: int | None = None,
    transport_security: TransportSecuritySettings | None = None,
    host: str = "127.0.0.1",
) -> Starlette:
    """Return an instance of the StreamableHTTP server app."""
    return self._lowlevel_server.streamable_http_app(
        streamable_http_path=streamable_http_path,
        json_response=json_response,
        stateless_http=stateless_http,
        event_store=event_store,
        retry_interval=retry_interval,
        transport_security=transport_security,
        host=host,
        auth=self.settings.auth,
        token_verifier=self._token_verifier,
        auth_server_provider=self._auth_server_provider,
        custom_starlette_routes=self._custom_starlette_routes,
        debug=self.settings.debug,
    )

list_prompts async 

list_prompts() -> list[Prompt]

List all available prompts.

Source code in src/mcp/server/mcpserver/server.py 
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async def list_prompts(self) -> list[MCPPrompt]:
    """List all available prompts."""
    prompts = self._prompt_manager.list_prompts()
    return [
        MCPPrompt(
            name=prompt.name,
            title=prompt.title,
            description=prompt.description,
            arguments=[
                MCPPromptArgument(
                    name=arg.name,
                    description=arg.description,
                    required=arg.required,
                )
                for arg in (prompt.arguments or [])
            ],
            icons=prompt.icons,
        )
        for prompt in prompts
    ]

get_prompt async 

get_prompt(
    name: str,
    arguments: dict[str, Any] | None = None,
    context: Context[LifespanResultT, Any] | None = None,
) -> GetPromptResult | InputRequiredResult

Get a prompt by name with arguments.

An InputRequiredResult returned by the prompt function is passed through unchanged (the 2026-07-28 multi-round-trip flow); the retry's answers arrive on ctx.input_responses, with ctx.request_state carrying the echoed opaque state.

Source code in src/mcp/server/mcpserver/server.py 
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async def get_prompt(
    self, name: str, arguments: dict[str, Any] | None = None, context: Context[LifespanResultT, Any] | None = None
) -> GetPromptResult | InputRequiredResult:
    """Get a prompt by name with arguments.

    An `InputRequiredResult` returned by the prompt function is passed
    through unchanged (the 2026-07-28 multi-round-trip flow); the retry's
    answers arrive on `ctx.input_responses`, with `ctx.request_state`
    carrying the echoed opaque state.
    """
    if context is None:
        context = Context(mcp_server=self, subscriptions=self._subscriptions)
    try:
        prompt = self._prompt_manager.get_prompt(name)
        if not prompt:
            raise ValueError(f"Unknown prompt: {name}")

        rendered = await prompt.render(arguments, context)
        if isinstance(rendered, InputRequiredResult):
            return rendered

        return GetPromptResult(
            description=prompt.description,
            messages=pydantic_core.to_jsonable_python(rendered),
        )
    except MCPError:
        raise
    except Exception as e:
        logger.exception(f"Error getting prompt {name}")
        raise ValueError(str(e)) from e

require_client_extension 

require_client_extension(
    ctx: ServerRequestContext[Any, Any], identifier: str
) -> None

Assert the connected client declared support for identifier.

Call this from an extension's handler or intercept_tool_call before offering extension-specific behaviour. Raises MCPError with the -32021 (missing required client capability) code and a requiredCapabilities payload when the client did not declare the extension, per SEP-2133.

Parameters:

Name Type Description Default
ctx ServerRequestContext[Any, Any]

The current request context.

 required
identifier str

The extension identifier the client must have declared.

 required

Raises:

Type Description
MCPError

With code MISSING_REQUIRED_CLIENT_CAPABILITY if the client did not advertise identifier.
Source code in src/mcp/server/mcpserver/server.py 
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def require_client_extension(ctx: ServerRequestContext[Any, Any], identifier: str) -> None:
    """Assert the connected client declared support for `identifier`.

    Call this from an extension's handler or `intercept_tool_call` before
    offering extension-specific behaviour. Raises `MCPError` with the
    `-32021` (missing required client capability) code and a
    `requiredCapabilities` payload when the client did not declare the
    extension, per SEP-2133.

    Args:
        ctx: The current request context.
        identifier: The extension identifier the client must have declared.

