Subagent Engineering

A subagent is a separate context with its own system prompt. It inherits none of the caller's conversation, and everything it reads is discarded when it stops — only what it returns survives. Isolation is the reason to pay for one, and the return value is the entire product.

A subagent definition is a system prompt. Invoke prompt-engineering for the wording, instruction budget, and timelessness rules that govern every line written here. This skill covers only what is specific to the subagent artifact — the delegation decision, the frontmatter, and the prompt the agent wakes up with.

Delegate only where the isolation pays

• Delegate work whose output is far larger than its conclusion — a wide search, a long file, a verbose test run. The caller pays for the summary instead of the transcript.
• Delegate to restrict, too. A tool allowlist or a cheaper model binds the whole run, which no instruction inside the main conversation achieves.
• Keep the work in the main conversation when it needs back-and-forth, when several phases share the same context, or when the change is small and targeted. A subagent starts cold: it pays in startup latency and in rediscovering what the caller already knows.
• Write a skill instead when the artifact wanted is reusable instruction text that runs in the caller's own context. A skill carries procedure; a subagent carries a context boundary.
• A subagent runs its own system prompt; an output style modifies the main agent's. A style appends its body to the session prompt and, under keep-coding-instructions: false, drops the # Doing tasks section — it never gives the work a context of its own.
• Check the built-ins before writing a definition — Explore for read-only search on Haiku, Plan for read-only research, and general-purpose, which is what an Agent call gets when it names no type.
• A subagent cannot spawn a subagent. Every fan-out is decided by the caller, so an agent that discovers more work reports it rather than delegating it.

Write the description as routing code

• Claude sees only name and description when it decides to delegate. The body loads after that decision, so no line in it rescues a description that never fires.
• State what the agent does, then when to invoke it. A description that names only a domain gives the model nothing to fire on.
• Discriminate against the neighboring agents by naming the exclusion — "not for general code review, use code-reviewer". A concrete exclusion beats abstract precision.
• Keep execution steps out. The description is read to decide whether to delegate, never how to execute, and steps there widen the trigger while changing nothing about the run.
• Reach for "use proactively" only where unprompted delegation is wanted. It buys automatic routing and pays in precision; an agent that fires on the wrong requests costs more than one that waits to be asked.

Description tuning against observed misfires, the constraint block that stops scope creep, the efficiency and return-length caps that stop context bloat, A/B comparison between versions, the fan-out iteration loop and the parallel-session bias it carries, and the criteria separating a fix from a split from a rebuild: [${CLAUDE_SKILL_DIR}/references/iteration.md]. Read it when a working agent fires on the wrong requests, never fires, underperforms on part of its scope, or is being tuned across a fan-out.

Set the fields that govern the run

• name takes lowercase letters, numbers, and hyphens, caps at 64 characters, matches the filename stem, and must not contain "anthropic" or "claude". claude-code-guide ships as a built-in, so an author copying that naming style writes a name the loader rejects. description caps at 1024 characters; neither field accepts < or >.
• Grant the narrowest tools set the task needs. An omitted tools field inherits everything the parent holds. The allowlist is the enforcement; a prose constraint in the body is not.
• Name every skill the agent needs in skills. A subagent inherits none from the parent, and the field injects the full skill text rather than making it invocable.
• permissionMode restricts and never escalates. A parent running auto makes the field inert, and a parent's bypassPermissions is inherited and cannot be revoked from the definition.
• Match model to the work, not to the caller. inherit is the default, so a cheap high-volume agent pays the caller's price until the field says otherwise. A per-invocation model and CLAUDE_CODE_SUBAGENT_MODEL both outrank it.
• The Agent tool's name parameter makes the spawn a teammate while agent teams are enabled, and a teammate's idle notification carries no output. Spawn without a name when the caller needs the result back.
• hooks, mcpServers, and permissionMode are ignored without warning in a plugin-bundled agent. Copy the file into .claude/agents/ to use them.

The full field list, the Agent tool parameters, permission-mode semantics, storage and scope priority, the hooks schema, and the SDK settingSources requirement: [${CLAUDE_SKILL_DIR}/references/spec.md]. Read it when setting a field this section does not cover, or when deciding where the definition file lives.

