Apply the request body limit to the SSE and OAuth endpoints (#3336)

docs/migration.md

@@ -755,7 +755,7 @@ Transport-specific parameters have been moved off the `MCPServer` constructor an
 - `sse_path`, `message_path` - SSE transport paths, on `run(transport="sse", ...)` and `sse_app()`
 - `streamable_http_path` - StreamableHTTP endpoint path, on `run(transport="streamable-http", ...)` and `streamable_http_app()`
 - `json_response`, `stateless_http` - StreamableHTTP behavior, same two places; each also removes a server-to-client channel, see [Server-initiated sampling, elicitation, and roots raise `NoBackChannelError`](#server-initiated-sampling-elicitation-and-roots-raise-nobackchannelerror)
-- `max_request_body_size` - StreamableHTTP request-body limit, same two places
+- `max_request_body_size` - HTTP request-body limit, on `run()` for both HTTP transports and on both app methods
 - `event_store`, `retry_interval` - StreamableHTTP event handling, same two places
 - `transport_security` - DNS rebinding protection, on `run()` for both HTTP transports and on both app methods
 

docs/run/index.md

@@ -67,7 +67,7 @@ Each transport has its own keyword arguments, all on `run()`:
 * `streamable_http_path`: where the MCP endpoint lives. Default `/mcp`.
 * `json_response=True`: answer each POST with a single JSON body instead of an SSE stream. That body has room for the response and nothing else, so a tool that calls back into the client mid-request (`ctx.elicit()`, sampling) raises `NoBackChannelError` on this leg, and notifications tied to the in-flight call (progress from `ctx.report_progress()`, per-call log messages) are dropped; the standalone `GET` stream still carries unrelated ones.
 * `stateless_http=True`: a fresh transport per request, no session tracking.
-* `max_request_body_size`: largest accepted POST body in bytes. Defaults to 4 MiB; larger requests
+* `max_request_body_size`: largest accepted request body in bytes. Defaults to 4 MiB; larger requests
   receive HTTP 413 before parsing or session creation. Raise it only when legitimate MCP messages
   exceed that size.
 * `event_store`, `retry_interval`, `transport_security`: resumability and DNS-rebinding protection. They can wait, until you deploy somewhere other than localhost; **[Deploy & scale](deploy.md)** covers `transport_security`.

src/mcp/server/auth/routes.py

@@ -17,6 +17,7 @@ from mcp.server.auth.handlers.token import TokenHandler
 from mcp.server.auth.middleware.client_auth import ClientAuthenticator
 from mcp.server.auth.provider import OAuthAuthorizationServerProvider
 from mcp.server.auth.settings import ClientRegistrationOptions, RevocationOptions
+from mcp.server.transport_security import DEFAULT_MAX_REQUEST_BODY_SIZE, RequestBodyLimitMiddleware
 from mcp.shared.auth import JWT_BEARER_GRANT_TYPE, OAuthMetadata, ProtectedResourceMetadata
 from mcp.shared.inbound import MCP_PROTOCOL_VERSION_HEADER
 
@@ -51,17 +52,24 @@ REVOCATION_PATH = "/revoke"
 ID_JAG_GRANT_PROFILE = "urn:ietf:params:oauth:grant-profile:id-jag"
 
 
-def cors_middleware(
+def _cors(app: ASGIApp, allow_methods: list[str]) -> ASGIApp:
-    handler: Callable[[Request], Response | Awaitable[Response]],
+    return CORSMiddleware(
-    allow_methods: list[str],
+        app=app,
-) -> ASGIApp:
-    cors_app = CORSMiddleware(
-        app=request_response(handler),
         allow_origins="*",
         allow_methods=allow_methods,
         allow_headers=[MCP_PROTOCOL_VERSION_HEADER],
     )
-    return cors_app
+
+
+def _body_limited(app: ASGIApp) -> ASGIApp:
+    return RequestBodyLimitMiddleware(app, DEFAULT_MAX_REQUEST_BODY_SIZE)
+
+
+def cors_middleware(
+    handler: Callable[[Request], Response | Awaitable[Response]],
+    allow_methods: list[str],
+) -> ASGIApp:
+    return _cors(request_response(handler), allow_methods)
 
 
 def create_auth_routes(
@@ -84,11 +92,13 @@ def create_auth_routes(
         supports_identity_assertion=identity_assertion_enabled,
     )
     client_authenticator = ClientAuthenticator(provider)
+    token_handler = TokenHandler(provider, client_authenticator, identity_assertion_enabled=identity_assertion_enabled)
 
