kevinslin/agstack/debug-test-failure

Debug Test Failure

Overview

Use this skill to answer one question: did the current branch's changes introduce the observed test failure, or did the failure already exist at the baseline before the user's or agent's changes?

Workflow

1. Capture the current context.

• Read repository instructions and respect prohibited commands.
• Record the current repo path, branch, git status --short, and test command.
• If the user specified test files, test names, or a test command, use that targeted test command.
• If the user did not specify tests, run the repository's full test suite or the closest documented "all tests" command.

1. Identify the comparison baseline.

• Determine "me" from the user's explicit identity if provided; otherwise use git config user.email and git config user.name.
• Determine the branch base with the upstream or default branch, for example:

git merge-base HEAD @{upstream}
git merge-base HEAD origin/main
git merge-base HEAD origin/master

• Inspect branch-only commits in chronological order:

git log --reverse --format='%H%x09%an%x09%ae%x09%s' <base>..HEAD

• Select the earliest commit in that branch-only list whose author is not "me".
• If every branch-only commit is authored by "me", use <base> as the comparison commit.
• If the selected non-me commit still includes earlier branch changes, call that out in the final report.

1. Create an isolated worktree.

• Follow dev.worktrees conventions when available: store worktrees under ~/.worktrees/<repo>/.
• Use a detached worktree so no branch pointer moves:

git worktree add --detach ~/.worktrees/<repo>/debug-test-failure-<short-sha> <baseline-commit>

• Do not place the worktree inside the repository.
• If the path already exists, use a clear suffix such as debug-test-failure-<short-sha>-2.

1. Run comparable tests.

• Run the same selected test command in the baseline worktree.
• If the current worktree has not already run that command, run it there too.
• Keep logs separate, for example /tmp/debug-test-failure-current.log and /tmp/debug-test-failure-baseline.log.
• Install or bootstrap dependencies only as required by repository instructions.
• Do not run unrelated precommit hooks or broad cleanup commands unless the user explicitly asked for them.

1. Compare results.

• If the baseline passes and the current worktree fails, report that the current branch likely introduced the failure.
• If both fail with the same failure, report that the failure likely predates the current changes or is environmental/flaky.
• If the baseline fails differently, report that the result is inconclusive and separate the baseline failure from the current failure.
• If either run cannot execute, report the exact blocker and avoid claiming causality.

1. Report the outcome.

• Include the current branch, baseline commit, baseline worktree path, test command, and pass/fail result for each worktree.
• State the causality conclusion in one sentence.
• Include the most relevant failure excerpt, not the full log.
• Keep the baseline worktree available until the report is complete; remove it only if the user asked for cleanup or the artifact is clearly unnecessary.