Spawn Pi Subagents

You can spawn new pi instances as subagents. This is the primary mechanism for parallelizing work, isolating risky operations, and running long tasks without blocking the main session.

Each subagent is an independent pi process running in a detached tmux session. They share the same filesystem and tools as the main agent but have their own context window — they don't see your conversation history.

When to Use

• Run multiple investigations or analyses in parallel
• Isolate complex or risky operations (code generation, refactoring)
• Execute long-running tasks without blocking the main session
• Get a fresh context window for focused, specific tasks
• Delegate work when implementing tasks from tasks.json
• Run Mixture-of-Experts analyses with multiple specialized agents

When NOT to Use

• Simple one-off commands (just run them directly)
• Tasks that require interactive user input (subagents are headless)
• When you need to share conversation context (subagents start fresh)
• When the user explicitly requests alternative tools (screen, nohup)

Core Concept

Main Agent (you)
    │
    ├── spawns → Subagent A (tmux session: task-code-review)
    │               └── pi -p "Review auth module" → output file
    │
    ├── spawns → Subagent B (tmux session: task-refactor)
    │               └── pi -p "Refactor user service" → output file
    │
    └── spawns → Subagent C (tmux session: task-tests)
                    └── pi -p "Write tests for login" → output file

All three run simultaneously. You poll for completion and collect results.

Quick Start

Spawn a Single Subagent

bash
# 1. Create a detached tmux session
tmux new-session -d -s subagent-scout -n worker

# 2. Send the pi command (runs autonomously)
tmux send-keys -t subagent-scout \
  'pi -p "Find all files that import the auth module and summarize their purpose" \
   > /tmp/scout.out 2>&1 && echo "___DONE___" >> /tmp/scout.out' C-m

Wait for Completion

bash
# Poll until the DONE marker appears
while ! grep -q "___DONE___" /tmp/scout.out 2>/dev/null; do
  sleep 2
done

# Read the results
cat /tmp/scout.out

Clean Up

bash
# Kill the session and remove temp files
tmux kill-session -t subagent-scout
rm /tmp/scout.out

Complete Workflow

bash
# === SPAWN ===
tmux new-session -d -s subagent-example
tmux send-keys -t subagent-example \
  'pi -p "Analyze the dependency graph in this project and identify circular deps" \
   > /tmp/example.out 2>&1 && echo "___DONE___" >> /tmp/example.out' C-m

# === WAIT ===
echo "Waiting for subagent..."
while ! grep -q "___DONE___" /tmp/example.out 2>/dev/null; do
  sleep 2
done

# === COLLECT ===
echo "=== Subagent Results ==="
grep -v "___DONE___" /tmp/example.out

# === CLEANUP ===
tmux kill-session -t subagent-example 2>/dev/null
rm /tmp/example.out

Running Subagents with Prompt Files

For complex tasks with multi-line prompts, use a prompt file instead of inline text:

bash
# Write the prompt to a file
cat > /tmp/prompt-task.txt << 'ENDOFPROMPT'
You are a code reviewer. Review the following files for security issues:

Files to review:
- src/auth/login.ts
- src/auth/middleware.ts
- src/auth/utils.ts

Focus on:
1. Authentication bypasses
2. Token handling vulnerabilities
3. Input validation gaps
4. Secrets management

For each finding, provide:
- File and line number
- Severity (CRITICAL/HIGH/MEDIUM/LOW)
- Description and fix recommendation
ENDOFPROMPT

# Spawn the subagent using the prompt file
tmux new-session -d -s subagent-review
tmux send-keys -t subagent-review \
  'cat /tmp/prompt-task.txt | pi --append-system-prompt -p "$(cat)" \
   > /tmp/review.out 2>&1 && echo "___DONE___" >> /tmp/review.out' C-m

Parallel Subagents

Spawn multiple subagents simultaneously for parallel work:

bash
# Define tasks
declare -A tasks=(
  ["architect"]="Review the project for architectural issues and design patterns"
  ["security"]="Audit the codebase for security vulnerabilities"
  ["performance"]="Identify performance bottlenecks and optimization opportunities"
  ["maintainer"]="Evaluate code quality, readability, and documentation"
)