     # Create routes
     # Allow CORS requests for endpoints meant to be hit by the OAuth client
     # (with the client secret). This is intended to support things like MCP Inspector,
-    # where the client runs in a web browser.
+    # where the client runs in a web browser. CORS is the outermost wrapper so that
+    # responses produced by inner layers (such as a 413) still carry CORS headers.
     routes = [
         Route(
             "/.well-known/oauth-authorization-server",
@@ -102,17 +112,12 @@ def create_auth_routes(
             AUTHORIZATION_PATH,
             # do not allow CORS for authorization endpoint;
             # clients should just redirect to this
-            endpoint=AuthorizationHandler(provider).handle,
+            endpoint=_body_limited(request_response(AuthorizationHandler(provider).handle)),
             methods=["GET", "POST"],
         ),
         Route(
             TOKEN_PATH,
-            endpoint=cors_middleware(
+            endpoint=_cors(_body_limited(request_response(token_handler.handle)), ["POST", "OPTIONS"]),
-                TokenHandler(
-                    provider, client_authenticator, identity_assertion_enabled=identity_assertion_enabled
-                ).handle,
-                ["POST", "OPTIONS"],
-            ),
             methods=["POST", "OPTIONS"],
         ),
     ]
@@ -125,10 +130,7 @@ def create_auth_routes(
         routes.append(
             Route(
                 REGISTRATION_PATH,
-                endpoint=cors_middleware(
+                endpoint=_cors(_body_limited(request_response(registration_handler.handle)), ["POST", "OPTIONS"]),
-                    registration_handler.handle,
-                    ["POST", "OPTIONS"],
-                ),
                 methods=["POST", "OPTIONS"],
             )
         )
@@ -138,10 +140,7 @@ def create_auth_routes(
         routes.append(
             Route(
                 REVOCATION_PATH,
-                endpoint=cors_middleware(
+                endpoint=_cors(_body_limited(request_response(revocation_handler.handle)), ["POST", "OPTIONS"]),
-                    revocation_handler.handle,
-                    ["POST", "OPTIONS"],
-                ),
                 methods=["POST", "OPTIONS"],
             )
         )

src/mcp/server/lowlevel/server.py

@@ -65,12 +65,8 @@ from mcp.server.context import HandlerResult, ServerMiddleware, ServerRequestCon
 from mcp.server.models import InitializationOptions
 from mcp.server.runner import serve_dual_era_loop
 from mcp.server.streamable_http import EventStore
-from mcp.server.streamable_http_manager import (
+from mcp.server.streamable_http_manager import StreamableHTTPASGIApp, StreamableHTTPSessionManager
-    DEFAULT_MAX_REQUEST_BODY_SIZE,
+from mcp.server.transport_security import DEFAULT_MAX_REQUEST_BODY_SIZE, TransportSecuritySettings
-    StreamableHTTPASGIApp,
-    StreamableHTTPSessionManager,
-)
-from mcp.server.transport_security import TransportSecuritySettings
 from mcp.shared._stream_protocols import ReadStream, WriteStream
 from mcp.shared.exceptions import MCPDeprecationWarning
 from mcp.shared.message import SessionMessage

src/mcp/server/mcpserver/server.py

@@ -87,9 +87,9 @@ from mcp.server.request_state import RequestStateBoundary, RequestStateSecurity
 from mcp.server.sse import SseServerTransport
 from mcp.server.stdio import stdio_server
 from mcp.server.streamable_http import EventStore
-from mcp.server.streamable_http_manager import DEFAULT_MAX_REQUEST_BODY_SIZE, StreamableHTTPSessionManager
+from mcp.server.streamable_http_manager import StreamableHTTPSessionManager
 from mcp.server.subscriptions import InMemorySubscriptionBus, ListenHandler, SubscriptionBus
-from mcp.server.transport_security import TransportSecuritySettings
+from mcp.server.transport_security import DEFAULT_MAX_REQUEST_BODY_SIZE, TransportSecuritySettings
 from mcp.shared.exceptions import MCPError
 from mcp.shared.uri_template import UriTemplate
 
@@ -365,6 +365,7 @@ class MCPServer(Generic[LifespanResultT]):
         port: int = ...,
         sse_path: str = ...,
         message_path: str = ...,
+        max_request_body_size: int = ...,
         transport_security: TransportSecuritySettings | None = ...,
     ) -> None: ...
 
@@ -1031,6 +1032,7 @@ class MCPServer(Generic[LifespanResultT]):
         port: int = 8000,
         sse_path: str = "/sse",
         message_path: str = "/messages/",
+        max_request_body_size: int = DEFAULT_MAX_REQUEST_BODY_SIZE,
         transport_security: TransportSecuritySettings | None = None,
     ) -> None:
         """Run the server using SSE transport."""
@@ -1039,6 +1041,7 @@ class MCPServer(Generic[LifespanResultT]):
         starlette_app = self.sse_app(
             sse_path=sse_path,
             message_path=message_path,
+            max_request_body_size=max_request_body_size,
             transport_security=transport_security,
             host=host,
         )
@@ -1093,6 +1096,7 @@ class MCPServer(Generic[LifespanResultT]):
         *,
         sse_path: str = "/sse",
         message_path: str = "/messages/",
+        max_request_body_size: int = DEFAULT_MAX_REQUEST_BODY_SIZE,
         transport_security: TransportSecuritySettings | None = None,
         host: str = "127.0.0.1",
     ) -> Starlette:
@@ -1105,7 +1109,9 @@ class MCPServer(Generic[LifespanResultT]):
                 allowed_origins=["http://127.0.0.1:*", "http://localhost:*", "http://[::1]:*"],
             )
 