Write the prompt the agent wakes up with

• The body is the whole system prompt. The agent receives it plus basic environment details and nothing else — no conversation history, no caller reasoning. Whatever the task depends on is restated here or passed at invocation.
• Say who fans out. An agent able to read its own scope will widen it: "the caller fans out, this agent does not — audit what the prompt assigned, never re-derive the scope."
• Specify the output format and name the verdict vocabulary. The caller acts on the return mechanically, so give a closed set of verdicts and require one per item. An agent reporting in free prose reports inconsistently.
• State what done looks like. Without a completion criterion the agent returns early on the first ambiguity, or works past the point the caller needed.
• Say that the summary is the product. Locate with Grep before reading whole files, and return findings rather than the material they came from.

Creation methods, tool sets by agent type, model selection and resolution order, the prompt skeleton, and four agent-type templates: [${CLAUDE_SKILL_DIR}/references/creation.md]. Read it before writing a new definition file.

Calibrate reviewing and auditing agents

An agent that finds problems will find problems. Left uncalibrated it treats an empty report as a failed run and manufactures findings to look diligent — which costs more than the review saves, because every false finding buys a human ruling. State the control explicitly in the prompt:

• Name the empty result as a success. "A clean audit that names what it checked is a useful result." Without this the agent infers the opposite.
• Do not scale findings to input size. A 25-line document audited to two findings is a correct report, not a lazy one. This bounds invention, never suppression: a diff with forty real violations reports forty. Volume is a failure only when the findings were manufactured to produce it.
• Split the burden of proof by finding type. On bright-line rules, flag every violation — a dismissed finding is cheaper than a missed one. On judgment calls the burden is on the finding: where the agent cannot argue it, the verdict is OK and nothing is reported.
• Require evidence for claims of absence. "No caller", "no test", "not used anywhere" carry the exact command run and what it returned. An absence with no sweep behind it is not reported.
• Separate "not worth reporting" from "could not check". OK closes a judgment call the agent weighed and rejected. Unverified is for a claim it could not settle — the test would not run, the platform was unavailable, the evidence was out of scope — and it names what would settle it. A finding is never silently dropped for lack of evidence, and never promoted to confirmed without it.

Run mutating agents isolated

An agent whose method mutates the tree — a test auditor that breaks code to prove a test catches it, a migration prover, anything running a negative control — needs isolation: worktree, not merely permission to edit. The risk is not two agents colliding but one agent crashing mid-mutation and stranding a broken tree that its caller believes is clean.

• Never end a run with a mutation in place. If a tool error or a timeout interrupts a control, restoring the tree is the first action before anything else.
• Restore and verify. Reverse the edit, then confirm with git diff that the file is back to its pre-mutation state. An unverified restore is an unrestored file.
• Mutate the subject, never the instrument. A test auditor changes the code under test, never the test.

Coordinate several agents through the return path

• A standalone subagent injects its full output into the caller's context; a teammate returns only what it sends. Three verbose standalone agents can exhaust the caller, which is what makes a team cheaper past a small fan-out.
• Use a team where the work shares findings or has ordering, and carry the ordering as blockedBy on a shared task list. Independent one-shot work stays cheaper as standalone spawns.
• Every task description stands alone. Teammates share no conversation history, so file paths, identifiers, and acceptance criteria live in the description itself.

Pipeline, parallel fan-out, orchestrator-workers, agent teams end to end, worktree isolation, background execution, the Agent SDK, and six complete agent definitions: [${CLAUDE_SKILL_DIR}/references/patterns.md]. Read it when more than one agent is involved, or when the run is backgrounded or driven from the SDK.

Measure before other people depend on it

Score trigger accuracy separately from output quality. They break for different reasons and take different fixes — a wrong description misroutes, a wrong body misreports — and a single overall impression hides which one failed. The requests that should route elsewhere are where a description fails, and they are the cases nobody runs by accident.

The five weighted scoring dimensions with their score guides, the quality thresholds, the five-level testing protocol, and regression benchmarking: [${CLAUDE_SKILL_DIR}/references/evaluation.md]. Read it when scoring an agent, or before other people depend on one.

Symptom-to-cause-to-fix for discovery failures, tool permissions, stalled teammates, background agents, worktree cleanup, hooks, and the SDK: [${CLAUDE_SKILL_DIR}/references/troubleshooting.md]. Read it when an agent fails outright rather than underperforms.