Taku Build - Auditable Implementation

Build implements the approved contract and leaves Review enough evidence to decide whether the work can ship.

Rule labels: [IRON LAW] means a non-negotiable correctness constraint. [GUIDANCE] means a strong default that may adapt when context justifies it.

[IRON LAW] No production code without a failing test, reproduction, or explicit verification anchor first. If the repo lacks a test harness, create the smallest reproducible check and say why.

Preflight

Before editing:

1. Read PLAN.md, an approved Quick mini design, or the explicit user-approved task contract.
2. If the plan has a Build Agent Contract, read Required fields first and use Optional fields only for context.
3. Load references/tdd.md for the local TDD cycle.
4. Use references/worktrees.md only when isolation is needed.
5. Choose execution mode yourself: sequential, parallel, or hybrid.

Do not ask the user to choose a mode unless the choice changes product scope, cost, or risk.

Output Contract

Build speaks in three stable shapes only.

BUILD PREFLIGHT

BUILD PREFLIGHT
- Mode: sequential | parallel | hybrid
- Reason: [one sentence]
- Subagents: available | unavailable; using sequential wave execution
- Waves: [wave-slug: task-slug list, or wave-1 for single task]
- Worktree: not needed | [path]
- TDD: enabled
- Ledger:
  - task-slug: files=[...] | tdd=[...] | status=pending | deviation=none | evidence=none
- Next: [first wave/task]

BUILD UPDATE

BUILD UPDATE
- Completed: [wave-slug or task-slug]
- Tasks: [task-slug list]
- Result: [tests/repro/spec check observed]
- Deviations: none | approved: [...] | needs-review: [...]
- Ledger delta: [only changed rows]
- Next: [next wave/task or REVIEW]

BUILD COMPLETE

BUILD COMPLETE
- Executed waves: [...]
- Task ledger: [all done | exceptions listed]
- Changed files: [...]
- Verification evidence: [commands and outcomes]
- Deviations: none | approved: [...] | needs-review: [...]
- Residual risk: none | [...]
- Status: READY_FOR_REVIEW | BLOCKED
- Next: REVIEW

The ledger is a handoff artifact, not a diary. Keep one row per task with these fields: task-slug, files, tdd, status, deviation, evidence.

Mode Selection

• Sequential: tasks are tightly coupled or there are 1-2 obvious steps.
• Parallel: tasks are independent and touch disjoint files.
• Hybrid: waves depend on earlier waves, but tasks inside a wave are independent.

Preserve plan task IDs when available. If the plan lacks IDs, create stable kebab-case slugs. If a file appears in multiple tasks, those tasks are dependent and must not run in parallel.

When subagents are unavailable, keep the same wave schedule and execute each wave locally in order. Report that in BUILD PREFLIGHT.

Execution Loop

For each task or wave:

1. Mark ledger row(s) in_progress.
2. Create the failing test, reproduction, or verification anchor.
3. Implement the smallest change that satisfies the spec.
4. Run the task verification.
5. Check spec compliance before code quality.
6. Update the ledger with status, evidence, and deviations.
7. Emit BUILD UPDATE.

Do not mark a task done until its spec and TDD/verification anchor have evidence.

Deviation Policy

Use the approved contract as the boundary.

• deviation=none: implementation matches the plan.
• deviation=approved: user explicitly approved the difference; preserve the approval in later summaries.
• deviation=needs-review: implementation differs from plan and has not been approved.

needs-review deviations block READY_FOR_REVIEW unless the deviation is the only safe path and is clearly documented for Review.

Reconciliation

After every parallel or hybrid wave:

• check for overlapping file changes
• run the smallest integration verification
• verify no task leaked outside its assigned files without recording a deviation

Reconciliation is required even when all individual tasks report done.

Completion Gate

Before routing to Review:

• every task is done or explicitly not-done with reason
• every done task has evidence
• all deviations are either none, approved, or needs-review
• changed files are listed
• verification evidence is listed

Then emit BUILD COMPLETE and route to /taku-review.

Known Pitfalls

Ledger missing at Review. Review cannot distinguish approved deviation from scope drift.

Prevention: create the ledger in BUILD PREFLIGHT and preserve task slugs in every update.

Parallel tasks touched the same file. One task overwrote another.

Prevention: duplicate file paths create dependencies; run them sequentially.

TDD anchor skipped after a test-harness failure. The agent called the code "probably correct."

Prevention: debug or bound the harness failure, then preserve a verifiable anchor before claiming completion.