-        sse = SseServerTransport(message_path, security_settings=transport_security)
+        sse = SseServerTransport(
+            message_path, security_settings=transport_security, max_request_body_size=max_request_body_size
+        )
 
         async def handle_sse(scope: Scope, receive: Receive, send: Send):  # pragma: no cover
             # Add client ID from auth context into request context if available

src/mcp/server/sse.py

@@ -52,6 +52,8 @@ from starlette.types import Receive, Scope, Send
 
 from mcp.server.auth.middleware.bearer_auth import AuthenticatedUser, AuthorizationContext, authorization_context
 from mcp.server.transport_security import (
+    DEFAULT_MAX_REQUEST_BODY_SIZE,
+    RequestBodyLimitMiddleware,
     TransportSecurityMiddleware,
     TransportSecuritySettings,
 )
@@ -79,7 +81,12 @@ class SseServerTransport:
     _session_owners: dict[UUID, AuthorizationContext]
     _security: TransportSecurityMiddleware
 
-    def __init__(self, endpoint: str, security_settings: TransportSecuritySettings | None = None) -> None:
+    def __init__(
+        self,
+        endpoint: str,
+        security_settings: TransportSecuritySettings | None = None,
+        max_request_body_size: int = DEFAULT_MAX_REQUEST_BODY_SIZE,
+    ) -> None:
         """Creates a new SSE server transport, which will direct the client to POST
         messages to the relative path given.
 
@@ -87,6 +94,9 @@ class SseServerTransport:
             endpoint: A relative path where messages should be posted
                     (e.g., "/messages/").
             security_settings: Optional security settings for DNS rebinding protection.
+            max_request_body_size: Maximum size in bytes for POSTed message bodies. Requests that
+                declare or stream a larger body receive HTTP 413. Defaults to 4 MiB, matching
+                `StreamableHTTPSessionManager`.
 
         Note:
             We use relative paths instead of full URLs for several reasons:
@@ -103,6 +113,9 @@ class SseServerTransport:
 
         super().__init__()
 
+        if max_request_body_size <= 0:
+            raise ValueError("max_request_body_size must be a positive number of bytes")
+
         # Validate that endpoint is a relative path and not a full URL
         if "://" in endpoint or endpoint.startswith("//") or "?" in endpoint or "#" in endpoint:
             raise ValueError(
@@ -118,6 +131,7 @@ class SseServerTransport:
         self._read_stream_writers = {}
         self._session_owners = {}
         self._security = TransportSecurityMiddleware(security_settings)
+        self._post_message_app = RequestBodyLimitMiddleware(self._handle_post_message, max_request_body_size)
         logger.debug(f"SseServerTransport initialized with endpoint: {endpoint}")
 
     @asynccontextmanager
@@ -203,6 +217,17 @@ class SseServerTransport:
             self._session_owners.pop(session_id, None)
 
     async def handle_post_message(self, scope: Scope, receive: Receive, send: Send) -> None:
+        """ASGI application for the message endpoint.
+
+        Only POST is accepted (other methods get 405), and bodies larger than
+        `max_request_body_size` are answered with 413 before the message is handled.
+        """
+        if scope["method"] != "POST":
+            response = Response(status_code=405, headers={"Allow": "POST"})
+            return await response(scope, receive, send)
+        await self._post_message_app(scope, receive, send)
+
+    async def _handle_post_message(self, scope: Scope, receive: Receive, send: Send) -> None:
         logger.debug("Handling POST message")
         request = Request(scope, receive)
 

src/mcp/server/streamable_http_manager.py

@@ -4,25 +4,25 @@ from __future__ import annotations
 
 import contextlib
 import logging
-from collections import deque
 from collections.abc import AsyncIterator
-from typing import TYPE_CHECKING, Any, Final
+from typing import TYPE_CHECKING, Any
 from uuid import uuid4
 
 import anyio
 from anyio.abc import TaskStatus
 from mcp_types import DEFAULT_NEGOTIATED_VERSION, INVALID_REQUEST, ErrorData, JSONRPCError
 from mcp_types.version import HANDSHAKE_PROTOCOL_VERSIONS
-from starlette.datastructures import Headers
 from starlette.requests import Request
 from starlette.responses import Response
-from starlette.types import ASGIApp, Message, Receive, Scope, Send
+from starlette.types import Receive, Scope, Send
 
 from mcp.server._streamable_http_modern import handle_modern_request
 from mcp.server.auth.middleware.bearer_auth import AuthenticatedUser, AuthorizationContext, authorization_context
 from mcp.server.connection import Connection
 from mcp.server.runner import serve_connection, serve_loop
 from mcp.server.streamable_http import MCP_SESSION_ID_HEADER, EventStore, StreamableHTTPServerTransport
+from mcp.server.transport_security import DEFAULT_MAX_REQUEST_BODY_SIZE as DEFAULT_MAX_REQUEST_BODY_SIZE
+from mcp.server.transport_security import RequestBodyLimitMiddleware as RequestBodyLimitMiddleware
 from mcp.server.transport_security import TransportSecuritySettings
 from mcp.shared._compat import resync_tracer
 from mcp.shared.inbound import MCP_PROTOCOL_VERSION_HEADER
@@ -34,9 +34,6 @@ if TYPE_CHECKING:
 
 logger = logging.getLogger(__name__)
 
