Set Up Claude Code Hooks Safely

## TL;DR

Claude Code hooks are useful when a small, deterministic action should run
around a Claude Code lifecycle event. Set them up safely by narrowing the event,
scoping the matcher, reviewing the command like code, keeping secrets out of
configuration, and starting with non-mutating behavior. Hooks that can block
tools, write files, run local commands, or contact external systems need a
clear owner and an easy disable path.

## Prerequisites & Requirements

- [ ] {"task": "Trusted workspace", "description": "The repository and local automation are trusted enough to run project commands"}
- [ ] {"task": "Single purpose", "description": "The hook has one narrow job and one owner"}
- [ ] {"task": "Scoped event", "description": "The lifecycle event and matcher are as specific as the workflow allows"}
- [ ] {"task": "Reviewed command", "description": "The command or script is readable, versioned, and reviewed before enablement"}
- [ ] {"task": "Rollback path", "description": "A teammate can disable or remove the hook without reverse-engineering it"}

## Core Concepts Explained

### Hooks run at lifecycle points

Claude Code hooks attach automation to specific lifecycle moments. That makes
them more deterministic than asking Claude to remember a workflow, but it also
means a hook can run when the matching event occurs, not only when a user thinks
about it.

### Matchers are a safety boundary

A broad matcher can make a hook run more often than expected. Treat the event
and matcher as part of the security review. A hook that only reacts to one
well-understood event is easier to audit than one that reacts to many tools,
paths, or situations.

### Commands are automation code

The command behind a hook can read local data, transform output, write files, or
call other tools depending on what it does. Review it like any other project
automation. If the command is too long to understand quickly, move it into a
versioned script with comments and ownership.

### Settings and permissions shape behavior

Claude Code settings and permissions help define what is allowed in a workspace.
Hook setup should fit those boundaries instead of bypassing them. If the team
has different rules for personal and project-level configuration, document where
the hook belongs.

## Step-by-Step Safe Setup

1. **Write the purpose first.** State what the hook should do, when it should
   run, and what it must never do. This prevents a small convenience hook from
   growing into broad automation.

2. **Choose the narrowest lifecycle event.** Pick the event that matches the
   actual need. A notification, reminder, or summary hook is usually lower risk
   than a hook that blocks tool use or mutates project files.

3. **Scope the matcher.** Limit the hook to the relevant tool, path, or event
   pattern when the hook system supports that distinction. Review broad matchers
   before sharing them with a team.

4. **Start non-mutating.** First make the hook report what it would do, write to
   local review output, or notify the user. Add file writes, network calls, or
   blocking behavior only after the team sees stable results.

5. **Review data flow.** Identify what the hook receives, what it logs, and
   whether it can send data outside the machine. Keep secrets and customer data
   out of hook input, output, and persistent logs.

6. **Document ownership.** Record who maintains the hook, where the command or
   script lives, what dependencies it needs, and how to disable it quickly.

7. **Test in a small workspace.** Enable the hook in a low-risk project or user
   scope before committing shared configuration. Confirm it does not loop,
   block unrelated work, or create noisy output.

8. **Review before sharing.** Project-level hooks should go through normal code
   review. The reviewer should understand the event, matcher, command,
   dependency footprint, data flow, and rollback path.

## Safety Matrix

| Hook behavior | Risk | Review focus |
| --- | --- | --- |
| Notification or reminder | Lower | Noise, log content, personal data |
| Local read-only check | Medium | Files read, command output, runtime cost |
| Blocking pre-tool guardrail | Medium | False positives, bypass path, owner |
| File-writing automation | Higher | Scope, backups, formatting, rollback |
| Network-capable hook | Higher | Destination, credentials, retained data |
| Package or build command | Higher | Untrusted scripts, dependency side effects |

## Review Checklist

- [ ] {"task": "Small command", "description": "The command or script is short enough to review or is versioned with clear comments"}
- [ ] {"task": "No embedded secrets", "description": "Configuration does not contain tokens, passwords, private URLs, or customer data"}
- [ ] {"task": "Scoped trigger", "description": "The hook event and matcher are narrower than a global catch-all"}
- [ ] {"task": "Human-visible result", "description": "Output explains what happened and how to disable the hook"}
- [ ] {"task": "No hidden network call", "description": "Any external destination is documented and approved"}
- [ ] {"task": "Team rollback", "description": "A maintainer can disable the hook if it blocks work or leaks noisy output"}

## Troubleshooting

- **The hook fires too often**: narrow the matcher or split the hook into
  separate event-specific entries.
- **The hook blocks good work**: make it advisory first, then add blocking only
  for high-confidence cases.
- **The hook output exposes sensitive data**: redact file contents and command
  output before logging, or disable persistent logs.
- **The command is hard to review**: move it into a named script, document its
  inputs and outputs, and add ownership.
- **Different teammates see different behavior**: clarify whether the hook is a
  user setting or project setting, then document the expected scope.

## Duplicate Check

This guide focuses on safe hook setup and review practice. Existing entries
include individual hook examples, hook generator commands, and broader workflow
guides that mention hooks, but they do not provide a source-backed safety guide
for introducing Claude Code hooks with scoped events, command review, privacy
review, and rollback planning.

## References

- Claude Code hooks - https://code.claude.com/docs/en/hooks
- Claude Code settings and permissions - https://code.claude.com/docs/en/settings#permissions
- Claude Code security - https://code.claude.com/docs/en/security
- Claude Code IAM - https://code.claude.com/docs/en/iam