[v1.x] Apply the request body limit to the SSE and OAuth endpoints

SseServerTransport now takes max_request_body_size (default 4 MiB, the
same default and validation as StreamableHTTPSessionManager) and answers
413 before session lookup or parsing when a request declares or streams a
larger body. The message endpoint only ever handled POST bodies, so it
now answers 405 (Allow: POST) to other methods instead of treating them
like a POST. FastMCP forwards its existing max_request_body_size setting
to the SSE transport, so one setting governs both HTTP transports.

The create_auth_routes endpoints (/token, /revoke, /register and
/authorize) are wrapped in RequestBodyLimitMiddleware at the route
declarations and answer 413 to bodies over the 4 MiB default before any
form or JSON parsing. On the CORS-enabled routes the limit sits inside the
CORS wrapper so a 413 still carries CORS headers; cors_middleware itself is
unchanged. The middleware no longer special-cases POST, since some of these
routes also accept OPTIONS or HEAD and their handlers read the body either
way.

Differences from the main change:
- FastMCP reuses its existing max_request_body_size setting for the SSE
  app instead of adding keywords to sse_app()/run(); no new FastMCP
  parameter.
- /register reads its body via request.json() on this line; the same
  wrapper applies unchanged.
- Tests use httpx and this line's dict-based SSE scope helper.

Author: maxisbey

docs/server.md

@@ -1253,7 +1253,7 @@ The FastMCP server instance accessible via `ctx.fastmcp` provides access to serv
   - `host` and `port` - Server network configuration
   - `mount_path`, `sse_path`, `streamable_http_path` - Transport paths
   - `stateless_http` - Whether the server operates in stateless mode
-  - `max_request_body_size` - Maximum Streamable HTTP POST body size in bytes
+  - `max_request_body_size` - Maximum HTTP request body size in bytes (Streamable HTTP and SSE)
   - And other configuration options
 
 ```python
@@ -1418,9 +1418,9 @@ Note that `uv run mcp run` or `uv run mcp dev` only supports server using FastMC
 
 > **Note**: Streamable HTTP transport is the recommended transport for production deployments. Use `stateless_http=True` and `json_response=True` for optimal scalability.
 
-Streamable HTTP POST bodies are limited to 4 MiB by default. Larger requests receive HTTP 413
-before parsing or session creation. If your server intentionally accepts larger MCP messages,
-configure the smallest suitable byte limit:
+HTTP request bodies (Streamable HTTP and SSE) are limited to 4 MiB by default. Larger requests
+receive HTTP 413 before parsing or session creation. If your server intentionally accepts larger MCP
+messages, configure the smallest suitable byte limit:
 
 ```python
 mcp = FastMCP("Large messages", max_request_body_size=8 * 1024 * 1024)

src/mcp/server/auth/routes.py

@@ -18,6 +18,7 @@
 from mcp.server.auth.provider import OAuthAuthorizationServerProvider
 from mcp.server.auth.settings import ClientRegistrationOptions, RevocationOptions
 from mcp.server.streamable_http import MCP_PROTOCOL_VERSION_HEADER
+from mcp.server.streamable_http_manager import DEFAULT_MAX_REQUEST_BODY_SIZE, RequestBodyLimitMiddleware
 from mcp.shared.auth import OAuthMetadata
 
 
@@ -53,17 +54,24 @@ def validate_issuer_url(url: AnyHttpUrl):
 REVOCATION_PATH = "/revoke"
 
 
-def cors_middleware(
-    handler: Callable[[Request], Response | Awaitable[Response]],
-    allow_methods: list[str],
-) -> ASGIApp:
-    cors_app = CORSMiddleware(
-        app=request_response(handler),
+def _cors(app: ASGIApp, allow_methods: list[str]) -> ASGIApp:
+    return CORSMiddleware(
+        app=app,
         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(
         revocation_options,
     )
     client_authenticator = ClientAuthenticator(provider)
+    token_handler = TokenHandler(provider, client_authenticator)
 
     # 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",
@@ -102,15 +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(
-                TokenHandler(provider, client_authenticator).handle,
-                ["POST", "OPTIONS"],
-            ),
+            endpoint=_cors(_body_limited(request_response(token_handler.handle)), ["POST", "OPTIONS"]),
             methods=["POST", "OPTIONS"],
         ),
     ]
@@ -123,10 +130,7 @@ def create_auth_routes(
         routes.append(
             Route(
                 REGISTRATION_PATH,
-                endpoint=cors_middleware(
-                    registration_handler.handle,
-                    ["POST", "OPTIONS"],
-                ),
+                endpoint=_cors(_body_limited(request_response(registration_handler.handle)), ["POST", "OPTIONS"]),
                 methods=["POST", "OPTIONS"],
             )
         )
@@ -136,10 +140,7 @@ def create_auth_routes(
         routes.append(
             Route(
                 REVOCATION_PATH,
-                endpoint=cors_middleware(
-                    revocation_handler.handle,
-                    ["POST", "OPTIONS"],
-                ),
+                endpoint=_cors(_body_limited(request_response(revocation_handler.handle)), ["POST", "OPTIONS"]),
                 methods=["POST", "OPTIONS"],
             )
         )

src/mcp/server/fastmcp/server.py

@@ -107,6 +107,7 @@ class Settings(BaseSettings, Generic[LifespanResultT]):
     stateless_http: bool
     """Define if the server should create a new transport per request."""
     max_request_body_size: int
+    """Maximum request body size in bytes for the Streamable HTTP endpoint and the SSE message endpoint."""
 
