Claude Code hooks are event-triggered commands that run at defined points in a coding session. They can block a selected action, run a check after a tool call, or perform work when a session starts or ends. Used carefully, hooks make specific rules repeatable; they do not guarantee that every workflow is safe.
The practical question is: How do Claude Code hooks work, and can you use them to make Claude Code safer? Yes, for rules tied to events the hook can observe and control. The key is to choose the right event, keep its matcher narrow, and understand how failures are handled.
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How Claude Code hooks work
A hook connects a command or script to a Claude Code lifecycle event. Depending on the event, it can run before or after a tool action, or at a session-related point. Anthropic’s overview describes eight events, but that count is specific to that overview; check the current reference for the available event inventory and exact behavior.
For example, a pre-action hook can inspect a proposed tool call and block it when a defined condition is met. A post-action hook can run a formatter after a file edit. A session-start hook can load context, while a completion-oriented hook can check whether a task meets a stated condition. These jobs happen at different times, so a hook that runs after an action cannot prevent that action’s side effects.
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Anthropic’s Claude Code hook overview describes hooks as configurable through JSON settings and shows examples of event-specific matchers and responses.
Choose the event that can enforce your rule
Start by writing the desired invariant as a testable condition: what must be true, and at what point must Claude Code check it? Choose the earliest event capable of enforcing that condition.
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| Timing | Useful for | Important limit |
|---|---|---|
| Before a tool action | Checking or blocking a covered command or edit before it runs | Only applies when the event and matcher cover the proposed action |
| After a tool action | Follow-up work such as formatting or checking a changed file | Cannot undo side effects simply by running afterward |
| Session start | Loading relevant context or setup information | Does not enforce a rule on every later action |
| Task completion | Checking a defined condition of done | Specify what happens when the check fails |
Anthropic’s Help Center guidance describes examples such as blocking dangerous commands before execution, formatting after edits, and checking task completion. Treat them as patterns, not blanket protection.
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Scope hooks deliberately with settings and matchers
The overview describes project, user, and local-project JSON settings. Project configuration is appropriate when a team intends to share and review a rule in the repository. User settings suit personal preferences; local-project settings can keep personal project configuration out of shared policy. Confirm current precedence and organizational policy in the live documentation before relying on a particular setting being applied.
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Matchers narrow the tool calls that trigger a hook. The overview’s examples include exact, case-sensitive tool names such as Write, alternatives such as Write|Edit, wildcard matching, and argument patterns such as Bash(npm test*). A broader matcher may run on more actions than intended; start with the narrowest pattern that covers the invariant and verify both matching and non-matching cases.
Understand hook inputs, outputs, and failure behavior
Anthropic’s overview says hooks receive JSON on standard input. It describes common session fields—including a session ID, transcript path, working directory, permission mode, and event name—and says tool-related events can include the tool name and input. Exact fields and response contracts depend on the event, so check the current reference before building an implementation around a schema.
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The overview describes exit code 0 as success, with stdout handled according to the event; it describes exit code 2 as a blocking error whose stderr becomes the error message. Other nonzero outcomes are described as non-blocking errors shown in verbose mode. Structured JSON can also provide event-dependent control fields. Do not assume one event’s output behavior applies to another.
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Use hooks alongside permissions and review
Hooks are one layer of policy enforcement, not a universal sandbox. Their coverage depends on the selected event and matcher, the correctness of the script, and the event’s response contract. They also do not replace examining proposed commands and code.
Anthropic’s security guidance emphasizes user review of proposed commands and code, while its Help Center recommends pre-approving common safe commands through permission settings as an auditable alternative to skipping permissions wholesale. Use permission controls for the broader approval boundary, and hooks for deterministic checks tied to specific events.
For a blocking rule, keep the condition understandable and narrow, then test both what it should stop and what it should allow. For an after-edit formatter, ensure the command is appropriate for the affected files and understand that it runs after the edit. For a completion check, define the failure path so a failed check cannot be mistaken for a successful result.
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A practical way to design a hook rule
- State the invariant. Write the condition in a form a script can check, such as “this covered command must not run unless the required condition holds.”
- Choose the event. Use a pre-action event when the rule must stop an action, a post-action event for safe follow-up, or a session/completion event for setup or a definition-of-done check.
- Set the narrowest scope. Decide whether the rule belongs in shared project configuration or in personal settings, and use a matcher limited to the intended tools or arguments.
- Define failure behavior. Decide whether a failed check blocks, reports an error, or merely informs, using the event’s documented response contract.
- Test both sides. In a disposable project, verify that the intended action triggers the hook and an unrelated action does not.
- Keep human review. Use hooks for repeatable checks, while retaining permission controls and review for decisions that require context.
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