[v1.x] fix(streamable-http): reject duplicate JSON-RPC ids with 409

The MCP base protocol requires that a request ID "MUST NOT have been
previously used by the requestor within the same session". Before this
change a duplicate POST silently overwrote the prior _request_streams
entry, leaving the original in-flight request hanging forever.

Mirror the existing GET_STREAM_KEY collision branch and return
409 Conflict, keeping the prior stream untouched.

Closes #2655

Author: truffle-dev

src/mcp/server/streamable_http.py

@@ -443,7 +443,9 @@ async def _validate_accept_header(self, request: Request, scope: Scope, send: Se
             return False
         return True
 
-    async def _handle_post_request(self, scope: Scope, request: Request, receive: Receive, send: Send) -> None:
+    async def _handle_post_request(  # noqa: C901, PLR0915
+        self, scope: Scope, request: Request, receive: Receive, send: Send
+    ) -> None:
         """Handle POST requests containing JSON-RPC messages."""
         writer = self._read_stream_writer
         if writer is None:  # pragma: no cover
@@ -531,7 +533,18 @@ async def _handle_post_request(self, scope: Scope, request: Request, receive: Re
             )
 
             # Extract the request ID outside the try block for proper scope
-            request_id = str(message.root.id)  # pragma: no cover
+            request_id = str(message.root.id)
+            # The MCP base protocol requires that "the request ID MUST NOT have been previously
+            # used by the requestor within the same session". If a client violates this, the
+            # prior stream would be silently overwritten and the in-flight request would hang,
+            # so reject the duplicate and leave the existing request untouched.
+            if request_id in self._request_streams:
+                response = self._create_error_response(
+                    f"Conflict: request id {request_id!r} is already in flight on this session",
+                    HTTPStatus.CONFLICT,
+                )
+                await response(scope, receive, send)
+                return
             # Register this stream for the request ID
             self._request_streams[request_id] = anyio.create_memory_object_stream[EventMessage](0)  # pragma: no cover
             request_stream_reader = self._request_streams[request_id][1]  # pragma: no cover

tests/server/test_streamable_http_manager.py

@@ -1,17 +1,24 @@
 """Tests for StreamableHTTPSessionManager."""
 
 import json
+from http import HTTPStatus
 from typing import Any
 from unittest.mock import AsyncMock, patch
 
 import anyio
 import pytest
+from starlette.requests import Request
 from starlette.types import Message
 
 from mcp.server import streamable_http_manager
 from mcp.server.lowlevel import Server
-from mcp.server.streamable_http import MCP_SESSION_ID_HEADER, StreamableHTTPServerTransport
+from mcp.server.streamable_http import (
+    MCP_SESSION_ID_HEADER,
+    EventMessage,
+    StreamableHTTPServerTransport,
+)
 from mcp.server.streamable_http_manager import StreamableHTTPSessionManager
+from mcp.shared.message import SessionMessage
 from mcp.types import INVALID_REQUEST
 
 
@@ -390,3 +397,78 @@ def test_session_idle_timeout_rejects_non_positive():
 def test_session_idle_timeout_rejects_stateless():
     with pytest.raises(RuntimeError, match="not supported in stateless"):
         StreamableHTTPSessionManager(app=Server("test"), session_idle_timeout=30, stateless=True)
+
+
+@pytest.mark.anyio
+async def test_handle_post_rejects_duplicate_request_id():
+    """Reject a POST whose JSON-RPC id matches an in-flight request on the same session.
+
+    The MCP base protocol forbids reusing a request ID within a session. Prior to the
+    fix, the second POST silently overwrote the prior ``_request_streams`` entry,
+    leaving the first request hanging forever. Now the duplicate is rejected with
+    409 Conflict and the prior in-flight stream is preserved untouched.
+    """
+    transport = StreamableHTTPServerTransport(mcp_session_id=None)
+
+    # The early ``writer is None`` guard reads this; the duplicate-id branch never
+    # actually sends on it, so a real stream is sufficient.
+    read_writer, read_reader = anyio.create_memory_object_stream[SessionMessage | Exception](0)
+    transport._read_stream_writer = read_writer
+
+    # Seed an in-flight request with id "1". The duplicate-id check must leave this
+    # pair in place.
+    in_flight_pair = anyio.create_memory_object_stream[EventMessage](0)
+    transport._request_streams["1"] = in_flight_pair
+
+    body = json.dumps({"jsonrpc": "2.0", "method": "tools/list", "id": 1, "params": {}}).encode()
+
+    body_sent = False
+
+    async def mock_receive() -> Message:
+        nonlocal body_sent
+        if body_sent:  # pragma: no cover
+            await anyio.sleep_forever()
+        body_sent = True
+        return {"type": "http.request", "body": body, "more_body": False}
+
+    sent_messages: list[Message] = []
+    response_body = b""
+
+    async def mock_send(message: Message) -> None:
+        nonlocal response_body
+        sent_messages.append(message)
+        if message["type"] == "http.response.body":
+            response_body += message.get("body", b"")
+
+    scope = {
+        "type": "http",
+        "method": "POST",
+        "path": "/mcp",
+        "headers": [
+            (b"content-type", b"application/json"),
+            (b"accept", b"application/json, text/event-stream"),
+        ],
+    }
+
+    request = Request(scope, mock_receive)
+    await transport._handle_post_request(scope, request, mock_receive, mock_send)
+
+    response_start = next(
+        (msg for msg in sent_messages if msg["type"] == "http.response.start"),
+        None,
+    )
+    assert response_start is not None, "Should have sent a response"
+    assert response_start["status"] == HTTPStatus.CONFLICT
+
+    error = json.loads(response_body)
+    assert error["jsonrpc"] == "2.0"
+    assert error["error"]["code"] == INVALID_REQUEST
+    assert "already in flight" in error["error"]["message"]
+
+    # The pre-existing in-flight stream must remain untouched.
+    assert transport._request_streams["1"] is in_flight_pair
+
+    in_flight_pair[0].close()
+    in_flight_pair[1].close()
+    read_writer.close()
+    read_reader.close()