pi-mono/packages/coding-agent/examples/extensions/subagent

Subagent Example

Delegate tasks to specialized subagents with isolated context windows.

Features

• Isolated context: Each subagent runs in a separate pi process
• Streaming output: See tool calls and progress as they happen
• Parallel streaming: All parallel tasks stream updates simultaneously
• Markdown rendering: Final output rendered with proper formatting (expanded view)
• Usage display: Shows turns, tokens, cost, and context usage inside each subagent result
• Abort support: Ctrl+C propagates to kill subagent processes

Structure

subagent/
├── README.md            # This file
├── index.ts             # The extension (entry point)
├── agents.ts            # Agent discovery logic
├── agents/              # Sample agent definitions
│   ├── scout.md         # Fast recon, returns compressed context
│   ├── planner.md       # Creates implementation plans
│   ├── reviewer.md      # Code review
│   └── worker.md        # General-purpose (full capabilities)
└── prompts/             # Workflow presets (prompt templates)
    ├── implement.md     # scout -> planner -> worker
    ├── scout-and-plan.md    # scout -> planner (no implementation)
    └── implement-and-review.md  # worker -> reviewer -> worker

Installation

From the repository root, symlink the files:

# Symlink the extension (must be in a subdirectory with index.ts)
mkdir -p ~/.pi/agent/extensions/subagent
ln -sf "$(pwd)/packages/coding-agent/examples/extensions/subagent/index.ts" ~/.pi/agent/extensions/subagent/index.ts
ln -sf "$(pwd)/packages/coding-agent/examples/extensions/subagent/agents.ts" ~/.pi/agent/extensions/subagent/agents.ts

# Symlink agents
mkdir -p ~/.pi/agent/agents
for f in packages/coding-agent/examples/extensions/subagent/agents/*.md; do
  ln -sf "$(pwd)/$f" ~/.pi/agent/agents/$(basename "$f")
done

# Symlink workflow prompts
mkdir -p ~/.pi/agent/prompts
for f in packages/coding-agent/examples/extensions/subagent/prompts/*.md; do
  ln -sf "$(pwd)/$f" ~/.pi/agent/prompts/$(basename "$f")
done

Security Model

This tool executes a separate pi subprocess with a delegated system prompt and tool/model configuration.

Project-local agents (.pi/agents/*.md) are repo-controlled prompts that can instruct the model to read files, run bash commands, etc.

Default behavior: Only loads user-level agents from ~/.pi/agent/agents.

To enable project-local agents, pass agentScope: "both" (or "project"). Only do this for repositories you trust.

When running interactively, the tool prompts for confirmation before running project-local agents unless the session trust context reports trusted and the agents come from that session project's .pi/agents directory. Agents loaded from another task cwd still require confirmation. Set confirmProjectAgents: false to disable confirmation.

Usage

Single agent

Use scout to find all authentication code

Parallel execution

Run 2 scouts in parallel: one to find models, one to find providers

Chained workflow

Use a chain: first have scout find the read tool, then have planner suggest improvements

Workflow prompts

/implement add Redis caching to the session store
/scout-and-plan refactor auth to support OAuth
/implement-and-review add input validation to API endpoints

Tool Modes

Mode	Parameter	Description
Single	{ agent, task }	One agent, one task
Parallel	{ tasks: [...] }	Multiple agents run concurrently (max 8, 4 concurrent)
Chain	{ chain: [...] }	Sequential with {previous} placeholder

Output Display

Collapsed view (default):

• Status icon (✓/✗/⏳) and agent name
• Last 5-10 items (tool calls and text)
• Usage stats: 3 turns ↑input ↓output RcacheRead WcacheWrite $cost ctx:contextTokens model

Expanded view (Ctrl+O):

• Full task text
• All tool calls with formatted arguments
• Final output rendered as Markdown
• Per-task usage (for chain/parallel)

Parallel mode streaming:

• Shows all tasks with live status (⏳ running, ✓ done, ✗ failed)
• Updates as each task makes progress
• Distinguishes pending, running, completed, failed, and aborted tasks
• Returns each completed task summary to the parent model with its own 50 KB limit
• Returns failure diagnostics from stderr/error messages when a child exits before producing output

Tool call formatting (mimics built-in tools):

• $ command for bash
• read ~/path:1-10 for read
• grep /pattern/ in ~/path for grep
• etc.

Agent Definitions

Agents are markdown files with YAML frontmatter:

---
name: my-agent
description: What this agent does
tools: read, grep, find, ls
model: claude-haiku-4-5
---

System prompt for the agent goes here.

When model is omitted, the subagent inherits the dispatching session's active model and thinking level.

Locations:

• ~/.pi/agent/agents/*.md - User-level (with agentScope: "user" or "both")
• .pi/agents/*.md - Project-level (only with agentScope: "project" or "both")

Project agents override user agents with the same name when agentScope: "both".

Sample Agents

Agent	Purpose	Model	Tools
scout	Fast codebase recon	Haiku	read, grep, find, ls, bash
planner	Implementation plans	Sonnet	read, grep, find, ls
reviewer	Code review	Sonnet	read, grep, find, ls, bash
worker	General-purpose	Sonnet	(all default)

Workflow Prompts

Prompt	Flow
/implement <query>	scout → planner → worker
/scout-and-plan <query>	scout → planner
/implement-and-review <query>	worker → reviewer → worker

Error Handling

• Exit code != 0 or signal termination: Tool returns an error with preserved stderr/output
• Spawn errors: Tool returns the spawn diagnostic
• stopReason "error": LLM error is propagated with its error message
• Abort: Ctrl+C sends SIGTERM, escalates to SIGKILL after five seconds if needed, and preserves partial details/usage
• Chain mode: Stops at the first failing step and reports which step failed

Limitations

• Output truncated to last 10 items in collapsed view (expand to see all)
• Single and chain output is capped at 50 KB/2,000 lines; full results remain in tool details
• Parallel output keeps a separate 50 KB budget per task, allowing up to roughly 400 KB across eight tasks
• Child usage is displayed in tool details but is not added to the calling session's usage totals
• Agents are snapshotted from each task's effective cwd when the tool invocation starts; relative task cwd values resolve from the session cwd
• Parallel mode limited to 8 tasks, 4 concurrent