    Raises:
        MCPError: With code `MISSING_REQUIRED_CLIENT_CAPABILITY` if the client
            did not advertise `identifier`.
    """
    client_params = ctx.session.client_params
    declared = client_params.capabilities.extensions if client_params else None
    if not declared or identifier not in declared:
        data = MissingRequiredClientCapabilityErrorData(
            required_capabilities=ClientCapabilities(extensions={identifier: {}})
        )
        raise MCPError(
            code=MISSING_REQUIRED_CLIENT_CAPABILITY,
            message=f"Client did not declare required extension {identifier!r}",
            data=data.model_dump(by_alias=True, mode="json", exclude_none=True),
        )

Audio

Helper class for returning audio from tools.

Source code in src/mcp/server/mcpserver/utilities/types.py 
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class Audio:
    """Helper class for returning audio from tools."""

    def __init__(
        self,
        path: str | Path | None = None,
        data: bytes | None = None,
        format: str | None = None,
    ):
        if not bool(path) ^ bool(data):  # pragma: no cover
            raise ValueError("Either path or data can be provided")

        self.path = Path(path) if path else None
        self.data = data
        self._format = format
        self._mime_type = self._get_mime_type()

    def _get_mime_type(self) -> str:
        """Get MIME type from format or guess from file extension."""
        if self._format:
            return f"audio/{self._format.lower()}"

        if self.path:
            suffix = self.path.suffix.lower()
            return {
                ".wav": "audio/wav",
                ".mp3": "audio/mpeg",
                ".ogg": "audio/ogg",
                ".flac": "audio/flac",
                ".aac": "audio/aac",
                ".m4a": "audio/mp4",
            }.get(suffix, "application/octet-stream")
        return "audio/wav"  # default for raw binary data

    def to_audio_content(self) -> AudioContent:
        """Convert to MCP AudioContent."""
        if self.path:
            with open(self.path, "rb") as f:
                data = base64.b64encode(f.read()).decode()
        elif self.data is not None:
            data = base64.b64encode(self.data).decode()
        else:  # pragma: no cover
            raise ValueError("No audio data available")

        return AudioContent(type="audio", data=data, mime_type=self._mime_type)

to_audio_content 

to_audio_content() -> AudioContent

Convert to MCP AudioContent.

Source code in src/mcp/server/mcpserver/utilities/types.py 
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def to_audio_content(self) -> AudioContent:
    """Convert to MCP AudioContent."""
    if self.path:
        with open(self.path, "rb") as f:
            data = base64.b64encode(f.read()).decode()
    elif self.data is not None:
        data = base64.b64encode(self.data).decode()
    else:  # pragma: no cover
        raise ValueError("No audio data available")

    return AudioContent(type="audio", data=data, mime_type=self._mime_type)

Image

Helper class for returning images from tools.

Source code in src/mcp/server/mcpserver/utilities/types.py 
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class Image:
    """Helper class for returning images from tools."""

    def __init__(
        self,
        path: str | Path | None = None,
        data: bytes | None = None,
        format: str | None = None,
    ):
        if path is None and data is None:  # pragma: no cover
            raise ValueError("Either path or data must be provided")
        if path is not None and data is not None:  # pragma: no cover
            raise ValueError("Only one of path or data can be provided")

        self.path = Path(path) if path else None
        self.data = data
        self._format = format
        self._mime_type = self._get_mime_type()

    def _get_mime_type(self) -> str:
        """Get MIME type from format or guess from file extension."""
        if self._format:
            return f"image/{self._format.lower()}"

        if self.path:
            suffix = self.path.suffix.lower()
            return {
                ".png": "image/png",
                ".jpg": "image/jpeg",
                ".jpeg": "image/jpeg",
                ".gif": "image/gif",
                ".webp": "image/webp",
            }.get(suffix, "application/octet-stream")
        return "image/png"  # default for raw binary data

    def to_image_content(self) -> ImageContent:
        """Convert to MCP ImageContent."""
        if self.path:
            with open(self.path, "rb") as f:
                data = base64.b64encode(f.read()).decode()
        elif self.data is not None:
            data = base64.b64encode(self.data).decode()
        else:  # pragma: no cover
            raise ValueError("No image data available")

        return ImageContent(type="image", data=data, mime_type=self._mime_type)

to_image_content 

to_image_content() -> ImageContent

Convert to MCP ImageContent.

Source code in src/mcp/server/mcpserver/utilities/types.py 
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def to_image_content(self) -> ImageContent:
    """Convert to MCP ImageContent."""
    if self.path:
        with open(self.path, "rb") as f:
            data = base64.b64encode(f.read()).decode()
    elif self.data is not None:
        data = base64.b64encode(self.data).decode()
    else:  # pragma: no cover
        raise ValueError("No image data available")

    return ImageContent(type="image", data=data, mime_type=self._mime_type)