Subagent Delegation

Subagent Delegation

A subagent is a fresh pi session with its own context, tool allowlist, and (optionally) a different model. The parent pays for the child's final answer only - every intermediate read, grep, and bash stays in the child's session. That makes subagents a context-compression tool, not a parallelism gimmick.

This skill is the policy for the parent side of that contract: when to spawn a child, how to brief it, and what NOT to delegate. For managing long-running children see subagent-background.

When to delegate

Reach for subagent when at least one of these is true:

• Noisy discovery. The next move would rg / read a bunch of files to answer a question you need one sentence from. Classic fit for the explore agent.
• Unknown-sized exploration. You don't know how many files you'll have to read to answer "where does X get initialized?" - cap the blast radius by handing it to a child.
• Fan-out. Two or more independent questions/tasks that don't depend on each other. Multiple subagent calls in one assistant turn run concurrently (see subagent-background).
• Cheaper model is enough. The subtask is mechanical (grep, count, list files, summarize a doc). Push it to a weak local model via modelOverride while the parent stays on a stronger model.
• Planning before coding. Non-trivial change on an unfamiliar codebase - let the plan agent produce a file-level plan you can execute inline.
• Dangerous or experimental changes. An agent with isolation: worktree gives you a sandbox the parent workspace never sees until you decide to apply it.

Do NOT delegate when:

• You already have the file open / know the exact lines. Spawning a child to do read path --offset N --limit 40 is pure overhead.
• The task needs the parent's in-memory context (prior tool results, user clarifications, unsaved reasoning). The child starts from zero - explaining all that in task is more expensive than doing it inline.
• The task is one tool call. A single rg or read is cheaper than spawning a session.
• You are a subagent. Nesting is disabled by design - the subagent tool is not exposed to children.

Pick the right agent type

Rules of thumb:

• Start with explore for any read-only question. It's the cheapest and can't damage the tree.
• Reach for plan before a multi-file change - a file-level plan in the parent is worth more than three rounds of "wait, which file?".
• Use general-purpose only when the child actually needs to write or run commands. Don't give it write access as insurance.

Run /agents in pi to see the currently loaded agents and their frontmatter.

Writing the task

The child session starts completely fresh. No chat history, no prior tool output, no memory of what you've already read. Everything it needs must be in the task string.

Structure every task with these four pieces:

1. Goal - one sentence saying what done looks like.
2. Constraints - paths to look in, paths to ignore, files that are off-limits, conventions to follow.
3. Known context - the handful of paths / symbols / line numbers you've already found. Saves the child from re-discovering them.
4. Expected answer shape - exactly what you want back. A list? A path + line number? A short plan? JSON?

Good task

Find every call site of `searchHandler` in src/ and list each as
`path:line - <one-line description of the surrounding block>`. Ignore dist/, node_modules/, and any \*.test.ts files.
Known: `searchHandler` is defined at src/api/search.ts:412. Return a bullet list, most-used file first.

Bad task

look at the search code and tell me about it

The bad version forces the child to guess what "the search code" means, pick an arbitrary depth of exploration, and invent an output shape. You'll get something, but it won't be what you needed.

Tips

• Lead with the deliverable. "Produce a list of …" beats "please investigate …". Weak models especially lock onto the first directive.
• Paste paths literally. src/api/search.ts:412 is cheaper to ground on than "the search handler somewhere in the api folder".
• Cap the scope. "Look only in src/api/**/*.ts" prevents the child from wandering into external/.
• State what NOT to do. "Do not edit files" for exploration tasks; "Do not run migrations" for db work.
• Forbid recursion explicitly if the child might try: "You cannot call subagent." (It can't - but saying so short-circuits attempts that burn turns.)

modelOverride - push grunt work to cheap models

modelOverride: "provider/model-id" runs the child on a different model than the parent. The pattern that matters for weak local models:

• Parent on a strong model (Claude, GPT, a big local model) handles reasoning, tool choice, and final synthesis.
• Child on a weak/cheap model (qwen3-6-35b-a3b, gpt-oss-20b, etc.) handles mechanical enumeration: grep, file listing, counting, "does this file import X?".

When to use it:

• The task is a closed-form lookup: "list every file matching …", "return the line number of …".
• The output is small and structured: a list, a count, a boolean, a single path.
• The child needs no judgment. If the answer requires weighing trade-offs, keep the parent's model.

When NOT to use it:

• Planning, design, or anything with "should we …?" in the prompt. Weak models will confidently pick wrong.
• Tasks that need tool-call accuracy (multi-step edits). The tool-arg-recovery / verify-before-claim extensions help, but strong models still win on complex tool use.
• When the child's answer will be fed straight back into a critical decision. Round-trip through the parent first.

Pair modelOverride with returnFormat: "json" and a strict schema in task when you need machine-readable output from a model that loves to narrate.

returnFormat: "json" - structured handoffs

Set returnFormat: "json" and the harness parses the child's final answer as JSON before returning. If parsing fails, you get the raw text plus an error flag - not a crash.

Use this when:

• You'll iterate over the result (items.forEach(...)-style).
• You're fanning out to multiple children and need to merge results.
• The child is on a weak model and you want the harness to catch prose-leakage.

Inside task, specify the schema literally:

Return a JSON object of the form:
{ "callers": [{ "path": "...", "line": 123, "context": "..." }] }
Do not include prose before or after the JSON.

Isolation

Every agent declares isolation: shared-cwd or isolation: worktree:

• shared-cwd (default for explore / plan / general-purpose) - child writes land in the parent's tree. Fast and simple; be careful with destructive tasks.
• worktree - child runs in a temporary git worktree. Use for risky refactors, experiments, or "try this and show me the diff" tasks. The parent sees the diff in the child's final answer.

You don't pass isolation at call time - it's baked into the agent definition. If you need worktree isolation and the agent you want is shared-cwd, either author a new agent or do the work inline with clear rollback.

Anti-patterns

• Don't delegate a single tool call. rg -n foo inline is one tool call; a subagent for the same question is a full session spin-up. Break-even is somewhere around "I'd need to read 3+ files to answer this."
• Don't re-explain the parent's entire context in task. If the task needs that much setup, it probably belongs inline.
• Don't use general-purpose as a default. Write access is a liability for read-only questions. Start with explore.
• Don't chain subagents serially when they're independent. Spawn them in the same turn - they run concurrently.
• Don't abandon background children. If you spawned with run_in_background: true, finish the handshake (see subagent-background). Orphans consume tokens until they exit.
• Don't trust weak-model output without verification. When you used modelOverride to downgrade, spot-check one or two claims with rg before acting on the full list.
• Don't nest. Subagents can't call subagent. If you're inside a child, return to the parent and let it fan out.

Quick reference