# Spawn all in parallel
for expert in "${!tasks[@]}"; do
  session="subagent-${expert}"
  task="${tasks[$expert]}"
  tmux new-session -d -s "$session"
  tmux send-keys -t "$session" \
    "pi -p '$task' > /tmp/${session}.out 2>&1 && echo '___DONE___' >> /tmp/${session}.out" C-m
  echo "Spawned: $session"
done

# Wait for ALL to complete
echo "Waiting for all subagents..."
for expert in "${!tasks[@]}"; do
  session="subagent-${expert}"
  while ! grep -q "___DONE___" "/tmp/${session}.out" 2>/dev/null; do
    sleep 2
  done
  echo "✅ $session complete"
done

# Aggregate results
echo "=== Aggregated Results ==="
for expert in "${!tasks[@]}"; do
  session="subagent-${expert}"
  echo ""
  echo "--- $expert ---"
  grep -v "___DONE___" "/tmp/${session}.out"
done

# Cleanup
for expert in "${!tasks[@]}"; do
  tmux kill-session -t "subagent-${expert}" 2>/dev/null
  rm -f "/tmp/subagent-${expert}.out"
done

Structured Output (JSON Mode)

For machine-parseable results, use pi's JSON mode:

bash
tmux new-session -d -s subagent-json
tmux send-keys -t subagent-json \
  'pi --mode json -p "List all exported functions in src/ with their signatures" \
   > /tmp/json-task.jsonl 2>&1 && echo "___DONE___" >> /tmp/json-task.jsonl' C-m

# Wait and collect
while ! grep -q "___DONE___" /tmp/json-task.jsonl 2>/dev/null; do sleep 2; done
cat /tmp/json-task.jsonl | grep -v "___DONE___"

Session Management

bash
# List all subagent sessions
tmux ls | grep "subagent-"

# Check if a specific subagent is still running
tmux has-session -t subagent-scout 2>/dev/null && echo "Running" || echo "Finished"

# Peek at live output (last 20 lines)
tmux capture-pane -t subagent-scout -p | tail -20

# Kill a stuck subagent
tmux kill-session -t subagent-scout

# Kill all subagent sessions
tmux ls | grep "subagent-" | cut -d: -f1 | xargs -I {} tmux kill-session -t {}

Subagent Patterns

Pattern 1: Scout (Read-Only Exploration)

Subagent explores code and reports back. No file modifications.

bash
tmux new-session -d -s scout-deps
tmux send-keys -t scout-deps \
  'pi -p "Map the full dependency graph of this project. List every module, what it imports, and who imports it." > /tmp/scout-deps.out 2>&1 && echo DONE >> /tmp/scout-deps.out' C-m

Pattern 2: Worker (Read-Write Implementation)

Subagent implements code changes. Always review changes after.

bash
tmux new-session -d -s worker-auth
tmux send-keys -t worker-auth \
  'pi -p "Implement JWT authentication middleware in src/auth/middleware.ts following the patterns in src/existing-middleware/. Include tests." > /tmp/worker-auth.out 2>&1 && echo DONE >> /tmp/worker-auth.out' C-m

Pattern 3: Reviewer (Post-Implementation Review)

Subagent reviews completed work and provides feedback.

bash
# After a worker completes, spawn a reviewer
tmux new-session -d -s reviewer-auth
tmux send-keys -t reviewer-auth \
  'pi -p "Review all changes in src/auth/ for correctness, security, and adherence to project patterns. Focus on the diff from main." > /tmp/reviewer-auth.out 2>&1 && echo DONE >> /tmp/reviewer-auth.out' C-m

Pattern 4: Pipeline (Sequential Subagents)

