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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Using transport=\"stdio\" closes real stdio, causing ValueError after server exits","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\nHi! I ran into an issue where running the server with `transport=\"stdio\"` causes subsequent stdio operations to fail after the server exits.\n\n### Minimal reproduction\n\n```python\nfrom mcp.server.fastmcp import FastMCP\n\nmcp = FastMCP(\"Demo\")\nmcp.run(transport=\"stdio\")\nprint(\"?\")\n```\n\nWhen using `Ctrl+D` to exit the server, the final print raises:\n\n```bash\nTraceback (most recent call last):\n  File \"...\", line 5, in \u003cmodule\u003e\n    print(\"?\")\nValueError: I/O operation on closed file.\n```\n\nI’m not sure if this is intended behavior though.\n\n### Likely cause\n\nIn stdio.py, stdio is wrapped like this:\n\u003chttps://github.com/modelcontextprotocol/python-sdk/blob/5301298225968ce0fa8ae62870f950709da14dc6/src/mcp/server/stdio.py#L44-L47\u003e\n\nWhen these wrappers are closed, they also close the sys.stdin.buffer / sys.stdout.buffer.\n\n### Proposed fix\n\n```python\n    if not stdin:\n        stdin_fd = os.dup(sys.stdin.fileno())\n        stdin_bin = os.fdopen(stdin_fd, \"rb\", closefd=True)\n        stdin = anyio.wrap_file(TextIOWrapper(stdin_bin, encoding=\"utf-8\"))\n    if not stdout:\n        stdout_fd = os.dup(sys.stdout.fileno())\n        stdout_bin = os.fdopen(stdout_fd, \"wb\", closefd=True)\n        stdout = anyio.wrap_file(TextIOWrapper(stdout_bin, encoding=\"utf-8\"))\n```\n\n### Python \u0026 MCP Python SDK\n\n```Text\nPython 3.10.15\nmcp==1.25.0\n```","author":{"url":"https://github.com/hyn0027","@type":"Person","name":"hyn0027"},"datePublished":"2026-01-22T23:15:42.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":3},"url":"https://github.com/1933/python-sdk/issues/1933"}