pi-sub-agent

A Pi package extension that adds a subagent tool for delegating work to specialized Pi subprocesses with isolated context windows.

Highlights

• Runs each delegated task in a separate pi --mode json -p --no-session subprocess.
• Supports single, parallel, and chain modes.
• Bundles default scout, planner, worker, reviewer, debugger, verifier, security-auditor, docs-writer, and refactorer agents.
• Discovers user agents from ~/.pi/agent/agents/*.md and optional project agents from .pi/agents/*.md.
• Provides /sub-agent-settings to view and edit each sub-agent's model and thinking effort.
• Streams progress, usage, tool-call summaries, final Markdown output, failure diagnostics, and structured result details.
• Sends delegated task prompts to child Pi processes over stdin instead of exposing prompt text in process arguments.
• Truncates each included LLM-facing subagent output to Pi's default tool limits (2,000 lines / 50KB), writes full truncated text to a private temp file, and preserves structured details for rendering.
• Prevents recursive subagent fan-out by removing the subagent tool from child allowlists and blocking nested subagent invocations.
• Requires confirmation before running project-local agents; non-interactive runs must explicitly set confirmProjectAgents: false.
• Registers the /sub-agent-settings slash command, but does not bundle prompt templates.

Requirements

• Pi coding agent 0.74 or newer.
• The pi executable available to the parent process (normal when the package runs inside Pi).
• Credentials for the active parent Pi model, plus any model explicitly selected by a custom agent.

Installation

After publication:

pi install npm:pi-sub-agent

From a local checkout:

npm install
npm run check
pi install ./ -l

For one-off testing without installing:

pi -e ./extensions/index.ts

Usage

Ask Pi to delegate to a bundled or user-defined agent:

Use the scout subagent to locate the authentication entry points.

The tool accepts exactly one mode:

{ "agent": "scout", "task": "Find all authentication code" }

{
  "tasks": [
    { "agent": "scout", "task": "Review database models" },
    { "agent": "planner", "task": "Review CLI entry points" }
  ]
}

{
  "chain": [
    { "agent": "scout", "task": "Find code relevant to OAuth" },
    { "agent": "planner", "task": "Plan OAuth support using this context:\n{previous}" },
    { "agent": "worker", "task": "Implement the plan:\n{previous}" }
  ]
}

Parallel mode is limited to 8 tasks total, with up to 4 running at once. Chain mode is limited to 8 sequential steps.

Delegation best practices

• Keep each delegated task self-contained: include the goal, relevant constraints, expected output, and any file paths already known.
• Prefer scout for discovery, planner for read-only plans, worker for implementation, reviewer for code review, debugger for failures, verifier for evidence-gathering checks, security-auditor for threat-focused review, docs-writer for documentation updates, and refactorer for behavior-preserving cleanup.
• Use parallel mode for independent research tasks; use chain mode only when each step needs the previous step's output.
• Keep chain handoffs concise. The full subagent details remain available in the tool result, but the next subagent only receives the previous final text through {previous}.
• Review project-local agents before enabling agentScope: "project" or "both", especially in repositories you did not create.

Tool parameters

| Field | Applies to | Description | | --- | --- | --- | | agent | Single | Agent name to run. Use with task. | | task | Single | Task text passed to the selected agent. Use with agent. | | tasks | Parallel | Array of { agent, task, cwd? } items. | | chain | Chain | Array of { agent, task, cwd? } steps, maximum 8. {previous} is replaced with the prior step's final output. | | agentScope | All | "user" (default), "project", or "both". Bundled agents are always available. | | confirmProjectAgents | All | Defaults to true; asks before running project-local agents when UI support exists. In non-interactive runs, project-local agents are blocked unless this is explicitly set to false. | | cwd | All | Default working directory override for every subprocess in the call; per-task or per-step cwd overrides it. |

cwd overrides on single calls, top-level parallel/chain calls, or individual parallel/chain tasks are resolved relative to the parent Pi working directory. A leading @ is accepted and stripped, matching Pi file-reference conventions.