Chain subagents where the output of one feeds the next:

bash
# Step 1: Analyze
tmux new-session -d -s pipe-analyze
tmux send-keys -t pipe-analyze \
  'pi -p "Analyze src/ for all TODO comments and group by category" > /tmp/pipe-step1.out 2>&1 && echo DONE >> /tmp/pipe-step1.out' C-m

# Wait for step 1
while ! grep -q "DONE" /tmp/pipe-step1.out 2>/dev/null; do sleep 2; done

# Step 2: Prioritize (uses step 1's output)
TODO_LIST=$(grep -v "DONE" /tmp/pipe-step1.out)
tmux new-session -d -s pipe-prioritize
tmux send-keys -t pipe-prioritize \
  "echo '$TODO_LIST' | pi -p 'Prioritize these TODOs by impact and effort. Suggest the top 5 to tackle first.' > /tmp/pipe-step2.out 2>&1 && echo DONE >> /tmp/pipe-step2.out" C-m

Timeouts and Safety

Always set timeouts for subagents to prevent runaway sessions:

bash
# Wrapper function with timeout
spawn_with_timeout() {
  local session=$1
  local command=$2
  local timeout_sec=${3:-300}  # default 5 minutes

  tmux new-session -d -s "$session"
  tmux send-keys -t "$session" "$command" C-m

  # Background killer
  (
    sleep "$timeout_sec"
    if tmux has-session -t "$session" 2>/dev/null; then
      echo "⏰ Timeout: killing $session after ${timeout_sec}s" >> "/tmp/${session}.out"
      echo "___TIMEOUT___" >> "/tmp/${session}.out"
      tmux kill-session -t "$session"
    fi
  ) &
}

# Usage
spawn_with_timeout "subagent-risky" \
  'pi -p "Run a complex analysis that might be slow" > /tmp/subagent-risky.out 2>&1 && echo DONE >> /tmp/subagent-risky.out' \
  120  # 2 minute timeout

Best Practices

• Name sessions clearly: subagent-<purpose>-<timestamp> or task-<id>
• Always capture output to a file: Tmux pane buffers are limited and volatile
• Use DONE markers: ___DONE___ or DONE makes polling reliable
• Set timeouts: Kill sessions that hang beyond a reasonable time
• Clean up: Always remove sessions and temp files when done
• Check disk space: Output files can grow large with verbose tasks
• Review worker output: Always inspect code changes made by worker subagents
• Use prompt files for complex tasks: Avoid quoting issues with inline prompts
• Never spawn from within a subagent: Subagents should not spawn their own subagents (infinite fanout risk)
• Limit parallelism: Don't spawn more than 5-8 subagents at once (system resource limits)

Troubleshooting

Subagent never finishes

bash
# Check if it's still running
tmux has-session -t subagent-name && echo "Still running" || echo "Crashed"

# Check the last output
tmux capture-pane -t subagent-name -p | tail -30

# Kill and retry if stuck
tmux kill-session -t subagent-name

Output file is empty

bash
# Check if the command started correctly
tmux capture-pane -t subagent-name -p

# Verify pi is on PATH inside tmux
tmux send-keys -t subagent-name 'which pi' C-m
sleep 1
tmux capture-pane -t subagent-name -p

"Command not found: pi"

The tmux session may not inherit your shell's PATH. Use the full path:

bash
tmux send-keys -t subagent-name \
  '$(which pi) -p "task" > /tmp/output.out 2>&1 && echo DONE >> /tmp/output.out' C-m

Quoting issues with complex prompts

Use a prompt file instead of inline text (see "Running Subagents with Prompt Files" above).

Integration with Other Skills

This skill is a foundational primitive used by higher-level skills:

• mixture-of-experts — Spawns multiple expert subagents in parallel, aggregates results
• implement-tasks — Spawns worker subagents to independently implement tasks from tasks.json
• explore — Can spawn scout subagents for parallel codebase exploration
• pr-review — Can spawn reviewer subagents for automated PR analysis

When using mixture-of-experts or implement-tasks, follow their specific patterns for spawning and aggregating. This skill provides the underlying tmux/pi mechanics.