     # resource settings
     warn_on_duplicate_resources: bool
@@ -835,6 +836,7 @@ def sse_app(self, mount_path: str | None = None) -> Starlette:
         sse = SseServerTransport(
             normalized_message_endpoint,
             security_settings=self.settings.transport_security,
+            max_request_body_size=self.settings.max_request_body_size,
         )
 
         async def handle_sse(scope: Scope, receive: Receive, send: Send):  # pragma: no cover

src/mcp/server/sse.py

@@ -53,6 +53,7 @@ async def handle_sse(request):
 
 import mcp.types as types
 from mcp.server.auth.middleware.bearer_auth import AuthenticatedUser, AuthorizationContext, authorization_context
+from mcp.server.streamable_http_manager import DEFAULT_MAX_REQUEST_BODY_SIZE, RequestBodyLimitMiddleware
 from mcp.server.transport_security import (
     TransportSecurityMiddleware,
     TransportSecuritySettings,
@@ -81,7 +82,12 @@ 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.
@@ -90,6 +96,9 @@ def __init__(self, endpoint: str, security_settings: TransportSecuritySettings |
             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:
@@ -106,6 +115,9 @@ def __init__(self, endpoint: str, security_settings: TransportSecuritySettings |
 
         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(
@@ -121,6 +133,7 @@ def __init__(self, endpoint: str, security_settings: TransportSecuritySettings |
         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
@@ -214,7 +227,18 @@ async def response_wrapper(scope: Scope, receive: Receive, send: Send):
             self._read_stream_writers.pop(session_id, None)
             self._session_owners.pop(session_id, None)
 
-    async def handle_post_message(self, scope: Scope, receive: Receive, send: Send) -> None:  # pragma: no cover
+    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:  # pragma: no cover
         logger.debug("Handling POST message")
         request = Request(scope, receive)
 

src/mcp/server/streamable_http_manager.py

@@ -29,7 +29,7 @@
 logger = logging.getLogger(__name__)
 
 DEFAULT_MAX_REQUEST_BODY_SIZE: Final = 4 * 1024 * 1024
-"""Default maximum Streamable HTTP request body size in bytes (4 MiB)."""
+"""Default maximum HTTP request body size in bytes (4 MiB)."""
 
 
 class StreamableHTTPSessionManager:
@@ -65,7 +65,7 @@ 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.
     """
 
@@ -371,7 +371,7 @@ def __init__(self, app: ASGIApp, max_body_size: int) -> None:
         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":
+        if scope["type"] != "http":
             await self.app(scope, receive, send)
             return
 

tests/server/auth/test_error_handling.py

@@ -14,6 +14,7 @@
 
 from mcp.server.auth.provider import AuthorizeError, RegistrationError, TokenError
 from mcp.server.auth.routes import create_auth_routes
+from mcp.server.streamable_http_manager import DEFAULT_MAX_REQUEST_BODY_SIZE
 
 # TODO(Marcelo): This TYPE_CHECKING shouldn't be here, but pytest doesn't seem to get the module correctly.
 if TYPE_CHECKING:
@@ -302,3 +303,59 @@ 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: httpx.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: httpx.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": "unauthorized_client", "error_description": "Missing client_id"}
+
+
+@pytest.mark.anyio
+async def test_cors_preflight_is_still_answered(client: httpx.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: httpx.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/fastmcp/test_server.py

@@ -3,6 +3,7 @@
 from typing import TYPE_CHECKING, Any
 from unittest.mock import patch
 
+import httpx
 import pytest
 from pydantic import AnyUrl, BaseModel
 from starlette.routing import Mount, Route
@@ -1505,3 +1506,17 @@ def test_streamable_http_app_passes_the_configured_request_body_limit_to_its_man
     mcp.streamable_http_app()
 
     assert mcp.session_manager.max_request_body_size == 8
+
+
+@pytest.mark.anyio
+async def test_sse_app_applies_the_configured_request_body_limit() -> None:
+    """FastMCP forwards its request-body setting to the SSE message endpoint: larger POSTs get HTTP 413."""
+    mcp = FastMCP(host="0.0.0.0", max_request_body_size=8)
+    transport = httpx.ASGITransport(app=mcp.sse_app())
+    async with httpx.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