Agent files

Agents are Markdown files with YAML frontmatter:

---
name: my-agent
description: What this agent does
tools: read, grep, find, ls
# Optional: model: provider/model-id
# Optional: thinking: off|minimal|low|medium|high|xhigh
---

System prompt for the agent goes here.

Discovery order is bundled extension agents first, then user agents, then project agents. Later sources override earlier agents with the same name.

tools may be a comma-separated string or a YAML list. Tool lists narrow the parent Pi session's active tool allowlist; omitted tools inherit the parent active tools. A subagent never enables a tool that is disabled in the parent session, and the subagent tool itself is always removed from child allowlists to avoid recursive delegation. Pi's read-only grep, find, and ls tools may be disabled in the parent session by default; enable them in the parent tool allowlist when you want bundled read-only agents such as scout, planner, reviewer, and security-auditor to use repository search. debugger and verifier additionally request bash for diagnostics and checks. Implementation-oriented bundled agents (worker, docs-writer, and refactorer) omit tools so they inherit the parent session's active tools. model and thinking are optional and inherit independently: an agent with a custom model but no thinking still uses the parent session's thinking level. Set thinking: off to explicitly disable inherited reasoning effort for that subagent. Legacy model: provider/model-id:high entries are parsed as model: provider/model-id plus thinking: high.

Unreadable agent files, missing required name/description metadata, invalid metadata types, and malformed YAML frontmatter are skipped so one bad agent file does not break discovery.

Bundled agents

| Agent | Purpose | Tools | | --- | --- | --- | | scout | Fast codebase reconnaissance and compressed context handoff. | read, grep, find, ls | | planner | Read-only implementation planning with acceptance criteria and verification steps. | read, grep, find, ls | | worker | General-purpose implementation using isolated context. | Parent active tools | | reviewer | Read-only code quality, correctness, and maintainability review. | read, grep, find, ls | | debugger | Systematic root-cause investigation for failures and regressions. | read, grep, find, ls, bash | | verifier | Runs checks and reports evidence before completion claims. | read, grep, find, ls, bash | | security-auditor | Read-only security review of trust boundaries and unsafe behavior. | read, grep, find, ls | | docs-writer | Documentation updates grounded in code and existing docs. | Parent active tools | | refactorer | Behavior-preserving cleanup and simplification. | Parent active tools |

| Scope | Loaded agents | | --- | --- | | user (default) | bundled + ~/.pi/agent/agents/*.md | | project | bundled + nearest .pi/agents/*.md | | both | bundled + user + nearest project agents |

Slash commands

/sub-agent-settings

Opens an interactive settings window listing the bundled and user-defined sub-agents visible in the default user scope. In print/JSON modes where no interactive UI is available, the command exits with a warning instead of opening the settings window. Each row shows the current model and thinking effort, for example:

reviewer  openai/gpt-5.5 • high

Set either field to inherit to use the parent Pi session's active model or thinking level. Edits to user agents update their markdown frontmatter. Edits to bundled agents create or update a same-named user override in ~/.pi/agent/agents/, so package files are not modified.

Security model

Project-local agents are repository-controlled prompts. They can request tools within the parent session's active tool allowlist and can instruct a subagent to read files, run shell commands, or edit code when those tools remain enabled. Keep agentScope at the default "user" unless you trust the repository. With the default confirmProjectAgents: true, the extension confirms before running project-local agents when UI is available and blocks them in non-interactive runs. Set confirmProjectAgents: false only when you have already reviewed and trust the project agents.

Each subagent is a normal child pi invocation in the selected cwd, so Pi packages and extensions enabled for that working directory still follow Pi's standard package security model. Install only trusted Pi packages and avoid cwd overrides into repositories whose Pi configuration you have not reviewed.

Recursive delegation is intentionally blocked. Child Pi processes receive PI_SUB_AGENT_DEPTH=1; if a child session somehow invokes subagent again, the extension returns a clear error before spawning another process.

Delegated task text is written to the child Pi process over stdin instead of being appended to command-line arguments, reducing process-list exposure and avoiding OS argument-length limits during large chain handoffs.

