Hooks and Settings — wiring external behavior into tool execution

There are places where you want external behavior to run before or after an LLM agent's tool call. Auto-formatter execution, change watching, logging, policy validation, secret-leak blocking — that kind of work.

1. About Claude Code Hooks

Claude Code provides a hook system. At defined points in the model's tool-call lifecycle, it runs user-defined shell commands. The places fit policy blocking, automation, and external notifications.

Frequently mentioned events:

A hook is defined as a shell command, takes JSON input, and returns its outcome via exit code and stdout. Exit code 0 passes; any other value blocks or warns.

2. The settings.json hierarchy

When the same key exists at multiple layers, they merge by a defined precedence.

3. Permission model

{
  "permissions": {
    "allow": ["Bash(git status)", "Read"],
    "ask":   ["Bash(*)"],
    "deny":  ["Bash(rm -rf *)"]
  }
}

• allow — auto-allowed.
• ask — ask the user on call.
• deny — blocked.

Tool/argument pattern matching is supported. Too broad (e.g., Bash(*) auto-allowed) weakens security; too narrow piles up "ask" fatigue.

4. Hook definition

{
  "hooks": {
    "PreToolUse": [{
      "matcher": "Bash",
      "hooks": [{ "type": "command", "command": "scripts/check-bash.sh" }]
    }],
    "PostToolUse": [{
      "matcher": "Edit",
      "hooks": [{ "type": "command", "command": "scripts/format.sh" }]
    }]
  }
}

The script reads the event's JSON input from stdin, processes it, and returns an exit code. The exact input/output schema and matching syntax are safest to follow from the official docs verbatim.

Environment variables — settings or hooks can add or change environment variables. Avoid keeping credentials in plain-text files; combine with OS secret managers or external environment-variable loading.

5. Similar concepts in other tools

Git pre-commit · pre-push hooks — managed via .git/hooks/ or tools like husky · lefthook · pre-commit. The difference is being tied to git actions.

husky — a library frequently used for managing git hooks in the Node ecosystem.

pre-commit (Python · cross-language) — a polyglot git-hook tool. Combines with various lint/format tools.

lint-staged — runs lint and format only on staged files. Often combined with husky.

Cursor · Continue — other AI coding tools are gradually adding options to run external commands before/after work.

CI-side guard — checks at the PR stage in GitHub Actions, GitLab CI, and so on. The shape where CI catches what hooks miss.

6. Examples of guard scripts

Frequently mentioned hook places:

• Block secret leaks — block code changes that contain .env or credential patterns.
• Block destructive commands — irreversible commands like rm -rf or DB DROP.
• Auto-format — prettier, ruff, gofmt, clang-format after Edit.
• Auto-run tests — invoke unit tests when specific directories change.
• Notifications — desktop notifications and messenger messages on completion.

7. Portability · debugging

Portability:

• Windows · Unix — .cmd/.ps1 on Windows, .sh on Unix. Match shebang (#!/usr/bin/env bash) and permissions (chmod +x).
• PATH — commands invoked by the hook (prettier · ruff) must be on PATH. Care needed with venvs and nvm.
• Encoding — when Windows console encoding mismatches script output encoding, you get garbled output.

Debugging:

• Check where the tool's logs put hook script stdout/stderr.
• Use options like set -e at the start and clear error messages.
• Verify the meaning of exit codes before deciding pass/block.

8. Common pitfalls

Infinite hook loops — the hook calls a tool that re-triggers the hook. Narrow the matching condition.

Accidentally allowing every command — the mistake of putting Bash(*) in allow. The permission scope balloons.

Untrusted hook sources — taking someone else's settings as-is runs arbitrary commands in your environment. Apply only after code review.

Logging sensitive info — when hooks log command arguments and file contents, secrets leak. Mask and filter.

Ignoring OS differences — leaving Unix-only shell scripts as-is fails on Windows.

Layer-precedence confusion — when user, project, and enterprise settings merge, it's unclear which wins.

Missing block message — leaving a block result without a reason makes debugging hard. Provide friendly errors.

Version compatibility — hook event names and input schemas may change. Verify behavior on tool updates.

Closing thoughts

Hooks are powerful for auto-format, secret-leak blocking, and policy validation. That said, broadening permissions weakens security, and taking settings from untrusted sources can run arbitrary commands. For small operations, two layers — git-side hooks plus CI-side guards — are sufficient.

Next

• claude-md-pattern
• ai-coding-ides

We refer to Claude Code Settings · Claude Code Hooks · Git githooks · pre-commit · husky · lint-staged · GitHub Actions.