-DEFAULT_MAX_REQUEST_BODY_SIZE: Final = 4 * 1024 * 1024
-"""Default maximum Streamable HTTP request body size in bytes (4 MiB)."""
-
 
 class StreamableHTTPSessionManager:
     """Manages StreamableHTTP sessions with optional resumability via event store.
@@ -70,7 +67,7 @@ class StreamableHTTPSessionManager:
             retry_interval is also configured, ensure the idle timeout comfortably exceeds the retry interval to
             avoid reaping sessions during normal SSE polling gaps. Default is None (no timeout). A value of 1800
             (30 minutes) is recommended for most deployments.
-        max_request_body_size: Maximum size in bytes for Streamable HTTP POST request bodies. Requests that
+        max_request_body_size: Maximum size in bytes for Streamable HTTP request bodies. Requests that
             exceed this limit receive a 413 response before parsing or session creation. Defaults to 4 MiB.
     """
 
@@ -371,66 +368,6 @@ class StreamableHTTPSessionManager:
             await response(scope, receive, send)
 
 
-class RequestBodyLimitMiddleware:
-    """Reject oversized HTTP request bodies before invoking an ASGI application."""
-
-    def __init__(self, app: ASGIApp, max_body_size: int) -> None:
-        self.app = app
-        self.max_body_size = max_body_size
-
-    async def __call__(self, scope: Scope, receive: Receive, send: Send) -> None:
-        if scope["type"] != "http" or scope["method"] != "POST":
-            await self.app(scope, receive, send)
-            return
-
-        headers = Headers(scope=scope)
-        content_length = headers.get("content-length")
-        if content_length is not None:
-            try:
-                declared_size = int(content_length)
-            except ValueError:
-                pass
-            else:
-                if declared_size > self.max_body_size:
-                    response = Response("Request body too large", status_code=413)
-                    return await response(scope, receive, send)
-
-        received_body = bytearray()
-        received_request = False
-        body_complete = False
-        trailing_message: Message | None = None
-        while True:
-            message = await receive()
-            if message["type"] != "http.request":
-                trailing_message = message
-                break
-
-            received_request = True
-            body = message.get("body", b"")
-            if len(received_body) + len(body) > self.max_body_size:
-                response = Response("Request body too large", status_code=413)
-                return await response(scope, receive, send)
-            received_body.extend(body)
-            if not message.get("more_body", False):
-                body_complete = True
-                break
-
-        cached_messages: deque[Message] = deque()
-        if received_request:
-            cached_messages.append(
-                {"type": "http.request", "body": bytes(received_body), "more_body": not body_complete}
-            )
-        if trailing_message is not None:
-            cached_messages.append(trailing_message)
-
-        async def replay() -> Message:
-            if cached_messages:
-                return cached_messages.popleft()
-            return await receive()
-
-        await self.app(scope, replay, send)
-
-
 class StreamableHTTPASGIApp:
     """ASGI application for Streamable HTTP server transport."""
 

src/mcp/server/transport_security.py

@@ -1,13 +1,20 @@
-"""DNS rebinding protection for MCP server transports."""
+"""Request checks shared by the HTTP server transports: Host/Origin header validation and body size limits."""
 
 import logging
+from collections import deque
+from typing import Final
 
 from pydantic import BaseModel, Field
+from starlette.datastructures import Headers
 from starlette.requests import Request
 from starlette.responses import Response
+from starlette.types import ASGIApp, Message, Receive, Scope, Send
 
 logger = logging.getLogger(__name__)
 
+DEFAULT_MAX_REQUEST_BODY_SIZE: Final = 4 * 1024 * 1024
+"""Default maximum HTTP request body size in bytes (4 MiB)."""
+
 
 # TODO(Marcelo): We should flatten these settings. To be fair, I don't think we should even have this middleware.
 class TransportSecuritySettings(BaseModel):
@@ -114,3 +121,63 @@ class TransportSecurityMiddleware:
             return Response("Invalid Origin header", status_code=403)
 
         return None
+
+
+class RequestBodyLimitMiddleware:
+    """Reject oversized HTTP request bodies before invoking an ASGI application."""
+
+    def __init__(self, app: ASGIApp, max_body_size: int) -> None:
+        self.app = app
+        self.max_body_size = max_body_size
+
+    async def __call__(self, scope: Scope, receive: Receive, send: Send) -> None:
+        if scope["type"] != "http":
+            await self.app(scope, receive, send)
+            return
+
+        headers = Headers(scope=scope)
+        content_length = headers.get("content-length")
+        if content_length is not None:
+            try:
+                declared_size = int(content_length)
+            except ValueError:
+                pass
+            else:
+                if declared_size > self.max_body_size:
+                    response = Response("Request body too large", status_code=413)
+                    return await response(scope, receive, send)
+
+        received_body = bytearray()
+        received_request = False
+        body_complete = False
+        trailing_message: Message | None = None
+        while True:
+            message = await receive()
+            if message["type"] != "http.request":
+                trailing_message = message
+                break
+
+            received_request = True
+            body = message.get("body", b"")
+            if len(received_body) + len(body) > self.max_body_size:
+                response = Response("Request body too large", status_code=413)
+                return await response(scope, receive, send)
+            received_body.extend(body)
+            if not message.get("more_body", False):
+                body_complete = True
+                break
+
+        cached_messages: deque[Message] = deque()
+        if received_request:
+            cached_messages.append(
+                {"type": "http.request", "body": bytes(received_body), "more_body": not body_complete}
+            )
+        if trailing_message is not None:
+            cached_messages.append(trailing_message)
+
+        async def replay() -> Message:
+            if cached_messages:
+                return cached_messages.popleft()
+            return await receive()
+
+        await self.app(scope, replay, send)

tests/server/auth/test_error_handling.py

@@ -16,6 +16,7 @@ from starlette.applications import Starlette
 from mcp.server.auth.provider import AuthorizeError, RegistrationError, TokenError
 from mcp.server.auth.routes import create_auth_routes
 from mcp.server.auth.settings import ClientRegistrationOptions, RevocationOptions
+from mcp.server.transport_security import DEFAULT_MAX_REQUEST_BODY_SIZE
 from tests.server.mcpserver.auth.test_auth_integration import MockOAuthProvider
 
