modelcontextprotocol · Ashutosh0x · Feb 10, 2026 · Feb 10, 2026
diff --git a/src/mcp/server/mcpserver/server.py b/src/mcp/server/mcpserver/server.py
@@ -805,7 +805,6 @@ async def handle_sse(scope: Scope, receive: Receive, send: Send):  # pragma: no
                 await self._lowlevel_server.run(
                     streams[0], streams[1], self._lowlevel_server.create_initialization_options()
                 )
-            return Response()
 
         # Create routes
         routes: list[Route | Mount] = []
@@ -869,15 +868,19 @@ async def handle_sse(scope: Scope, receive: Receive, send: Send):  # pragma: no
             )
         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]
+            # Use an ASGI-compatible wrapper to avoid Starlette's
+            # BaseHTTPMiddleware wrapping the SSE handler as a regular
+            # endpoint.  BaseHTTPMiddleware expects http.response.body
+            # messages, but the SSE handler sends raw ASGI events,
+            # which triggers "AssertionError: Unexpected message".
+            class HandleSseAsgi:
+                async def __call__(self, scope: Scope, receive: Receive, send: Send) -> None:
+                    await handle_sse(scope, receive, send)
 
             routes.append(
                 Route(
                     sse_path,
-                    endpoint=sse_endpoint,
+                    endpoint=HandleSseAsgi(),
                     methods=["GET"],
                 )
             )

diff --git a/tests/issues/test_883_middleware.py b/tests/issues/test_883_middleware.py
@@ -0,0 +1,96 @@
+"""Regression test for issue #883: AssertionError when using Starlette middleware.
+
+BaseHTTPMiddleware expects http.response.body messages, but the SSE handler
+sends raw ASGI events, which triggers "AssertionError: Unexpected message"
+when the SSE endpoint is wrapped as a regular Starlette endpoint.
+
+The fix uses an ASGI-compatible callable class (HandleSseAsgi) instead of a
+Starlette endpoint wrapper, so the SSE handler bypasses middleware response
+body wrapping.
+"""
+
+import multiprocessing
+import socket
+from collections.abc import Generator
+
+import anyio
+import httpx
+import pytest
+import uvicorn
+from starlette.applications import Starlette
+from starlette.middleware import Middleware
+from starlette.middleware.base import BaseHTTPMiddleware, RequestResponseEndpoint
+from starlette.requests import Request
+from starlette.responses import Response
+
+from mcp.server.mcpserver import MCPServer
+from mcp.server.transport_security import TransportSecuritySettings
+from tests.test_helpers import wait_for_server
+
+
+class PassthroughMiddleware(BaseHTTPMiddleware):  # pragma: no cover
+    """A simple pass-through middleware that triggers BaseHTTPMiddleware wrapping."""
+
+    async def dispatch(self, request: Request, call_next: RequestResponseEndpoint) -> Response:
+        return await call_next(request)
+
+
+@pytest.fixture
+def server_port() -> int:
+    with socket.socket() as s:
+        s.bind(("127.0.0.1", 0))
+        return s.getsockname()[1]
+
+
+def run_server_with_middleware(server_port: int) -> None:  # pragma: no cover
+    """Create an MCP server wrapped in Starlette BaseHTTPMiddleware."""
+    mcp_server = MCPServer("test-883")
+    transport_security = TransportSecuritySettings(enable_dns_rebinding_protection=False)
+    sse_app = mcp_server.sse_app(transport_security=transport_security, host="0.0.0.0")
+
+    # This is the exact scenario that triggers #883:
+    # BaseHTTPMiddleware wrapping a Starlette app containing SSE endpoints
+    app = Starlette(middleware=[Middleware(PassthroughMiddleware)])
+    app.mount("/", sse_app)
+
+    server = uvicorn.Server(config=uvicorn.Config(app=app, host="127.0.0.1", port=server_port, log_level="error"))
+    server.run()
+
+
+@pytest.fixture()
+def middleware_server(server_port: int) -> Generator[None, None, None]:
+    proc = multiprocessing.Process(
+        target=run_server_with_middleware,
+        kwargs={"server_port": server_port},
+        daemon=True,
+    )
+    proc.start()
+    wait_for_server(server_port)
+    yield
+    proc.kill()
+    proc.join(timeout=2)
+
+
+@pytest.mark.anyio
+async def test_sse_with_middleware_no_assertion_error(middleware_server: None, server_port: int) -> None:
+    """Verify SSE endpoint works when Starlette BaseHTTPMiddleware is applied.
+
+    Before the fix, this would raise:
+        AssertionError: Unexpected message type 'http.response.body'
+    """
+    async with httpx.AsyncClient(base_url=f"http://127.0.0.1:{server_port}") as client:
+        with anyio.fail_after(5):
+            async with client.stream("GET", "/sse") as response:
+                assert response.status_code == 200
+                assert "text/event-stream" in response.headers.get("content-type", "")
+
+                # Read the first event to confirm SSE is streaming properly
+                line_number = 0
+                async for line in response.aiter_lines():  # pragma: no branch
+                    if line_number == 0:
+                        assert line == "event: endpoint"
+                    elif line_number == 1:
+                        assert line.startswith("data: /messages/?session_id=")
+                    else:
+                        return
+                    line_number += 1