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Claude Code hooks let you run commands at specific points in an agent session—for example, checking an action before it runs, formatting files after edits, or checking work at completion. They can make carefully chosen rules repeatable, but they do not make every workflow safe: protection depends on the event, matcher, script, and failure behavior, alongside Claude Code’s permission settings and human review.
How do Claude Code hooks work?
A hook is a command associated with a Claude Code lifecycle event. When that event occurs—and any configured matcher applies—Claude Code runs the command. Depending on the event and response, a hook can block a covered action, provide information, or run follow-up automation. Anthropic’s overview describes eight events, but that count reflects the article and should not be treated as a guaranteed current inventory.
Think of a hook as a way to enforce or automate one specific condition at one specific point. It is not a general-purpose security boundary: an action that does not trigger the chosen event and matcher is outside that hook’s coverage.
Which hook timing fits the rule you want to enforce?
| Timing | Suitable purpose | Important limit |
|---|---|---|
Before a tool action (PreToolUse) |
Check a proposed action and, where the event’s response contract permits, block it before execution. | Coverage depends on the matched tool call and the hook’s correct implementation. |
After a tool action (PostToolUse) |
Run follow-up work such as formatting after a file edit. | It runs after the action; it cannot prevent that action’s side effects merely by running afterward. |
| At session start | Load relevant context or perform session setup. | It is not a substitute for checking individual actions later in the session. |
At completion (Stop) |
Check whether a defined completion condition has been met. | Choose and test failure behavior; do not assume a check automatically guarantees correct or complete work. |
Anthropic’s guidance uses these event roles for examples including blocking dangerous commands before execution, formatting after edits, loading context when a session starts, and checking task completion. See the Anthropic hooks overview and the Claude Help Center’s Claude Code tips.
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How should you design hooks as safety invariants?
An invariant is a condition that must remain true, not merely a task to try. Write the rule in observable terms, place it at the earliest suitable event, and make the failure outcome explicit. The following are design patterns, not guarantees of complete protection.
Before an action: define what must be true
For a command or edit that should proceed only under a stated condition, use a narrowly matched pre-action hook where the event supports that decision. For example, a team might define which commands are permitted in a given context and have a check reject covered calls outside that policy. Verify the event-specific response contract before relying on a blocking result.
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After an edit: automate deterministic checks
A post-action hook can run a formatter or checker when doing so is safe and appropriate. Keep the operation predictable, and account for the fact that the edit has already occurred by the time this hook runs.
At completion: test the definition of done
Use a completion check for a condition that can be evaluated reliably, such as whether a required validation command succeeds. Decide what should happen when it fails, and confirm that the chosen event handles that outcome as expected.
Across projects: choose configuration scope deliberately
Anthropic’s overview describes JSON settings at project, user, and local-project levels. Put shared policy in project configuration only when the team intends to version and review it. User or local settings suit personal configuration. Check the current documentation for precedence and policy behavior before making assumptions about enterprise-wide enforcement.
How do matchers and configuration scope affect coverage?
Matchers narrow which tool calls trigger a hook. Anthropic’s overview gives case-sensitive examples including Write, Write|Edit, and Bash(npm test*). A broader matcher can apply to more actions than intended, while a narrow matcher can leave relevant actions uncovered. Match the rule to the actual tool and arguments you want to handle, then test both triggering and non-triggering cases.
The same overview describes project, user, and local-project JSON settings. A shared project setting can make a team rule reviewable alongside repository changes; a personal setting may be appropriate for an individual workflow. Do not assume settings take precedence in a particular way without checking the current documentation.
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What can a hook return, and how should failures be handled?
Anthropic’s overview describes hooks receiving JSON on standard input. It lists session fields such as the session ID, transcript path, working directory, permission mode, and event name; tool-related events can also include tool name and input. It describes exit code 0 as success, with standard output interpreted as JSON or added to context depending on the event; exit code 2 as a blocking error whose standard error becomes the error message; and other nonzero outcomes as non-blocking errors shown in verbose mode. Structured JSON can provide event-dependent control fields.
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These are overview-level descriptions, not a universal contract for every event or version. Check the current event reference for the exact input schema and response semantics before implementing a rule whose safety depends on them. Decide in advance whether a hook failure should block, warn, or permit the workflow to continue, and test that behavior rather than assuming all failures are fail-closed.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Can Claude Code hooks make Claude Code safer?
They can improve safety for covered actions by making selected checks repeatable and observable. They do not guarantee safety: coverage depends on the event and matcher, the script must be correct, and the event’s response contract determines what its result can do. A hook is one layer of policy enforcement, not a universal sandbox and not a replacement for permission controls or human judgment.
Anthropic’s security guidance emphasizes reviewing proposed commands and code. The Claude Help Center recommends pre-approving common safe commands through permission settings as an auditable alternative to skipping prompts wholesale. See Anthropic’s security guidance and the Claude Code power-user tips.
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- State the invariant. Write down the condition that must hold and which action the hook is meant to cover.
- Choose the earliest suitable event. Use a pre-action event for a rule that must prevent an action; use a post-action event for follow-up work, not prevention.
- Keep the matcher narrow. Confirm its exact spelling and scope, including case sensitivity and any argument pattern.
- Test in a disposable project. Verify both a call that should trigger the hook and a similar call that should not.
- Exercise success and failure paths. Confirm the hook’s output, exit behavior, and resulting Claude Code behavior for the specific event and version in use.
- Review shared configuration. If the rule is committed to a project, make its command and script part of the team’s normal review process.
For current implementation details, consult Anthropic’s hooks overview and verify the event-specific schema and behavior in the current official reference before deploying a blocking rule.
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