 
@@ -288,3 +289,61 @@ async def test_token_error_handling_refresh_token(
         data = refresh_response.json()
         assert data["error"] == "invalid_scope"
         assert data["error_description"] == "The requested scope is invalid"
+
+
+_FORM = "application/x-www-form-urlencoded"
+
+
+@pytest.mark.anyio
+@pytest.mark.parametrize(
+    ("method", "path", "content_type"),
+    [
+        ("POST", "/token", _FORM),
+        ("POST", "/revoke", _FORM),
+        ("POST", "/register", "application/json"),
+        ("POST", "/authorize", _FORM),
+        # The other methods these routes accept reach the same body-reading handlers.
+        ("OPTIONS", "/token", _FORM),
+        ("OPTIONS", "/revoke", _FORM),
+        ("OPTIONS", "/register", "application/json"),
+        ("HEAD", "/authorize", _FORM),
+    ],
+)
+async def test_oversized_request_body_returns_413(
+    client: httpx2.AsyncClient, method: str, path: str, content_type: str
+):
+    """Each endpoint that reads a request body rejects one over 4 MiB before parsing it, whatever the method."""
+    response = await client.request(
+        method, path, content=b"x" * (DEFAULT_MAX_REQUEST_BODY_SIZE + 1), headers={"Content-Type": content_type}
+    )
+    assert response.status_code == 413
+
+
+@pytest.mark.anyio
+async def test_request_body_within_the_limit_is_still_parsed(client: httpx2.AsyncClient):
+    """A small body is passed through to the handler intact: the form is parsed and its fields validated."""
+    response = await client.post("/token", data={"grant_type": "authorization_code"})
+    assert response.status_code == 401
+    assert response.json() == {"error": "invalid_client", "error_description": "Missing client_id"}
+
+
+@pytest.mark.anyio
+async def test_cors_preflight_is_still_answered(client: httpx2.AsyncClient):
+    """A CORS preflight to a body-limited endpoint is answered by the CORS layer as before."""
+    response = await client.options(
+        "/token", headers={"Origin": "https://client.example.com", "Access-Control-Request-Method": "POST"}
+    )
+    assert response.status_code == 200
+    assert response.headers["access-control-allow-origin"] == "*"
+
+
+@pytest.mark.anyio
+async def test_oversized_cross_origin_request_gets_413_with_cors_headers(client: httpx2.AsyncClient):
+    """The 413 is produced inside the CORS layer, so a browser client can still read it."""
+    response = await client.post(
+        "/token",
+        content=b"x" * (DEFAULT_MAX_REQUEST_BODY_SIZE + 1),
+        headers={"Content-Type": _FORM, "Origin": "https://client.example.com"},
+    )
+    assert response.status_code == 413
+    assert response.headers["access-control-allow-origin"] == "*"

tests/server/mcpserver/test_server.py

@@ -5,6 +5,7 @@ from typing import Any
 from unittest.mock import AsyncMock, MagicMock, patch
 
 import anyio
+import httpx2
 import pytest
 from inline_snapshot import snapshot
 from mcp_types import (
@@ -1812,6 +1813,19 @@ def test_streamable_http_no_redirect() -> None:
     assert streamable_routes[0].path == "/mcp", "Streamable route path should be /mcp"
 
 
+async def test_sse_app_applies_the_configured_request_body_limit() -> None:
+    """`sse_app(max_request_body_size=...)` rejects larger POSTs to the message endpoint with HTTP 413."""
+    app = MCPServer("test").sse_app(max_request_body_size=8, host="0.0.0.0")
+    transport = httpx2.ASGITransport(app=app)
+    async with httpx2.AsyncClient(transport=transport, base_url="http://localhost") as http:
+        response = await http.post(
+            "/messages/?session_id=12345678123456781234567812345678",
+            content=b"123456789",
+            headers={"Content-Type": "application/json"},
+        )
+    assert response.status_code == 413
+
+
 async def test_report_progress_delegates_to_session_report_progress():
     """Context.report_progress delegates to ServerSession.report_progress unconditionally.
 

