Use hooks for repeatable actions tied to a defined event; keep prompt rules for decisions that require context and judgment. In Claude Code, a configured command, HTTP, or MCP hook can run at its matching lifecycle event, but that does not guarantee the hook is correct, that its result reaches the main conversation, or that the resulting code is correct.
What hooks change—and what they do not
A prompt rule asks an agent to remember and follow guidance while working. A hook attaches a configured handler to a lifecycle event, such as a tool call, file edit, session start, or completion. Anthropic describes Claude Code hooks as user-defined commands, HTTP endpoints, or LLM prompts that fire on specific events in the session lifecycle: Claude Code hook configuration.
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The practical advantage is procedural: a matching event can trigger a handler without relying solely on the model to remember an instruction. The guarantee ends there. A handler can be misconfigured or enforce the wrong policy, and a check that runs successfully cannot establish that a feature is correct or ready to deploy.
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| Approach | Best suited to | How it is triggered | Important limit |
|---|---|---|---|
| Prompt rules, project instructions, and skills | Choices requiring interpretation, trade-offs, or project context | They guide the agent as it works; they are not, by themselves, event-triggered handlers | They depend on the agent applying the guidance appropriately |
| Configured command, HTTP, or MCP hook in Claude Code | Repeatable procedures with a defined event and observable result | A matching Claude Code lifecycle event invokes the configured handler | Execution does not prove the handler’s logic or the code is correct |
| Prompt or agent hook in Claude Code | Checks or responses that call for model judgment | A matching event invokes a model-based hook | These use Claude’s judgment; they are not deterministic rule execution |
Anthropic distinguishes the hook types explicitly: command, HTTP, and MCP hooks execute deterministically, while prompt and agent hooks use Claude’s judgment. See its guidance on hook types and configuration and Claude Code steering and instructions. “Deterministic” describes execution of the configured handler, not whether its decision is wise or its output is error-free.
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Good candidates for a hook
Automate work when you can name both the event that should trigger it and an outcome that can be checked. Anthropic’s Claude Code documentation describes these kinds of uses:
- Before a tool runs: Inspect a proposed command or write path and block a known dangerous command or invalid location, if the selected event and handler support that behavior.
- After an edit or tool action: Run a formatter or write an audit entry. Formatting establishes only that the formatter ran; it does not validate feature behavior.
- At session start or resume: Load useful context such as the current branch or project status. This supplies context; it does not validate later changes.
- At completion: Run an objective check or report that a required check failed. A passing check is evidence only for that check, not a substitute for review or proof of deployability.
For example, “run the formatter after a file edit” has a trigger and an observable action. “Make sure the change is good” does not define a mechanical test; it still requires review and judgment.
Keep judgment-heavy rules in instructions and review
Rules about architectural trade-offs, whether a dependency is appropriate, or which related files deserve review are contextual. Keep these in project instructions, skills, or the task prompt, and have a person review consequential decisions. A hook can support that workflow with a mechanical check, but do not treat a proposed workflow—such as enforcing every file dependency or reviewing all related files—as a guaranteed capability unless the specific agent documents that behavior.
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Choose and operate hooks carefully
- Make the trigger narrow: Match the event and scope that actually need the procedure; an overbroad automatic action can disrupt unrelated work.
- Define an observable result: Decide whether the handler should block, modify, log, or report, and ensure failures are visible to the people or agent that need to act.
- Separate enforcement from explanation: Use instructions to explain intent and a hook to automate the check. Do not assume a blocking action communicates the reason unless its result is surfaced.
- Review the policy and implementation: A reliably invoked hook can still encode an unsafe command, an incorrect path rule, or a misleading check.
- Check the product documentation: The lifecycle events, matchers, return behavior, and blocking semantics described here are Claude Code-specific. Other coding agents may expose different mechanisms or none at all.
A practical division of responsibility
- Write the rule: State the project requirement in instructions or the task prompt, including the rationale when interpretation matters.
- Identify the mechanical part: If a portion can be checked or carried out repeatedly with a clear trigger and result, consider a hook for that portion.
- Specify visibility and failure behavior: Decide how the handler reports success or failure and whether the agent must receive that result.
- Validate the hook itself: Check that it runs at the intended event, affects only the intended scope, and handles failure as expected.
- Review the work: Treat hook results as evidence about specific procedures, not as a blanket verdict on code quality or correctness.
Anthropic’s documentation describes eight Claude Code hook events covering the session lifecycle; event availability and behavior can change, so consult the current official configuration guide before building around a particular event.
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