[xiehuanyi]

Atomic sorting of Cython into stdout or another write-only stream currently emits no content. The validation read consumes the buffer; the deliberately ignored Python syntax error for Cython skips the rewind, so the subsequent copy reads EOF.

Rewind after the validation block for all accepted output. Add .pyx/.pxd regressions with non-atomic controls and a changelog entry.

Validation: the two atomic cases fail before and all four cases pass afterward. Real python -m isort --atomic --stdout restores the sorted output for both extensions; invalid Python still raises ExistingSyntaxErrors without output. The 20 API/IO tests and default unit suite pass (676 passed, one existing skip). Changed-file mypy/Flake8, Ruff check/format, the hug profile, repository pre-commit checks and Bandit pass.

Prepared and verified with Codex (GPT-6.1-sol) assistance.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.37%. Comparing base (92c5685) to head (7169382).
⚠️ Report is 9 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2721      +/-   ##
==========================================
- Coverage   99.38%   99.37%   -0.01%     
==========================================
  Files          41       40       -1     
  Lines        3230     3224       -6     
  Branches      690      689       -1     
==========================================
- Hits         3210     3204       -6     
  Misses         12       12              
  Partials        8        8              

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