Add Streamable HTTP request body limits (#3095)

tests/docs_src/test_asgi.py

@@ -43,6 +43,7 @@ async def test_streamable_http_app_takes_runs_options_except_port() -> None:
         "stateless_http",
         "event_store",
         "retry_interval",
+        "max_request_body_size",
         "transport_security",
         "host",
     }
@@ -56,6 +57,17 @@ async def test_a_request_before_the_session_manager_runs_is_rejected() -> None:
             await http.post("/mcp")
 
 
+async def test_streamable_http_app_applies_the_configured_request_body_limit() -> None:
+    """The documented `max_request_body_size` option rejects larger requests with HTTP 413."""
+    server = MCPServer("Notes")
+    app = server.streamable_http_app(max_request_body_size=8)
+    transport = httpx2.ASGITransport(app=app)
+    async with server.session_manager.run():
+        async with httpx2.AsyncClient(transport=transport, base_url="http://localhost") as http:
+            response = await http.post("/mcp", content=b"123456789")
+    assert response.status_code == 413
+
+
 async def test_mounting_at_the_root_keeps_the_default_path() -> None:
     """tutorial002: `Mount("/")` plus the default `streamable_http_path` leaves the endpoint at `/mcp`."""
     (mount,) = tutorial002.app.routes

tests/docs_src/test_legacy_clients.py

@@ -52,6 +52,7 @@ def test_streamable_http_app_has_no_era_knob() -> None:
         "stateless_http",
         "event_store",
         "retry_interval",
+        "max_request_body_size",
         "transport_security",
         "host",
     }

tests/server/test_streamable_http_manager.py

@@ -2,6 +2,7 @@
 
 import json
 import logging
+from collections.abc import Iterator
 from typing import Any
 from unittest.mock import AsyncMock, patch
 
@@ -9,7 +10,7 @@ import anyio
 import httpx2
 import pytest
 from mcp_types import INVALID_REQUEST, ListToolsResult, PaginatedRequestParams
-from starlette.types import Message, Scope
+from starlette.types import Message, Receive, Scope, Send
 
 from mcp import Client
 from mcp.client.streamable_http import streamable_http_client
@@ -17,7 +18,11 @@ 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 StreamableHTTPSessionManager
+from mcp.server.streamable_http_manager import (
+    DEFAULT_MAX_REQUEST_BODY_SIZE,
+    RequestBodyLimitMiddleware,
+    StreamableHTTPSessionManager,
+)
 
 
 @pytest.mark.anyio
@@ -71,9 +76,9 @@ async def test_handle_request_without_run_raises_error():
     manager = StreamableHTTPSessionManager(app=app)
 
     # Mock ASGI parameters
-    scope = {"type": "http", "method": "POST", "path": "/test"}
+    scope: Scope = {"type": "http", "method": "POST", "path": "/test", "headers": []}
 
-    async def receive():  # pragma: no cover
+    async def receive() -> Message:
         return {"type": "http.request", "body": b""}
 
     async def send(message: Message):  # pragma: no cover
@@ -86,6 +91,148 @@ async def test_handle_request_without_run_raises_error():
     assert "Task group is not initialized. Make sure to use run()." in str(excinfo.value)
 
 
+@pytest.mark.anyio
+async def test_oversized_content_length_is_rejected_before_body_read_or_session_creation() -> None:
+    """SDK-defined: an oversized declared body gets HTTP 413 before the server reads it or creates a session."""
+    manager = StreamableHTTPSessionManager(app=Server("test-size-limit"), max_request_body_size=8)
+    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": "POST",
+        "path": "/mcp",
+        "headers": [(b"content-length", b"9")],
+    }
+    async with manager.run():
+        await manager.handle_request(scope, receive, send)
+        assert manager._server_instances == {}
+
+    response_start = next(message for message in sent_messages if message["type"] == "http.response.start")
+    assert response_start["status"] == 413
+    receive.assert_not_awaited()
+
+
+@pytest.mark.anyio
+@pytest.mark.parametrize("headers", [[], [(b"content-length", b"invalid")], [(b"content-length", b"8")]])
+async def test_oversized_streamed_body_is_rejected_before_session_creation(
+    headers: list[tuple[bytes, bytes]],
+) -> None:
+    """SDK-defined: streamed bodies enforce the limit with missing, invalid, or understated length."""
+    manager = StreamableHTTPSessionManager(app=Server("test-streamed-size-limit"), max_request_body_size=8)
+    sent_messages: list[Message] = []
+    request_messages: Iterator[Message] = iter(
+        [
+            {"type": "http.request", "body": b"1234", "more_body": True},
+            {"type": "http.request", "body": b"56789", "more_body": False},
+        ]
+    )
+
+    async def receive() -> Message:
+        return next(request_messages)
+
+    async def send(message: Message) -> None:
+        sent_messages.append(message)
+
+    scope: Scope = {"type": "http", "method": "POST", "path": "/mcp", "headers": headers}
+    async with manager.run():
+        await manager.asgi_app(scope, receive, send)
+        assert manager._server_instances == {}
+
+    response_start = next(message for message in sent_messages if message["type"] == "http.response.start")
+    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"))
+    assert manager.max_request_body_size == DEFAULT_MAX_REQUEST_BODY_SIZE == 4 * 1024 * 1024
+
+
+@pytest.mark.parametrize("max_request_body_size", [0, -1])
+def test_request_body_limit_rejects_non_positive_values(max_request_body_size: int) -> None:
+    """SDK-defined: callers cannot disable request-size protection with a non-positive value."""
+    with pytest.raises(ValueError) as exc_info:
+        StreamableHTTPSessionManager(app=Server("test-invalid-size-limit"), max_request_body_size=max_request_body_size)
+    assert str(exc_info.value) == "max_request_body_size must be a positive number of bytes"
+
+
 class TestException(Exception):
     __test__ = False  # Prevent pytest from collecting this as a test class
     pass
@@ -122,7 +269,7 @@ async def test_stateful_session_cleanup_on_graceful_exit(running_manager: tuple[
         "headers": [(b"content-type", b"application/json")],
     }
 
-    async def mock_receive():  # pragma: no cover
+    async def mock_receive():
         return {"type": "http.request", "body": b"", "more_body": False}
 
     # Trigger session creation
@@ -180,7 +327,7 @@ async def test_stateful_session_cleanup_on_exception(running_manager: tuple[Stre
         "headers": [(b"content-type", b"application/json")],
     }
 
-    async def mock_receive():  # pragma: no cover
+    async def mock_receive():
         return {"type": "http.request", "body": b"", "more_body": False}
 
     # Trigger session creation
@@ -298,7 +445,7 @@ async def test_unknown_session_id_returns_404(caplog: pytest.LogCaptureFixture):
         }
 
         async def mock_receive():
-            return {"type": "http.request", "body": b"{}", "more_body": False}  # pragma: no cover
+            return {"type": "http.request", "body": b"{}", "more_body": False}
 
         with caplog.at_level(logging.INFO):
             await manager.handle_request(scope, mock_receive, mock_send)
@@ -378,7 +525,7 @@ async def test_idle_session_is_reaped(caplog: pytest.LogCaptureFixture, request:
             "headers": [(b"content-type", b"application/json")],
         }
 
-        async def mock_receive():  # pragma: no cover
+        async def mock_receive():
             return {"type": "http.request", "body": b"", "more_body": False}
 
         await manager.handle_request(scope, mock_receive, mock_send)