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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"`stdio_server` uses unbuffered memory streams which can cause server to block and become unresponsive","articleBody":"### Initial Checks\n\n- [x] I confirm that I'm using the latest version of MCP Python SDK\n- [x] I confirm that I searched for my issue in https://github.com/modelcontextprotocol/python-sdk/issues before opening this issue\n\n### Description\n\nThe MCP Python SDK's `stdio_server` becomes unresponsive during slow message processing operations, causing ping and/or new requests to timeout.\n\n#### Observed Behaviour\n\n* Server appears to 'freeze' and becomes unresponsive after running for extended periods\n* Ping requests timeout during slow operations\n* New requests cannot be processed while the server is handling long-running operations\n* After 67 minutes of operation with requests every 10 seconds, the server became completely unresponsive\n\n#### Expected Behaviour\n\n* Server should remain responsive to new requests (like pings) even while processing slow operations\n* Ping requests should not timeout due to message processing delays\n* The server should handle concurrent requests without blocking\n\n### Suspected Root Cause\n\nThe `stdio_server` uses `max_buffer_size=0` (supplied to `anyio.create_memory_object_stream`) by default, which creates synchronous handoff between the stdin reader and message processor. \n\nSee: https://github.com/modelcontextprotocol/python-sdk/blob/543961968c0634e93d919d509cce23a1d6a56c21/src/mcp/server/stdio.py#L57-L58\n\nWhen the message processor is slow or blocked, the stdin reader cannot read new messages from stdin, making the server appear unresponsive.\n\n### Example Code\n\n```python\n  import anyio\n  import pytest\n\n\n  @pytest.mark.anyio\n  async def test_server_becomes_unresponsive_with_slow_processor():\n      \"\"\"Demonstrates how server becomes unresponsive during slow processing.\"\"\"\n\n      # Simulates stdio_server with default max_buffer_size=0 (synchronous handoff)\n      send_stream, receive_stream = anyio.create_memory_object_stream(0)\n\n      async def stdin_reader():\n          # First message gets through\n          await send_stream.send(\"request_1\")\n\n          # Second message (like a ping) blocks until first is fully processed\n          await send_stream.send(\"ping\")  # This will block for entire processing time!\n\n      async def message_processor():\n          # Process first message\n          msg = await receive_stream.receive()\n\n          # Simulate slow processing (database query, API call, etc.)\n          await anyio.sleep(0.1)  # 100ms processing time\n\n          # During this time, stdin_reader is completely blocked\n          # No new messages (including pings) can be read!\n\n          ping = await receive_stream.receive()  # Finally unblocks stdin_reader\n\n      async with anyio.create_task_group() as tg:\n          tg.start_soon(message_processor)\n          await anyio.sleep(0.01)  # Let processor start waiting\n          tg.start_soon(stdin_reader)\n\n      send_stream.close()\n      receive_stream.close()\n```\n\n### Proposed Solution\n\nAllow users to configure `max_buffer_size \u003e 0` to enable buffering, with a default value (`0`) that preserves the current behaviour.\n\n```python\nasync def stdio_server(\n    stdin: anyio.AsyncFile[str] | None = None,\n    stdout: anyio.AsyncFile[str] | None = None,\n    max_buffer_size: int = 0, \n):\n    # ...\n    read_stream_writer, read_stream = anyio.create_memory_object_stream(max_buffer_size)\n    write_stream, write_stream_reader = anyio.create_memory_object_stream(max_buffer_size)\n```\n\n### Python \u0026 MCP Python SDK\n\n  - Python: 3.12\n  - MCP Python SDK: 1.12.4\n  - OS: macOS\n\n### Additional Context\n  \nThe issue manifests in long-running servers where message processing can occasionally be slow (database queries, file operations, API calls). \n\nThe fix should make `max_buffer_size` configurable so users can add buffering to prevent the stdin reader from blocking during slow operations.","author":{"url":"https://github.com/peteski22","@type":"Person","name":"peteski22"},"datePublished":"2025-09-01T19:35:49.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":1},"url":"https://github.com/1333/python-sdk/issues/1333"}