tests/server/test_sse_security.py

@@ -18,7 +18,7 @@ from mcp.server import Server
 from mcp.server.auth.middleware.bearer_auth import AuthenticatedUser
 from mcp.server.auth.provider import AccessToken
 from mcp.server.sse import SseServerTransport
-from mcp.server.transport_security import TransportSecuritySettings
+from mcp.server.transport_security import DEFAULT_MAX_REQUEST_BODY_SIZE, TransportSecuritySettings
 from mcp.shared._stream_protocols import WriteStream
 from mcp.shared.message import SessionMessage
 from tests.interaction.transports import StreamingASGITransport
@@ -204,9 +204,18 @@ def _authenticated_user(client_id: str, subject: str | None = None, issuer: str
 
 
 def _sse_scope(
-    method: str, path: str, user: AuthenticatedUser | None, *, query_string: bytes = b"", body: bytes = b""
+    method: str,
+    path: str,
+    user: AuthenticatedUser | None,
+    *,
+    query_string: bytes = b"",
+    body: bytes | list[bytes] = b"",
 ) -> tuple[Scope, Receive, Send, list[Message]]:
-    """Build an ASGI scope/receive/send triple for a request to the SSE transport."""
+    """Build an ASGI scope/receive/send triple for a request to the SSE transport.
+
+    `body` may be a list of chunks to deliver the request body over several `http.request` messages;
+    no Content-Length header is set either way.
+    """
     scope: Scope = {
         "type": "http",
         "method": method,
@@ -218,9 +227,11 @@ def _sse_scope(
     if user is not None:
         scope["user"] = user
     sent: list[Message] = []
+    chunks = list(body) if isinstance(body, list) else [body]
 
     async def receive() -> Message:
-        return {"type": "http.request", "body": body, "more_body": False}
+        chunk = chunks.pop(0)
+        return {"type": "http.request", "body": chunk, "more_body": bool(chunks)}
 
     async def send(message: Message) -> None:
         sent.append(message)
@@ -233,6 +244,10 @@ def _response_status(sent: list[Message]) -> int:
     return response_start["status"]
 
 
+def _response_body(sent: list[Message]) -> bytes:
+    return b"".join(msg.get("body", b"") for msg in sent if msg["type"] == "http.response.body")
+
+
 async def _post_message(transport: SseServerTransport, session_id: str, user: AuthenticatedUser | None) -> int:
     """POST a message to an SSE session as `user` and return the response status."""
     body = b'{"jsonrpc": "2.0", "id": 1, "method": "ping", "params": null}'
@@ -368,6 +383,80 @@ async def test_sse_post_with_a_disallowed_host_is_rejected_before_session_lookup
     assert _response_status(sent) == 421
 
 
+# A well-formed session ID that no live session owns.
+_UNKNOWN_SESSION = b"session_id=12345678123456781234567812345678"
+
+
+@pytest.mark.anyio
+async def test_sse_post_body_over_the_limit_returns_413():
+    """A POST body larger than max_request_body_size is answered with 413 before any session handling."""
+    transport = SseServerTransport("/messages/", max_request_body_size=8)
+    scope, receive, send, sent = _sse_scope(
+        "POST", "/messages/", None, query_string=_UNKNOWN_SESSION, body=b"123456789"
+    )
+
+    await transport.handle_post_message(scope, receive, send)
+    assert _response_status(sent) == 413
+    assert _response_body(sent) == b"Request body too large"
+
+
+@pytest.mark.anyio
+async def test_sse_post_body_limit_defaults_to_four_mib():
+    """Without an explicit limit, a body one byte over 4 MiB (and no Content-Length) is answered with 413."""
+    transport = SseServerTransport("/messages/")
+    body = b"x" * (DEFAULT_MAX_REQUEST_BODY_SIZE + 1)
+    scope, receive, send, sent = _sse_scope("POST", "/messages/", None, query_string=_UNKNOWN_SESSION, body=body)
+
+    await transport.handle_post_message(scope, receive, send)
+    assert _response_status(sent) == 413
+
+
+@pytest.mark.anyio
+async def test_sse_post_streamed_body_over_the_limit_returns_413():
+    """The limit counts bytes across body chunks, not just a declared Content-Length."""
+    transport = SseServerTransport("/messages/", max_request_body_size=8)
+    scope, receive, send, sent = _sse_scope(
+        "POST", "/messages/", None, query_string=_UNKNOWN_SESSION, body=[b"1234", b"56789"]
+    )
+
+    await transport.handle_post_message(scope, receive, send)
+    assert _response_status(sent) == 413
+
+
+@pytest.mark.anyio
+async def test_sse_post_within_the_limit_reaches_session_lookup():
+    """A body within the limit is passed on intact: an unknown session still gets its 404."""
+    transport = SseServerTransport("/messages/", max_request_body_size=64)
+    scope, receive, send, sent = _sse_scope(
+        "POST", "/messages/", None, query_string=_UNKNOWN_SESSION, body=[b'{"jsonrpc": ', b'"2.0"}']
+    )
+
+    await transport.handle_post_message(scope, receive, send)
+    assert _response_status(sent) == 404
+    assert _response_body(sent) == b"Could not find session"
+
+
+@pytest.mark.anyio
+@pytest.mark.parametrize("method", ["GET", "PUT"])
+async def test_sse_message_endpoint_answers_405_to_non_post(method: str):
+    """The message endpoint only accepts POST; other methods get 405 with an Allow header."""
+    transport = SseServerTransport("/messages/")
+    scope, receive, send, sent = _sse_scope(method, "/messages/", None, query_string=_UNKNOWN_SESSION, body=b"{}")
+
+    await transport.handle_post_message(scope, receive, send)
+    assert _response_status(sent) == 405
+    response_start = next(msg for msg in sent if msg["type"] == "http.response.start")
+    assert (b"allow", b"POST") in response_start["headers"]
+
+
+@pytest.mark.parametrize("max_request_body_size", [0, -1])
+def test_sse_transport_rejects_a_non_positive_body_limit(max_request_body_size: int):
+    """The body limit must be a positive number of bytes, matching StreamableHTTPSessionManager."""
+    with pytest.raises(ValueError) as exc_info:
+        SseServerTransport("/messages/", max_request_body_size=max_request_body_size)
+    assert str(exc_info.value) == "max_request_body_size must be a positive number of bytes"
+
+
 @pytest.mark.anyio
 async def test_sse_round_trip_delivers_posted_messages_and_streams_responses():
     """A POSTed JSON-RPC message reaches the server's read stream, and a message

