[jarhodes314]

Proposed changes

Adds /mutants-review [PR], a user-invoked agent skill in .agents/skills/, that runs cargo-mutants on a PR's diff and drafts tests for the mutants that survive.

/mutants-review <pr>
  check out the PR head in a temporary worktree
  cargo mutants --in-diff <pr diff> --test-tool nextest --all-features
  for each missed mutant
    gap            -> write the smallest test that catches it
    equivalent     -> one-line reason
    not worth it   -> one-line reason
  rerun cargo mutants on those mutants with the tests added
    the baseline proves each test passes on the real code
    caught.txt proves each test catches its mutant
  add [AI] comments to the user's pending review (after they approve)

The user's checkout is never touched, and nothing is visible to the PR author until the user submits the review.

Tried on #4373: cargo mutants --in-diff found 8 mutants (5 caught, 2 missed, 1 unviable). For the missed replace is_connection_closed -> bool with true, the skill wrote connection_refused_is_not_treated_as_a_closed_connection. The verify run reported that mutant caught and the unmutated baseline passed, and the test was added as a pending review comment.

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Improvement (general improvements like code refactoring that doesn't explicitly fix a bug or add any new functionality)
• Documentation Update (if none of the other choices apply)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Paste Link to the issue

Checklist

• I have read the CONTRIBUTING doc
• I have signed the CLA (in all commits with git commit -s. You can activate automatic signing by running just prepare-dev once)
• I ran just format as mentioned in CODING_GUIDELINES
• I used just check as mentioned in CODING_GUIDELINES
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

Further comments

The change is a single Markdown file that only runs when someone types /mutants-review; it touches no code, build or CI.

Every comment the skill posts is prefixed with [AI], so readers can tell which review comments were generated.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[github-actions]

Robot Results

✅ Passed	❌ Failed	⏭️ Skipped	Total	Pass %	⏱️ Duration
1047	0	8	1047	100	2h47m15.278827s

[rina23q]

[AI] question: Why not keep the skill in .agents/skills/ with a wrapper here, like pr-review?

---

from me: The PR description says:

Adds /mutants-review [PR], a user-invoked agent skill in .agents/skills/

So it seems this file is mislocated?

[jarhodes314]

Moved back to .agents/skills/mutants-review/SKILL.md, with a wrapper in .claude/commands/ like pr-review.

[rina23q]

[AI] issue: A mutant's line in missed.txt can be outside the diff, so posting the review can fail.

--in-diff keeps every mutant whose replaced code overlaps a changed line. But missed.txt gives the line where the replaced code starts. For a "replace function body" mutant, that is the first line of the body. If the PR changed only the end of a long function, that line is outside the diff hunks. GitHub rejects a review comment on such a line, so the REST call that creates the new review fails as a whole.

Suggested change

	Each untested behaviour becomes one comment, anchored on a mutated line (always inside the diff, since `--in-diff` produced it), `side: RIGHT`. Mutants killed by the same test share one comment.
	Each untested behaviour becomes one comment, anchored on a changed line of `pr.diff` that lies inside the mutated function, `side: RIGHT`. Mutants killed by the same test share one comment.

[jarhodes314]

Applied, thanks. The old wording wrongly claimed the mutated line is always inside the diff.