Starting from v1.12.0 (checked with v1.12.0 and v1.12.1) any request to the MCP server in the HTTP Streamable mode produces an error:
INFO: 127.0.0.1:32926 - "POST /mcp/ HTTP/1.1" 307 Temporary Redirect
[07/23/25 16:34:18] INFO Processing request of type ListToolsRequest server.py:625
INFO: 127.0.0.1:32926 - "POST /mcp HTTP/1.1" 200 OK
INFO Terminating session: None streamable_http.py:630
ERROR Error in message router streamable_http.py:880
╭────────────────── Traceback (most recent call last) ──────────────────╮ │ /usr/local/lib/python3.12/site-packages/mcp/server/streamable_http.py │ │ :831 in message_router │ │ │ │ 828 │ │ │ # Create a message router that distributes messages │ │ 829 │ │ │ async def message_router(): │ │ 830 │ │ │ │ try: │ │ ❱ 831 │ │ │ │ │ async for session_message in write_stream_r │ │ 832 │ │ │ │ │ │ # Determine which request stream(s) sho │ │ 833 │ │ │ │ │ │ message = session_message.message │ │ 834 │ │ │ │ │ │ target_request_id = None │ │ │ │ /usr/local/lib/python3.12/site-packages/anyio/abc/_streams.py:35 in │ │ __anext__ │ │ │ │ 32 │ │ │ 33 │ async def __anext__(self) -> T_co: │ │ 34 │ │ try: │ │ ❱ 35 │ │ │ return await self.receive() │ │ 36 │ │ except EndOfStream: │ │ 37 │ │ │ raise StopAsyncIteration │ │ 38 │ │ │ │ /usr/local/lib/python3.12/site-packages/anyio/streams/memory.py:111 │ │ in receive │ │ │ │ 108 │ async def receive(self) -> T_co: │ │ 109 │ │ await checkpoint() │ │ 110 │ │ try: │ │ ❱ 111 │ │ │ return self.receive_nowait() │ │ 112 │ │ except WouldBlock: │ │ 113 │ │ │ # Add ourselves in the queue │ │ 114 │ │ │ receive_event = Event() │ │ │ │ /usr/local/lib/python3.12/site-packages/anyio/streams/memory.py:93 in │ │ receive_nowait │ │ │ │ 90 │ │ │ │ 91 │ │ """ │ │ 92 │ │ if self._closed: │ │ ❱ 93 │ │ │ raise ClosedResourceError │ │ 94 │ │ │ │ 95 │ │ if self._state.waiting_senders: │ │ 96 │ │ │ # Get the item from the next sender │ ╰───────────────────────────────────────────────────────────────────────╯ ClosedResourceError
Rollback to 1.11.0 fix the issue.
Starting from v1.12.0 (checked with v1.12.0 and v1.12.1) any request to the MCP server in the HTTP Streamable mode produces an error:
Rollback to 1.11.0 fix the issue.