fix: stdio_server emits LF only on Windows stdout (fixes #2433)#2908
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kaXianc2-gom wants to merge 1 commit into
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fix: stdio_server emits LF only on Windows stdout (fixes #2433)#2908kaXianc2-gom wants to merge 1 commit into
kaXianc2-gom wants to merge 1 commit into
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Add newline='' to TextIOWrapper for stdout to prevent \n → \r\n translation on Windows, which corrupts newline-delimited JSON messages. The MCP protocol uses \n as the line delimiter. Emitting \r\n is a protocol-level impurity that breaks clients parsing JSON lines. stdin is intentionally left with the default newline=None so that universal-newline behaviour normalises \r\n → \n on read. Fixes modelcontextprotocol#2433 Co-Authored-By: Claude <noreply@anthropic.com>
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Summary
Fix #2433: Windows
TextIOWrapperinstdio_server()emits\r\ninstead of\n, corrupting newline-delimited JSON messages.Change
Add
newline=""toTextIOWrapper(sys.stdout.buffer, ...)so the MCP protocol line delimiter (\n) is preserved on Windows.src/mcp/server/stdio.py: +3 lines (1 line of code + 2 comment lines), −1 linetests/server/test_stdio.py: +18 lines (new testtest_mcpserver_run_stdio_emits_lf_not_crlf)stdinis intentionally left with the defaultnewline=Noneso universal-newline behaviour normalises\r\n→\non read.AI Assistance Disclosure
This contribution was developed with AI assistance (Claude / Anthropic Claude Code). The process was:
stdio.py(addnewline=""parameter)Verification Environment
Why this fix is correct
"json\n"on Windows stdout"json\r\n"❌"json\n"✅"json\r\n"from Windows stdin"json\n"✅