Subagent driven development

Subagent-Driven Development

The controller never writes the code itself. It dispatches work to fresh subagents, one task at a time, and gates every result. This keeps the controller's context small (it holds the plan + the ledger, not the diffs) and makes each task independently verifiable.

Loop

1. Plan → ledger, verification-first. Break the work into tasks. For each, decide how it will be verified (a command + the expected observation) before dispatching anyone — a task without a check is not yet planned. Record it on the ledger at creation time: ledger_add(title, verify: {command, expect}). The verify field is structural, not prose in a brief — it survives compaction, server restarts, and gets handed to the implementer and reviewer identically. The ledger is file-backed (under .ideal-harness/), so it survives both context compaction and an MCP-server restart — it is the controller's memory. Use the scout agent first when the plan needs locations ("where is X") — it returns a file:line table, not file dumps.
2. Independent second opinion, for non-trivial plans. Before dispatching any implementer, spawn plan-critic with the plan/ledger tasks — it runs at a different model tier (pinned in its own frontmatter) than the authoring conversation, so its critique is a genuinely different reasoning trace, not the same model re-reading its own output. Treat it like the reviewer gate: a blocker sends the plan back for revision; PASS or only minor issues let the loop continue. This is the harness's dual-model consensus gauntlet — skip it under the same bar as "When NOT to use" below (trivial plans, <3 tasks); it's a consensus gate, not mandatory friction on every change.
3. Per task, dispatch a fresh implementer agent. Hand it a self-contained brief: task spec, file paths, and the ledger task's verify command + expected observation (pull it from ledger_status, don't re-derive it). It writes its diff to a file (artifact), not into your context, runs the verification itself, and reports faithfully. Record the artifact on the ledger task.
4. Dispatch the reviewer agent. Hand it the same verify field from the ledger. It checks the artifact against the task spec on two axes: spec-compliance and quality — and verifies by default via the ledger_verify MCP tool (a real, policy-gated subprocess run that sets the task's status from the actual result), falling back to re-running verify.command itself only when ledger_verify can't auto-run it. Either way it never trusts the implementer's claim outright. It returns PASS or severity-tagged issues.
5. Fix loop. On issues, dispatch a fix subagent with the issues + artifact. Re-review. Cap iterations; if it won't converge, mark the task failed and escalate.
6. Mark done, move on. Update the ledger (ledger_update status=done artifact=...). Pick the next pending task.
7. Final broad review once all tasks are done.

Discipline

• File-based handoff. Artifacts pass as files/paths, never pasted into the controller's context.
• Durable ledger. Every state change is recorded so a resumed session continues exactly where it stopped (resumeFrom a checkpoint).
• Loop guard. If the same action repeats (loop_check reports stalled), stop and change strategy — do not burn budget.
• Spend cap. Gate expensive steps through spend_check; abort gracefully at the cap.
• One task at a time. Resist one-shotting the whole plan in a single subagent.

When NOT to use

Trivial single-file edits. The controller/subagent overhead isn't worth it below ~3 tasks.