tests/server/test_streamable_http_manager.py

@@ -10,7 +10,7 @@ import anyio
 import httpx2
 import pytest
 from mcp_types import INVALID_REQUEST, ListToolsResult, PaginatedRequestParams
-from starlette.types import Message, Receive, Scope, Send
+from starlette.types import Message, Scope
 
 from mcp import Client
 from mcp.client.streamable_http import streamable_http_client
@@ -18,11 +18,7 @@ from mcp.server import Server, ServerRequestContext, streamable_http_manager
 from mcp.server.auth.middleware.bearer_auth import AuthenticatedUser
 from mcp.server.auth.provider import AccessToken
 from mcp.server.streamable_http import MCP_SESSION_ID_HEADER, StreamableHTTPServerTransport
-from mcp.server.streamable_http_manager import (
+from mcp.server.streamable_http_manager import DEFAULT_MAX_REQUEST_BODY_SIZE, StreamableHTTPSessionManager
-    DEFAULT_MAX_REQUEST_BODY_SIZE,
-    RequestBodyLimitMiddleware,
-    StreamableHTTPSessionManager,
-)
 
 
 @pytest.mark.anyio
@@ -146,79 +142,6 @@ async def test_oversized_streamed_body_is_rejected_before_session_creation(
     assert response_start["status"] == 413
 
 
-@pytest.mark.anyio
-async def test_client_disconnect_while_streaming_request_body_is_replayed() -> None:
-    """SDK-defined: raw ASGI is required to prove a disconnect before body completion reaches the transport."""
-    disconnect: Message = {"type": "http.disconnect"}
-    request_messages: Iterator[Message] = iter(
-        [{"type": "http.request", "body": b"1234", "more_body": True}, disconnect]
-    )
-    received_messages: list[Message] = []
-
-    async def receive() -> Message:
-        return next(request_messages)
-
-    async def app(scope: Scope, receive: Receive, send: Send) -> None:
-        received_messages.append(await receive())
-        received_messages.append(await receive())
-
-    scope: Scope = {"type": "http", "method": "POST", "path": "/mcp", "headers": []}
-    middleware = RequestBodyLimitMiddleware(app, max_body_size=8)
-
-    await middleware(scope, receive, AsyncMock())
-
-    assert received_messages == [
-        {"type": "http.request", "body": b"1234", "more_body": True},
-        disconnect,
-    ]
-
-
-@pytest.mark.anyio
-async def test_client_disconnect_before_request_body_is_replayed() -> None:
-    """SDK-defined: raw ASGI proves a disconnect before the first body message reaches the transport."""
-    disconnect: Message = {"type": "http.disconnect"}
-    received_messages: list[Message] = []
-
-    async def receive() -> Message:
-        return disconnect
-
-    async def app(scope: Scope, receive: Receive, send: Send) -> None:
-        received_messages.append(await receive())
-
-    scope: Scope = {"type": "http", "method": "POST", "path": "/mcp", "headers": []}
-    middleware = RequestBodyLimitMiddleware(app, max_body_size=8)
-
-    await middleware(scope, receive, AsyncMock())
-
-    assert received_messages == [disconnect]
-
-
-@pytest.mark.anyio
-async def test_request_body_chunks_are_replayed_as_one_message() -> None:
-    """SDK-defined: raw ASGI proves chunk overhead is discarded before the body reaches the transport."""
-    request_messages: Iterator[Message] = iter(
-        [
-            {"type": "http.request", "body": b"12", "more_body": True},
-            {"type": "http.request", "body": b"34", "more_body": True},
-            {"type": "http.request", "body": b"56", "more_body": False},
-        ]
-    )
-    received_messages: list[Message] = []
-
-    async def receive() -> Message:
-        return next(request_messages)
-
-    async def app(scope: Scope, receive: Receive, send: Send) -> None:
-        received_messages.append(await receive())
-
-    scope: Scope = {"type": "http", "method": "POST", "path": "/mcp", "headers": []}
-    middleware = RequestBodyLimitMiddleware(app, max_body_size=8)
-
-    await middleware(scope, receive, AsyncMock())
-
-    assert received_messages == [{"type": "http.request", "body": b"123456", "more_body": False}]
-
-
 def test_request_body_limit_defaults_to_four_mib() -> None:
     """SDK-defined: Streamable HTTP request bodies are limited to 4 MiB by default."""
     manager = StreamableHTTPSessionManager(app=Server("test-default-size-limit"))

tests/server/test_transport_security.py

@@ -1,9 +1,17 @@
-"""Tests for the transport-security request validation middleware."""
+"""Tests for the request checks shared by the HTTP server transports."""
+
+from collections.abc import Iterator
+from unittest.mock import AsyncMock
 
 import pytest
 from starlette.requests import Request
+from starlette.types import Message, Receive, Scope, Send
 