Output limits

The text returned to the main model is truncated from the tail per included subagent output at Pi's default tool-output limits: 2,000 lines or 50KB, whichever is hit first. Parallel mode returns one bounded section per subagent rather than a shared 50KB budget, so every parallel task remains visible to the parent agent. When truncation occurs, the affected section includes a Full output saved to: temp-file path with 0600 permissions so the main agent can inspect the complete text if needed. Full subagent messages also remain in structured details so interactive rendering can show complete expanded output without flooding the model context.

Error handling

• Invalid requests return clear guidance and keep structured result details available to the main agent; partial or mixed mode arguments are rejected before spawning child Pi processes.
• Invalid cwd overrides are rejected before spawning child Pi processes and include the resolved path in diagnostics.
• Non-zero subprocess exits, stopReason: "error", stopReason: "aborted", and stopReason: "length" are treated as failed subagent runs.
• Subprocess launch failures, such as a missing pi executable, include the attempted command and OS error in the LLM-facing failure output.
• Malformed child stdout in JSON mode is captured as a diagnostic; if no JSON messages are produced, the run is treated as a failed subagent invocation instead of silently returning (no output).
• Failed subagent runs and pre-spawn failures (invalid arguments, nested calls, project-agent confirmation blocks, or task-limit violations) are marked as Pi tool errors without dropping streamed output, subprocess diagnostics, or per-agent details.
• Project-local agents are blocked in non-interactive runs unless confirmProjectAgents: false is set.
• Nested subagent calls are blocked before spawning another Pi process.
• Chain mode is capped at 8 steps and stops at the first failed step with diagnostic output; parallel mode reports per-task success and failure counts with failure diagnostics.
• Aborts propagate to child processes with SIGTERM and escalate to SIGKILL after 5 seconds if the subprocess does not exit.
• Child Pi processes terminated by an external signal are treated as failed runs and include the signal name in diagnostics.

Troubleshooting

If a subagent fails with Failed to start subagent process (pi): ..., make sure the pi executable is available to the parent Pi process. This is normally automatic when running through the installed Pi CLI. For custom wrappers or local development, either launch Pi through its normal executable or ensure pi is on PATH before invoking the extension.

Development

npm test
npm run typecheck
npm run lint
npm run check

npm publish runs npm run check automatically through prepublishOnly.

Public release readiness

This package follows Pi package conventions for public distribution:

• package.json declares an explicit pi.extensions entry (./extensions/index.ts) so Pi loads only the public extension entry point, not helper modules.
• Runtime Pi imports (@earendil-works/pi-ai, @earendil-works/pi-coding-agent, @earendil-works/pi-tui, and typebox) are declared as peer dependencies, matching Pi package guidance.
• The published npm tarball is limited to extensions/, README.md, CHANGELOG.md, LICENSE, and package.json via the files list.
• The package intentionally does not publish prompts/, skills/, or themes/; it only registers the subagent tool and /sub-agent-settings command.
• Before publishing, run npm run check and npm pack --dry-run to verify tests, linting, type checking, and packaged files.

Release verification checklist

Run these from the repository root immediately before publishing:

npm test
npm run typecheck
npm run lint
npm run check
npm audit --omit=dev
npm pack --dry-run

Confirm that npm audit --omit=dev reports no production dependency vulnerabilities and that npm pack --dry-run includes only the public runtime files: extensions/, README.md, CHANGELOG.md, LICENSE, and package.json. Do not publish local .pi/, tests/, generated coverage, or development-only files.

After installing the packed package in a scratch project, run a short interactive smoke test before publishing publicly:

1. Start Pi with the installed package enabled and confirm the startup header lists pi-sub-agent.
2. Run /sub-agent-settings and verify bundled agents are listed; exit without saving if no changes are needed.
3. Ask for a simple scout delegation in a disposable repository and confirm the child process streams progress and returns structured output.
4. If testing project-local agents, review the .pi/agents/*.md files first and leave confirmProjectAgents enabled unless the repository is trusted.

Key files:

• extensions/index.ts — Pi extension and subagent tool implementation.
• extensions/agents.ts — agent discovery and frontmatter loading.
• extensions/agents/*.md — bundled default agents.
• tests/subagent.test.ts — regression tests for discovery, Pi tool conventions, and package metadata.