Project 4 · A Fix Loop with a Real Checker

⚠️ Throwaway repo. Set a limit first. Do this entirely inside a worktree — the whole point is that a bad fix attempt can't touch your working tree — and cap attempts before beat one.

The scenario

You maintain a real internal library — say, a shared TypeScript utilities package consumed by three other services, with a CI pipeline that runs lint, typecheck, and a unit suite on every push. A dependency bump broke type inference in one exported function; CI is red. Project 2 taught you the conditional shape with an inline checker; this project adds the two things that make a fix loop trustworthy at scale: the maker never touches your real working tree, and the checker is a separate process from the one making the edit — never the same model call grading its own diff.

What you'll build

• Heartbeat: conditional, one beat per attempt.
• Isolation: every attempt runs in a fresh git worktree add ../fix-attempt-N, never the main checkout — a bad attempt is deleted, not reverted.
• Body: edit only the broken export and its type declarations.
• Checker: a separate invocation — a second agent session (or a plain CI script) that only reads the diff and the CI output, never edits anything, and returns pass/fail.
• Spine: fix-state.md — attempt number, worktree path, checker verdict per attempt.
• Human gate: merging the worktree's branch back is a manual step — the loop stops short of that, every time.

# Claude Code
> /loop In a fresh worktree (../fix-attempt-N), fix the type error CI flagged in
  src/format.ts. Run lint+typecheck+tests locally. Then, in a SEPARATE session,
  have the checker agent read only the diff and the CI log and verdict pass/fail —
  it may not edit anything. Record the verdict in fix-state.md. On pass, stop and
  report the worktree/branch for human review. On fail, delete the worktree and
  retry (max 4 attempts), then escalate.

# OpenCode
git worktree add ../fix-attempt-$i fix/type-error-$i
opencode run --cwd ../fix-attempt-$i "Fix the CI-flagged type error in src/format.ts only."
opencode run "You are the checker. Read the diff in ../fix-attempt-$i and the CI log.
  Do not edit anything. Verdict PASS or FAIL with a one-line reason." >> fix-state.md

Done when (provable stop)

• Every attempt ran in its own worktree; the main checkout was never touched by a fix.
• The checker's verdict came from a genuinely separate invocation that made zero edits.
• fix-state.md shows a truthful attempt count and each attempt's verdict.
• On a PASS, the loop stopped and handed a branch name to a human — it did not merge.
• If 4 attempts failed, the loop escalated instead of trying a 5th.

Going deeper

This is starters/ticket-to-pr-ready/ and starters/loop-harness-verification/ combined — a maker producing a patch, a genuinely independent verifier checking it, worktree isolation throughout. Read both kits once this project is done; the "genuinely independent" requirement (not just a different prompt in the same session) is the exact lesson loop-harness-verification was built to teach.

When it goes wrong

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Sources: worktree isolation from Step 8 (Panaversity, S1); maker–checker independence from Step 11 and Sydney Runkle's The Art of Loop Engineering (S6). Full attribution: resources/sources.md.