-from mcp.server.transport_security import TransportSecurityMiddleware, TransportSecuritySettings
+from mcp.server.transport_security import (
+    RequestBodyLimitMiddleware,
+    TransportSecurityMiddleware,
+    TransportSecuritySettings,
+)
 
 
 def _request(host: str | None, origin: str | None, content_type: str | None = "application/json") -> Request:
@@ -86,3 +94,111 @@ async def test_validate_request_ignores_content_type_on_get() -> None:
     middleware = TransportSecurityMiddleware(SETTINGS)
     response = await middleware.validate_request(_request("good.example", None, content_type="text/plain"))
     assert response is None
+
+
+@pytest.mark.anyio
+async def test_client_disconnect_while_streaming_request_body_is_replayed() -> None:
+    """SDK-defined: raw ASGI is required to prove a disconnect before body completion reaches the transport."""
+    disconnect: Message = {"type": "http.disconnect"}
+    request_messages: Iterator[Message] = iter(
+        [{"type": "http.request", "body": b"1234", "more_body": True}, disconnect]
+    )
+    received_messages: list[Message] = []
+
+    async def receive() -> Message:
+        return next(request_messages)
+
+    async def app(scope: Scope, receive: Receive, send: Send) -> None:
+        received_messages.append(await receive())
+        received_messages.append(await receive())
+
+    scope: Scope = {"type": "http", "method": "POST", "path": "/mcp", "headers": []}
+    middleware = RequestBodyLimitMiddleware(app, max_body_size=8)
+
+    await middleware(scope, receive, AsyncMock())
+
+    assert received_messages == [
+        {"type": "http.request", "body": b"1234", "more_body": True},
+        disconnect,
+    ]
+
+
+@pytest.mark.anyio
+async def test_client_disconnect_before_request_body_is_replayed() -> None:
+    """SDK-defined: raw ASGI proves a disconnect before the first body message reaches the transport."""
+    disconnect: Message = {"type": "http.disconnect"}
+    received_messages: list[Message] = []
+
+    async def receive() -> Message:
+        return disconnect
+
+    async def app(scope: Scope, receive: Receive, send: Send) -> None:
+        received_messages.append(await receive())
+
+    scope: Scope = {"type": "http", "method": "POST", "path": "/mcp", "headers": []}
+    middleware = RequestBodyLimitMiddleware(app, max_body_size=8)
+
+    await middleware(scope, receive, AsyncMock())
+
+    assert received_messages == [disconnect]
+
+
+@pytest.mark.anyio
+async def test_request_body_chunks_are_replayed_as_one_message() -> None:
+    """SDK-defined: raw ASGI proves chunk overhead is discarded before the body reaches the transport."""
+    request_messages: Iterator[Message] = iter(
+        [
+            {"type": "http.request", "body": b"12", "more_body": True},
+            {"type": "http.request", "body": b"34", "more_body": True},
+            {"type": "http.request", "body": b"56", "more_body": False},
+        ]
+    )
+    received_messages: list[Message] = []
+
+    async def receive() -> Message:
+        return next(request_messages)
+
+    async def app(scope: Scope, receive: Receive, send: Send) -> None:
+        received_messages.append(await receive())
+
+    scope: Scope = {"type": "http", "method": "POST", "path": "/mcp", "headers": []}
+    middleware = RequestBodyLimitMiddleware(app, max_body_size=8)
+
+    await middleware(scope, receive, AsyncMock())
+
+    assert received_messages == [{"type": "http.request", "body": b"123456", "more_body": False}]
+
+
+@pytest.mark.anyio
+@pytest.mark.parametrize("method", ["GET", "PUT", "OPTIONS", "HEAD", "DELETE"])
+async def test_request_body_limit_applies_to_every_method(method: str) -> None:
+    """SDK-defined: the limit is a property of the request body, not of the method that carries it."""
+    app = AsyncMock()
+    sent_messages: list[Message] = []
+    receive = AsyncMock(return_value={"type": "http.request", "body": b"123456789", "more_body": False})
+
+    async def send(message: Message) -> None:
+        sent_messages.append(message)
+
+    scope: Scope = {"type": "http", "method": method, "path": "/mcp", "headers": []}
+    middleware = RequestBodyLimitMiddleware(app, max_body_size=8)
+
+    await middleware(scope, receive, send)
+
+    assert [message["status"] for message in sent_messages if message["type"] == "http.response.start"] == [413]
+    app.assert_not_awaited()
+
+
+@pytest.mark.anyio
+async def test_request_body_limit_leaves_non_http_scopes_alone() -> None:
+    """SDK-defined: only HTTP requests carry a body to limit; other ASGI scopes go straight to the app."""
+    app = AsyncMock()
+    receive = AsyncMock()
+    send = AsyncMock()
+    scope: Scope = {"type": "lifespan"}
+    middleware = RequestBodyLimitMiddleware(app, max_body_size=8)
+
+    await middleware(scope, receive, send)
+
+    app.assert_awaited_once_with(scope, receive, send)
+    receive.assert_not_awaited()