Task Splitter — cheapest correct execution

Goal: finish the main task with the lowest total cost that still meets quality. Two levers: (1) route each piece of work to the cheapest capable tier, (2) pay each subagent's cold-start context once by batching same-tier work into one thread.

Read references/routing-rubric.md before scoring and references/batch-brief-template.md before writing any batch prompt.

Hard rules

• T0 work never spawns an agent. Deterministic steps (install, mkdir, mv, a config value, running a generator) run inline in the main thread. A cold-start subagent costs more than the command.
• Never use subagent_type: "fork" for cheap tiers. A fork always runs on the parent model and ignores model. Use general-purpose (or Explore for read-only search) with an explicit model and effort.
• This skill explicitly authorizes setting effort on every Agent call it makes, using the tier table. Always pass both model and effort.
• Verification by scripts, not models. Tests, lint, typecheck and build are run by the main thread. Do not spend a model call on reviewing code a script can check.
• Escalate only the failing subtask, one tier, once. After one escalation, stop and ask the user.
• Never batch two subtasks that edit the same file into different batches of the same wave.

Tier table (defaults — calibrate with the routing log)

Relative cost weights for the preview: haiku = 1, sonnet = 3, opus = 5; effort multiplier low = 1, medium = 1.5, high = 2.5. T0 = ~0.

Workflow

Phase 0 — Capture & persist

1. Restate the main task in ≤10 lines: goal, constraints, acceptance criteria, stack, out-of-scope.
2. Derive a slug. Look for an existing ledger in <project>/.claude/task-splitter/ (ls .claude/task-splitter/ 2>/dev/null). If one matches this task and has unfinished subtasks → resume from it (skip to the first phase whose work is not done).
3. Otherwise create <project>/.claude/task-splitter/<YYYY-MM-DD>-<slug>.md from templates/split-ledger.md and fill the Context section. Update it after every phase. In plan mode, if writing files outside the plan file is not allowed, put the ledger content into the plan file under a ## Ledger heading and write the real file as the first T0 step after approval.

Phase 1 — Light scout

Read only what the split needs: manifest (package.json, pyproject, etc.), entry points, the files the feature touches, existing utilities to reuse. If a broad search is needed, use ONE Explore agent with model: haiku, effort: low and ask for paths + one-line findings only. Record reusable utilities and exact file paths in the ledger — they become the context packets that stop cheap agents from re-exploring.

Phase 2 — Split

Break the work into atomic subtasks. Each subtask has: id (S1, S2…), goal (one sentence), files (exact paths, new or existing), depends_on (ids), done_check (a command like pnpm test foo or a precise criterion), type (short tag: install, scaffold, component, endpoint, test, migration, refactor, config, docs, debug, design…). Atomic = one agent can finish it without decisions that belong to another subtask. Pull design decisions out into their own early subtask (often T4) so later subtasks are mechanical and cheap.

Phase 3 — Score & route

Score each subtask with the 5-dimension rubric in references/routing-rubric.md (ambiguity, reasoning depth, blast radius, context needed, novelty; 0–2 each), apply the override rules, then map score → tier. Then check ~/.claude/task-splitter/routing-log.md (if it exists): if a type was escalated in ≥2 of its last 3 runs at a tier, start it one tier higher; if it passed first-time ≥3 times in a row one tier lower than you chose, consider dropping a tier.

Phase 4 — Batch

1. Build dependency waves: wave 1 = subtasks with no unmet deps; wave N = those whose deps are all in earlier waves.
2. Inside each wave, group by (model, effort), then by code area (same feature folder / module) so one agent loads related files once.
3. Limits per batch: ≤6 subtasks or one coherent area; no file edited by two batches in the same wave (move the conflicting subtask into the same batch or the next wave).
4. Merge tiny batches: a lone T1 subtask joins another T1 batch in the same wave if one exists; a lone T1 subtask that is nearly mechanical may be done inline instead.
5. T0 subtasks are grouped into a single inline step at the start of their wave.

Phase 5 — Cost preview (approval gate)

Write into the plan file (or show the user, outside plan mode):

| Wave | Batch | Model / effort | Subtasks | Est. cost |
|------|-------|----------------|----------|-----------|
| 1    | inline| main / shell   | S1, S2   | ~0        |
| 1    | B1    | haiku / low    | S3, S4   | 2         |
| 1    | B2    | sonnet / medium| S5       | 4.5       |
| 2    | B3    | opus / high    | S6       | 12.5      |
Total ≈ 19 units  (all-opus/high baseline ≈ 75 units → ~75% cheaper)

Est. cost = number of subtasks × model weight × effort multiplier. Include the all-opus/high baseline so the saving is visible. In plan mode, finish with ExitPlanMode; do not execute before approval.

Phase 6 — Execute

For each wave, in order:

1. Run the wave's T0 inline step.
2. Launch all batches of the wave in ONE message (parallel Agent calls): Agent(subagent_type: "general-purpose", model: <tier model>, effort: <tier effort>, description: "<wave>/<batch>: <short>", prompt: <brief>). Build every brief from references/batch-brief-template.md. The shared preamble must be byte-identical across all batches (prompt-cache friendly) — put variable content after it.
3. Wait for every batch of the wave to report before starting the next wave. Never write up or guess a pending agent's result.
4. Update the ledger with each batch's status and reported files.

Phase 7 — Verify & escalate

1. After each wave, the main thread runs the subtasks' done_check commands plus the project's cheap global checks (typecheck / lint / affected tests).
2. A failing subtask is re-run alone (or with other failures of the same new tier) at one tier higher, with the failing output and the previous agent's report attached.
3. Max one escalation per subtask. If it still fails, stop and ask the user with a short diagnosis. T4 failures go straight to the user.

Phase 8 — Record & learn

1. Mark the ledger complete: per-subtask final tier, pass/escalated, files changed.
2. Append one line per subtask to ~/.claude/task-splitter/routing-log.md (create the folder/file if missing; never inside the skill's git repo): YYYY-MM-DD | <project> | <type> | <start tier> | <final tier> | first-pass|escalated|failed
3. Report to the user in ≤10 lines: what was done, final cost units vs baseline, any escalations, anything left open.

Output conventions

• Keep the main thread lean: agents return short reports, not file contents.
• Give the user tables, not prose, for the split and the cost preview.
• If the whole task scores ≤3 total, or has ≤2 subtasks, say so and just do it directly — splitting it would cost more than it saves.