fix: stop newline translation corrupting stdio framing on Windows#3132
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fix: stop newline translation corrupting stdio framing on Windows#3132bmdhodl wants to merge 1 commit into
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stdio_server() wrapped sys.stdout.buffer in a TextIOWrapper with default newline handling, which translates "\n" to os.linesep on write. On Windows every outgoing frame was terminated with "\r\n", breaking the newline-delimited JSON framing required by the spec. Fix: newline="" on the stdout wrapper only. Outgoing frames must be spec-exact; stdin keeps universal-newline reading so CRLF from clients stays tolerated. The regression test asserts on the raw bytes written to stdout, so the platform's newline translation itself is under test: it fails on main on Windows (b'...}\r\n') and passes with the fix. Runs on the windows-latest CI cell. Fixes modelcontextprotocol#2433 Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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Summary
Fixes #2433. Supersedes stale #2552 (credit to its author for the direction).
stdio_server()wrapssys.stdout.bufferin aTextIOWrapperwith default newline handling, which translates\ntoos.linesepon write. On Windows every outgoing frame is terminated with\r\n, breaking the newline-delimited JSON framing required by the spec.Change
newline=""on the stdout wrapper only. Write-side frames must be spec-exact (\n-delimited); stdin keeps universal-newline reading so CRLF from clients stays tolerated (liberal in what we accept).How Has This Been Tested?
Reproduced the bug natively on Windows 11 (Python 3.13, current
main) with a raw-bytes client against a real stdio server subprocess: frames arrived asb'...}\r\n'before the fix,b'...}\n'after.The new regression test asserts on the raw bytes written to stdout, so the platform's newline translation itself is under test. Verified red/green on native Windows: fails on
mainwithb'{"jsonrpc":"2.0","id":1,"result":{}}\r\n', passes with the fix. It runs on thewindows-latestunit-test CI cell, which currently has no byte-level framing coverage (the test in #2552 assertedendswith(b"\n"), which cannot detect CRLF).Local gate on Windows: full suite 5438 passed / 15 skipped / 1 xfailed,
ruff check+ruff formatclean,pyright(pinned 1.1.405) 0 errors.Breaking Changes
None. Output on POSIX platforms is byte-identical; Windows output changes from out-of-spec
\r\nframing to the spec's\n.Types of changes
Checklist