Pi Subagents

Pi Subagents

The parent works directly by default. Invoke subagents only when the operator requested delegation in the current request or through applicable user/project instructions. Task size, complexity, risk, tool-call count, recipe fit, or an available specialist does not independently authorize delegation.

Once authorized, choose the smallest bounded shape that earns its token and elapsed-time overhead through concrete evidence, independent review, specialization, useful parallelism, or needed isolation. A single child is valid; writer, challenge, and review stages must each earn their overhead rather than becoming default ceremony. The parent keeps user intent, constraints, routing, arbitration, decisions, final acceptance, and publication authority, and may perform the work directly where it is the most efficient owner.

Children do not spawn subagents unless the parent explicitly delegated fanout and their resolved tools allow subagent.

Launch shape

workflowScript is code-driven: runs.run(...) for keyed steps, runs.all([...]) for fanout, plain JavaScript for branching and aggregation. Keep scripts portable: use top-level await, plain helpers, or explicit Promise chains, not nested async helpers. Legacy top-level chain / tasks inputs and durable .chain.md execution are inspection or migration material only.

Use runs.lanes(...) only inside a workflowScript, not as a top-level mode, when a broad, predeclared plan benefits from visible per-lane stages; otherwise use ordinary runs.run(...) / runs.all(...). See the canonical staged-lane example. Keep assignments bounded, but do not add stages or ceremony just to satisfy this skill.

When composing runs.run(...), runs.all(...), or runs.lanes(...), always supply a short verb + behavior display label derived from the task, unless the user supplied an explicit label; preserve that label. Keep the stable machine key independent (for example, issue2011-writer with label: "Fix workflow steering"). For runs.lanes, put labels on stage items, not lane objects. Use stage-appropriate labels for reviews and retained-child follow-ups too (for example, Review workflow steering). Generate labels in the orchestrator while composing the launch—no extra model call, runtime generator, or schema change. Native direct { agent, task } calls have no top-level label parameter; do not invent one or wrap a tiny single task in a workflow just to label it.

Use async/background by default. Set async:false only when the parent must block. Final reviews, validation gates, oracle checks, and publication checks stay async.

In an ordinary interactive session, yield after launching or triaging useful async lanes and let Pi wake the parent on completion; ordinary async subagents already have native completion notifications, so do not call bg_wait() merely because a child is active. Use blocking bg_wait() only for provider, detached, or other background work without a native notification when a headless/run-to-completion contract or a required same-turn artifact makes the result necessary before this turn ends. For “continue/orchestrate/work until done,” keep the lane board moving while a safe immediate action remains; if only async lanes are running, record the revisit trigger and yield.

Package agents appear in subagent({ action: "list" }). External CLI/job agents use their own runner contract. Do not pass native Pi child options to them unless that runner explicitly supports the option.

Read the reference for the branch

For exact API fields and worked examples, call subagent({action:"guide",topic:"tool-reference"}) or topic:"workflows". The compact tool definition is not the recipe catalog; use topic:"missions" for mission updates and schedules.

For an authorized complex delegated workflow, read prompting-and-roles.md and execution-controls.md, then load review-and-validation.md and constraints-and-recipes.md before launch or review.

Operating rules

• Avoid duplicate scouts, overlapping writers, and vague prompts without a concrete deliverable.
• Keep the parent on the ordinary strong default model. Route workers/scouts to a fast capable tier, serious reviews to a strong tier, and top reasoning to bounded read-only critique.
• Exact model names are deployment policy. Put them in user/project settings or profiles, not package guidance.
• Give every child a compact meta-prompt checklist: objective; repo/cwd/ref; authority/edit boundary; relevant files/contracts and constraints; success/acceptance criteria; validation; expected output/report; and stop/ask conditions. See references/prompting-and-roles.md.
• For mutation work, use an isolated lane/worktree when isolation, overlap, or concurrent juggling matters; keep one writer per cwd/worktree. See references/multi-lane-orchestration.md for lane mechanics.
• Keep long/high-output validation out of chat: prefer interactive_shell dispatch/background monitors, bounded logs, or subagent-owned reports; return a concise summary plus report path unless same-turn output is required. Do not use interactive_shell as an implicit fallback for a failed subagent lane; see references/execution-controls.md.
• Treat subagent workflow, child launch, prompt runtime, extension load, and child tooling setup failures as lane infrastructure blockers. Stop, report the exact failure and run/worktree state, verify a clean worktree or capture a partial diff, and use only a clear same-protocol retry or an owner-approved execution-mode fallback.
• For cross-codebase work, record the repo, explicit cwd, authority boundary, and expected output before launch.
• Make parallel prompts distinct by source seam, evidence, and decision. Do not clone prompts with only item numbers swapped.
• Prefer fresh-context review/validation fanout, then synthesize and apply fixes in the parent.
• For Pi extension repos under ~/.pi/agent/extensions, put lane worktrees outside extension auto-discovery, such as ~/.pi/agent/worktrees.
• Preserve capability ceilings, including child tool limits and allowed-agent restrictions.
• Preserve parent authority and escalate unresolved choices.
• Treat receipts, CI, review bots, and external-run records as evidence, not authority.
• For backlog maintenance, releases, merge queues, or other public-repo mutation policy, load the matching user/project skill. This package defines delegation primitives, not private policy.
• As a conservative orchestration policy, do not pass a hard toolBudget or tight usageBudget to mutation-capable workers. The default tool budget blocks read/search tools rather than mutation tools. If interrupted after a tool call starts, checkpoint after the current tool returns with changed files, build/test state, and commit or PR state.