background-implementer

name	background-implementer
description	Run bounded parallel implementation with isolated workers (native worktrees or Codex), respect a dependency DAG, and verify each worker's diff before integrating. Use only when the user explicitly requests background, parallel, or multi-agent implementation, such as "백그라운드 구현", "bg impl", or "병렬 구현". Do not trigger for ordinary implementation or code-writing requests.
trigger-keywords	["bg impl","백그라운드 구현","병렬 구현","parallel implementation","multi-agent implementation"]
tags	["implementation","parallel","multi-agent","worktree"]

Background Implementer

Implement independent tasks with isolated parallel workers, then integrate only the diffs that pass verification — in dependency order.

Mechanics (worker invocation, provider recipes, worktree isolation, .context/ layout, token discipline) live in references/orchestration.md. This file owns the workflow, the schema, and the integration gate.

1. Decompose into a task DAG

From the plan (or the request), extract independent units and their dependencies:

- id: T1
  feature: db-migration
  files: [migrations/…]
  depends_on: []
  provider: codex          # who implements it (see §3)
  verify: how to check it (test cmd, build, manual assertion)
- id: T2
  feature: models
  depends_on: [T1]

Typical ordering: migrations → models → handlers/services → frontend. Independent leaves (migrations, types, frontend scaffolding) parallelize; dependents wait.

2. Wave execution

Run tasks in dependency waves; parallelize within a wave, barrier between waves.

Wave 1 (parallel): migration + types + frontend scaffold
Wave 2 (needs migration): models
Wave 3 (needs models): handlers

Only start a wave once its dependencies are integrated and verified (§4).

3. Assign and isolate workers

Choose the provider per task, then give each writer its own git worktree — never let two workers edit the same files (see orchestration reference for the isolation recipe).

Provider	Best for	Isolation
host-native sub-agent (Claude)	complex logic, multi-file changes, deep codebase reading	isolation: worktree frontmatter / EnterWorktree
Codex exec	focused code gen, migrations, models	manual git worktree add per worker

Prefer the host's native worker + native worktree isolation. Use Codex only as a cross-tool fallback or when the user explicitly asks for it.

Pass each worker its task by file path (not inline), scoped to one id, with its verify command. Tell every worker: do not commit — leave the worktree dirty for the orchestrator to inspect and apply.

Codex worker recipe (note the global -a/-C placement — the common breakage):

codex -C "$WT" -a never exec -s workspace-write \
  -o "$PROJ/.context/impl/${ROUND}-codex-${FEATURE}.md" \
  - < "$PROJ/.context/impl/${ROUND}-codex-${FEATURE}.prompt.md"

workspace-write has no network by default and protects .git; install deps up front and do not rely on the worker committing. Use --output-schema if you need the worker's summary in a fixed JSON shape.

4. Integration gate

Prefer the host's native wait/notification. When a worker finishes, do not apply its work blindly:

1. Inspect the worker's notes and its actual worktree diff (git -C "$WT" diff).
2. Run the task's verify command against the worktree. A worker's claim of success is not evidence — the passing check is.
3. Reject overlapping, out-of-scope, or unverified changes.
4. Apply accepted diffs to the main checkout in dependency order, then remove the worktree.
5. Run the relevant tests after each integration; summarize remaining failures honestly.

Only fire-and-forget (launch and return immediately) when the user explicitly asks for it.

5. Iterate (optional)

If verification fails or the user requests fixes, run R02 on the affected tasks only. Stop when all tasks integrate cleanly and their verifications pass, when a fix needs a decision not yet authorized, or at the user's round/token bound.

Token efficiency

• Write task instructions to a .md file; pass the path, not the body.
• Workers return compact structured summaries; the orchestrator reads summaries, not full transcripts (open a worker's output only when the summary is insufficient).
• Keep prompts, logs, and summaries in .context/impl/ so the main conversation stays lean.

Best practices

Do

• Respect dependency order (migration → models → handlers).
• One worktree per writer; verify before integrating.
• Wait for bounded workers via notification, not polling.

Don't

• Tight-poll worker processes or run 10+ workers at once.
• Let multiple workers edit the same file.
• Hand unverified diffs to the main checkout, or hand unfinished verification back to the user unasked.

Safety

• Workers must not commit, push, or open PRs; the orchestrator integrates after review.
• -s danger-full-access / --dangerously-bypass-approvals-and-sandbox only inside an externally isolated or disposable worktree runner.
• Non-interactive workers cannot pause for approval — out-of-sandbox actions fail rather than prompt. Scope sandboxes deliberately.
• Preserve unrelated user changes; never discard a dirty worktree without inspecting it.

Version Notes

• Worker/provider recipes verified against current Claude Code sub-agents (native worktree isolation), codex-cli, and gemini-cli on 2026-07-23. Re-check codex exec --help before changing recipes; Codex flags move fast. See references/orchestration.md.