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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Progress notifications cause server to hang on stdio transport","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\nWhen using the low-level Server API (`mcp.server.lowlevel.Server`) with stdio transport, sending progress notifications during request handling causes the server to hang indefinitely and never send the final response.\n\n**What I am seeing:**\n- Server successfully sends progress notification via `write_stream.send()`\n- Server handler returns `CallToolResult` \n- The final response is never sent to the client\n- Client times out waiting for response\n- Server remains blocked and time out eventually\n\n### Example Code\n\n```Python\nimport asyncio\nfrom mcp.server.lowlevel import Server\nfrom mcp.server.stdio import stdio_server\nfrom mcp.types import *\n\nserver = Server(\"progress-bug-demo\")\n\n@server.list_tools()\nasync def handle_list_tools() -\u003e list[Tool]:\n    return [Tool(\n        name=\"test_tool\",\n        description=\"Demonstrates the bug\",\n        inputSchema={\"type\": \"object\", \"properties\": {}}\n    )]\n\n@server.call_tool()\nasync def handle_call_tool(name: str, arguments: dict) -\u003e list[TextContent]:\n    ctx = server.request_context\n    progress_token = ctx.meta.progressToken if ctx.meta else None\n    \n    if progress_token:\n        # This causes the server to hang\n        await ctx.session.send_progress_notification(\n            progress_token=progress_token,\n            progress=0.5,\n            total=1.0,\n            message=\"Working...\"\n        )\n    \n    # This response is never sent due to the hang\n    return [TextContent(type=\"text\", text=\"Task completed!\")]\n\nasync def main():\n    async with stdio_server() as (read_stream, write_stream):\n        await server.run(\n            read_stream,\n            write_stream,\n            server.create_initialization_options()\n        )\n\nif __name__ == \"__main__\":\n    asyncio.run(main())\n```\n\n### Python \u0026 MCP Python SDK\n\n```Text\nMCP: 1.10.1\nPython: 3.12.6\n```\n\nFrom deeper debugging looks like buffer size is the problem : https://github.com/modelcontextprotocol/python-sdk/blob/main/src/mcp/server/stdio.py#L57\n\nHave to test locally if increasing it will most likely fix the issue and if its causing more problems than just fixing this.","author":{"url":"https://github.com/abhishekgahlot2","@type":"Person","name":"abhishekgahlot2"},"datePublished":"2025-07-14T19:58:11.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":4},"url":"https://github.com/1141/python-sdk/issues/1141"}