Spawning Agents via CLI

Overview

Subagents don't have access to the Task tool. When you need to delegate work as a subagent, spawn Claude instances via CLI backgrounding:

cat [prompt-file] | claude -p --output-format json

Core principle: Spawn fresh Claude instances with clean context for independent work, get structured JSON responses.

When to Use

Use when:

• You're a subagent and need to delegate work (can't use Task tool)
• Context budget is tight and delegation would be more efficient
• You need parallel analysis of independent items
• Task benefits from fresh context without conversation history

Don't use when:

• You're the main Claude instance (use Task tool instead)
• You can efficiently do the work yourself with available tools
• The task requires shared conversation context that's hard to serialize
• You were just spawned to do a specific task - do the work, don't spawn another level

Recursion guard: If you were spawned via this technique, do the work you were spawned for. Don't spawn yet another agent unless the task genuinely requires parallel delegation.

Core Pattern

Before (baseline failure):

Subagent receives large analysis task
→ Tries to do everything itself
→ Burns context budget on work that could be delegated
→ May hit limits or inefficiency

After (with spawning):

Subagent receives large analysis task
→ Writes focused prompt to file
→ Backgrounds: cat prompt.txt | claude -p --output-format json
→ Polls BashOutput until complete
→ Parses JSON response and continues work

Quick Reference

Step	Command	Notes
1. Write prompt	Write tool	Clear, focused task description
2. Background spawn	cat prompt.txt | claude -p --output-format json	Use run_in_background: true
3. Poll output	BashOutput on bash_id	System reminder when ready
4. Parse response	Extract from stdout	JSON with result field

Implementation

Step 1: Write Focused Prompt

**Role:** You are analyzing TypeScript error patterns.

**Task:**
Search /path/to/code for all files with TypeScript errors and categorize by error type.

**Deliver:**
A JSON object with error categories and file counts.

Context passing:

• Use --add-dir /path/to/files if spawned agent needs file access
• Use --tools "Read,Grep,Glob" to limit available tools if needed
• Leave --system-prompt alone - let spawned agent use normal prompts

Step 2: Background the Spawn

cat task-prompt.txt | claude -p --output-format json

Use run_in_background: true in Bash tool. Capture the bash_id.

Step 3: Poll for Completion

System reminders notify you when output available:

<system-reminder>
Background Bash abc123 (...) Has new output available.
</system-reminder>

Poll with BashOutput tool:

BashOutput({ bash_id: "abc123" })
// Returns: { status: "completed", exit_code: 0, stdout: "{...}" }

Polling strategy:

• Wait 3-5 seconds between polls (use sleep command)
• System reminder appears when output ready
• Process ends when work complete, not at timeout

Step 4: Parse JSON Response

{
  "type": "result",
  "subtype": "success",
  "is_error": false,
  "result": "your actual answer here",
  "session_id": "...",
  "duration_ms": 7125,
  "total_cost_usd": 0.18
}

Extract the result field for the actual answer.

Error handling:

• Check is_error: true for failures
• Check exit_code from BashOutput (0 = success)
• Empty/malformed JSON = spawned agent failed

Common Rationalizations

Excuse	Reality
"I can just do this myself"	You'll burn context budget on work that should be delegated
"Spawning seems complicated"	4 steps: write prompt, background, poll, parse. Takes ~10 seconds.
"I should spawn an agent for this simple task"	If YOU were just spawned, do the work. Don't recurse infinitely.
"I'll load the files directly"	Loading large files wastes context. Spawn agent with file access instead.

Common Mistakes

❌ Forgetting You Don't Have Task Tool

You are a subagent
→ Tries Task tool
→ Tool not available

Fix: Use CLI spawning instead.

❌ Not Passing File Access

cat prompt.txt | claude -p --output-format json
→ Spawned agent: "I don't have access to /some/path"

Fix: Add --add-dir /some/path to command.

❌ Constant Polling Without Sleep

while not complete:
    check BashOutput every few milliseconds
    → Burns through tool calls

Fix: Use sleep 3 between polls. System reminders notify when ready.

❌ Not Handling JSON Parse Errors

Parse stdout directly
→ Malformed JSON crashes

Fix: Check exit_code: 0 and is_error: false before parsing.

Real-World Impact

Context efficiency: Spawning agents with fresh context can be 50-100x more efficient than loading large files into current context.

Parallel work: Spawn multiple agents for independent tasks, poll all, gather results.

Subagent capability: Subagents can now delegate just like main instances (via different mechanism).