MCP server in the HTTP Streamable mode broken #1190
Description
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.