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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Simple echo MCP server fails to start with `mcp.run()`","articleBody":"**Describe the bug**\nUnable to start MCP server\n\n**To Reproduce**\n```python\nfrom mcp.server.fastmcp import FastMCP\n\nmcp = FastMCP(\n    \"Echo Server\",\n    port=9000,\n    debug=True,\n    log_level=\"DEBUG\",\n)\n\n\n@mcp.tool()\ndef echo(text: str) -\u003e str:\n    \"\"\"Echo the input text\"\"\"\n    return text\n\n\nif __name__ == \"__main__\":\n    mcp.run()\n```\n\n```shell\n$ uv run mcp_server.py        \n[03/20/25 16:31:00] DEBUG    Using selector: EpollSelector   \n\u003c---HANG---\u003e\n```\n\n```shell\n$ curl http://localhost:9000\ncurl: (7) Failed to connect to localhost port 9000 after 0 ms: Could not connect to server\n```\n\n**Expected behavior**\nServer starts\n\n### Additional information\nStack trace on SIGINT\n```\n  File \"/home/sirianni/src/example/.venv/lib/python3.12/site-packages/anyio/_backends/_asyncio.py\", line 2303, in run\n    return runner.run(wrapper())\n           ^^^^^^^^^^^^^^^^^^^^^\n  File \"/home/sirianni/.local/share/uv/python/cpython-3.12.9-linux-x86_64-gnu/lib/python3.12/asyncio/runners.py\", line 118, in run\n    return self._loop.run_until_complete(task)\n           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n  File \"/home/sirianni/.local/share/uv/python/cpython-3.12.9-linux-x86_64-gnu/lib/python3.12/asyncio/base_events.py\", line 678, in run_until_complete\n    self.run_forever()\n  File \"/home/sirianni/.local/share/uv/python/cpython-3.12.9-linux-x86_64-gnu/lib/python3.12/asyncio/base_events.py\", line 645, in run_forever\n    self._run_once()\n  File \"/home/sirianni/.local/share/uv/python/cpython-3.12.9-linux-x86_64-gnu/lib/python3.12/asyncio/base_events.py\", line 1961, in _run_once\n    event_list = self._selector.select(timeout)\n                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n  File \"/home/sirianni/.local/share/uv/python/cpython-3.12.9-linux-x86_64-gnu/lib/python3.12/selectors.py\", line 468, in select\n    fd_event_list = self._selector.poll(timeout, max_ev)\n                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n  File \"/home/sirianni/.local/share/uv/python/cpython-3.12.9-linux-x86_64-gnu/lib/python3.12/asyncio/runners.py\", line 157, in _on_sigint\n    raise KeyboardInterrupt()\nKeyboardInterrupt\n\nDuring handling of the above exception, another exception occurred:\n\nTraceback (most recent call last):\n  File \"/home/sirianni/src/example/mcp_server.py\", line 18, in \u003cmodule\u003e\n    mcp.run()\n  File \"/home/sirianni/src/example/.venv/lib/python3.12/site-packages/mcp/server/fastmcp/server.py\", line 154, in run\n    anyio.run(self.run_stdio_async)\n  File \"/home/sirianni/src/example/.venv/lib/python3.12/site-packages/anyio/_core/_eventloop.py\", line 74, in run\n    return async_backend.run(func, args, {}, backend_options)\n           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n  File \"/home/sirianni/src/example/.venv/lib/python3.12/site-packages/anyio/_backends/_asyncio.py\", line 2302, in run\n    with Runner(debug=debug, loop_factory=loop_factory) as runner:\n         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n  File \"/home/sirianni/.local/share/uv/python/cpython-3.12.9-linux-x86_64-gnu/lib/python3.12/asyncio/runners.py\", line 62, in __exit__\n    self.close()\n  File \"/home/sirianni/.local/share/uv/python/cpython-3.12.9-linux-x86_64-gnu/lib/python3.12/asyncio/runners.py\", line 70, in close\n    _cancel_all_tasks(loop)\n  File \"/home/sirianni/.local/share/uv/python/cpython-3.12.9-linux-x86_64-gnu/lib/python3.12/asyncio/runners.py\", line 199, in _cancel_all_tasks\n    to_cancel = tasks.all_tasks(loop)\n                ^^^^^^^^^^^^^^^^^^^^^\n  File \"/home/sirianni/.local/share/uv/python/cpython-3.12.9-linux-x86_64-gnu/lib/python3.12/asyncio/tasks.py\", line 59, in all_tasks\n    scheduled_tasks = list(_scheduled_tasks)\n                      ^^^^^^^^^^^^^^^^^^^^^^\n  File \"/home/sirianni/.local/share/uv/python/cpython-3.12.9-linux-x86_64-gnu/lib/python3.12/_weakrefset.py\", line 64, in __iter__\n    with _IterationGuard(self):\n         ^^^^^^^^^^^^^^^^^^^^^\nKeyboardInterrupt\n```","author":{"url":"https://github.com/sirianni","@type":"Person","name":"sirianni"},"datePublished":"2025-03-20T20:32:09.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":1},"url":"https://github.com/337/python-sdk/issues/337"}