fix: use os.dup() to prevent stdio transport from closing real stdin/stdout#2166
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giulio-leone wants to merge 2 commits intomodelcontextprotocol:mainfrom
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February 28, 2026 15:41
…stdout When stdio_server() wraps sys.stdin.buffer / sys.stdout.buffer in TextIOWrapper, closing or GC'ing the wrapper also closes the underlying buffer, making sys.stdin/sys.stdout unusable after the server exits. Use os.dup() to duplicate the file descriptors before wrapping, so the wrappers own independent descriptors that can be safely closed without affecting the original stdio handles. Fixes modelcontextprotocol#1933
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Closing in favor of #2172 which has a more comprehensive fix. |
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Summary
Running an MCP server with
transport="stdio"causes subsequent stdio operations to fail withValueError: I/O operation on closed fileafter the server exits.Root Cause
In
stdio.py,TextIOWrapper(sys.stdin.buffer)wraps the real stdin buffer directly. When the wrapper is garbage-collected or closed, it also closessys.stdin.buffer, makingsys.stdinunusable.Fix
Use
os.dup()to duplicate the file descriptor before wrapping:This creates an independent file descriptor that can be safely closed without affecting the original
sys.stdin/sys.stdout.Verification
print()works after server exitFixes #1933