Dispatching subagents

Claude Code has three ways to start a subagent. They differ most in what the launching costs the parent.

	Agent tool	Workflow tool	`claude -p` children (`scripts/headless_fanout.py`)
parent cost	one parent turn per launch and one per completion	one tool call per run	one Bash call per run
concurrency	20, and a 21st launch errors (it does not queue)	min(16, vCPUs − 2): 2 on a 4-vCPU CCotw container	RAM-bound, about 210 MB per child: 40 ran clean on 4 vCPU / 15 GB
child context	session prefix (~32–55K tokens), plus parent transcript chunks if a context hook is installed	session prefix	what you pass: about 1.2K tokens with `--system-prompt` and no tools, 39K with the default prompt
tools	agent type's tools, the session's MCP connectors, hooks	as Agent	built-in tools named in `--tools`, write paths allowlisted; no MCP, hooks or skills
effort	session's level, or the `effort:` in a named agent type's frontmatter	`agent(prompt, {effort})`	`--effort` (measured: low gave 0 thinking tokens, high gave 112–197)
needs user opt-in	no	yes: an explicit request, ultracode, or a skill that calls it	no
cost visibility	none per child	none per child	`total_cost_usd` and tokens per child, in a ledger
output check	yours	in-script	file plus JSON Schema, or CLI-validated `--json-schema`

dispatching-subagents