Bisect

Drive git bisect to isolate the first commit that introduced a bug, then report it. The skill stops at the report. Fixing belongs to the user with full context.

When to use

• User asks to "bisect", "find the commit that broke X", says "this used to work", or names a regression in a specific test/build target.
• ci-debug-loop exhausted log analysis without identifying a clear cause.
• A behavior, test, or build that was green at a known earlier point is now red.

Do not use when the bug is obviously in the most recent commit (inspect it), or when the reproducer is too fragile/flaky to drive an automated bisect (fix the flake first).

Inputs (confirm before starting)

Per confirm-before-implementing, gather these before touching git bisect:

• Symptom: one-line description of what's broken (e.g. "TestExchangeToken fails with unauthorized").
• Reproducer: a shell command that exits 0 when the bug is absent, 1 when present. If the user can't supply one, build it together (see Reproducer strategy).
• Known-bad ref: default HEAD.
• Known-good ref: default oldest commit within the last 2 weeks:

  git rev-list --since='2 weeks ago' --reverse HEAD | head -1

  If the user has a tag, branch, or specific date in mind, use it instead.

Reproducer strategy

The reproducer must work at every commit in the bisect range. The reproducer that exists today often won't: APIs change, test files don't yet exist on old commits, etc. Pick the highest-applicable strategy:

1. External reproducer (preferred). A self-contained shell script in $TMPDIR (e.g. /tmp/repro.sh) that builds and exercises the project from outside the source tree. Hits a stable surface (CLI flag, HTTP endpoint, exported function with stable signature) that survived the bisect range. Examples:

   # CLI: build and check observable behavior
   go build -o /tmp/foo ./cmd/foo && /tmp/foo --some-flag | grep -q expected
   # HTTP: spin up server, hit endpoint, kill
   go run ./cmd/server & PID=$!; sleep 1; curl -fsS localhost:8080/health; kill $PID

   This survives any in-tree changes because the reproducer doesn't live in the tree.
2. Carry an in-tree reproducer file across checkouts. Stash the reproducer test file, git stash apply it at each step, run, undo. Workable but brittle; checkouts during bisect can conflict with the stash. Use only when an external reproducer is impossible.
3. Narrow the range first. If the reproducer fundamentally requires an API that didn't exist before commit X, set the known-good to X and bisect within the API-stable window. The bisect then can't find regressions older than X. Accept that as a scope limit, don't fight it.

If none of the three apply, bisect is the wrong tool. Read the diff manually instead.

Workflow

1. Resolve refs and reproducer. Confirm the reproducer exits 1 at HEAD (bug present) and 0 at known-good (bug absent). If either fails, the inputs are wrong. Stop and re-gather.
2. Estimate cost. Count commits in range:

   git rev-list --count <good>..<bad>

   Steps ≈ log2(N). If more than ~8 steps (>256 commits), ask the user to narrow the known-good; bisect time grows quickly.
3. Snapshot working state. Auto-stash uncommitted changes including untracked files:

   STASH_REF=""
   if ! git diff --quiet || ! git diff --cached --quiet || [ -n "$(git ls-files --others --exclude-standard)" ]; then
     git stash push -u -m "bisect-auto-$(date +%s)" && STASH_REF=$(git rev-parse stash@{0})
   fi
   ORIG_HEAD=$(git rev-parse --abbrev-ref HEAD)
   trap 'git bisect reset >/dev/null 2>&1; git checkout "$ORIG_HEAD" 2>/dev/null; [ -n "$STASH_REF" ] && git stash pop 2>/dev/null' EXIT INT TERM

   The trap restores state on success, error, or interrupt.
4. Build the wrapper script. Write to $TMPDIR/bisect-wrapper.sh:

   #!/bin/sh
   set -e
   
   # Skip commits explicitly marked as intentionally broken.
   if git log -1 --format=%B | grep -qF '[skip-bisect]'; then
     exit 125
   fi
   
   # Optional project regen, uncomment per project needs.
   # task generate || exit 125
   # buf generate || exit 125
   
   # Run the reproducer. Exit 0 = good, 1 = bad, 125 = skip.
   <USER REPRODUCER COMMAND HERE>

   chmod +x the wrapper. Compile errors and missing-tool failures from the reproducer should propagate as exit codes the wrapper passes through; commits that can't even build typically return non-zero from the build step. Convert those to 125 by guarding with || exit 125 if the user wants to skip them rather than treat them as bad.
5. Run bisect.

   git bisect start <bad> <good>
   git bisect run "$TMPDIR/bisect-wrapper.sh"

   git bisect run drives to completion automatically. Capture the output.
6. Read the result. The last line of git bisect run output is <sha> is the first bad commit. Capture the SHA:

   FIRST_BAD=$(git bisect log | awk '/first bad commit/ {print $2}' | head -1)

   If git bisect log doesn't have it (older git), parse the run output instead.
7. Reset state. git bisect reset returns HEAD to the original ref. The trap will fire on exit and restore the stash.
8. Generate report (see Output).

Edge cases

• Flaky reproducer. If the same commit returns different exit codes on repeat runs, bisect is unreliable. Loop the reproducer N times (e.g. 5) and treat any failure as bad. Wrap the user's command in for i in 1 2 3 4 5; do <repro> || exit 1; done; exit 0. If still flaky, fix the flake first.
• Stale generated files after git checkout between steps. Add a regen step at the top of the wrapper (task generate, buf generate, npm run generate) before the reproducer.
• [skip-bisect] commits. Wrapper greps the message and exits 125. This relies on the commit-per-phase rule's marker convention; assume well-formed history.
• Compile/setup failures on old commits often surface as non-zero exits. By default git bisect run treats those as "bad". If they're really "can't tell", convert to 125 by guarding setup with || exit 125 in the wrapper.
• Working tree dirt. Handled by step 3's auto-stash + trap. Don't skip the trap; Ctrl-C without it leaves the user mid-bisect.
• Merges in the range. git bisect handles them by linearizing via the commit graph. No special action.
• Refactor renames. The offending commit may be a rename or move that exposes a latent bug elsewhere. Note this in the report; don't claim the rename is the root cause.

Output

Produce a single Markdown report:

## Offending commit
**<sha7>** — <subject>
Author: <name> | Date: <YYYY-MM-DD>
PR: <url if found via `gh pr list --search "<sha>" --state merged`>

## Diff
<output of `git show --stat <sha>` followed by `git show <sha>`>

## Commit message
<full body from `git log -1 --format=%B <sha>`>

## Suggested next step
- [ ] Write a regression test that reproduces <symptom> at HEAD (so the bisect is repeatable)
- [ ] Revert + reimplement, OR fix forward (user's call)
- [ ] Open issue / PR comment if the offending commit was already merged and shipped

Do not auto-revert, auto-fix, or push anything. The skill ends at the report.

Cross-references

• commit-per-phase rule defines the [skip-bisect] marker the wrapper looks for.
• ci-debug-loop skill is the typical escalation source. When log analysis stalls, hand the failing test/command to this skill as the reproducer.
• verify-when-complete skill produces good reproducer one-liners (task test -- -run '^TestX$').

Anti-patterns

• Running bisect without a deterministic reproducer. Guesses propagate exponentially.
• Skipping the auto-stash and trap. Leaves the user mid-bisect on Ctrl-C.
• Treating compile-error commits as "bad" without checking. That's a setup failure, not the bug.
• Auto-fixing the offending commit. Out of scope for this skill.
• Per ~/.claude/rules/probe-not-assume.md: confirm via tool/command before recommending; do not infer.