Claude Hooks

Claude Hooks

Overview

Hooks let you automate Claude Code: run tests automatically after edits, block dangerous operations, enforce quality gates before Claude declares done. Without hooks, Claude relies purely on its training and your instructions. With hooks, you build enforced guardrails and automation into the workflow itself.

When to Use

• Want Claude Code to auto-run tests after editing files
• Need to prevent certain risky operations (e.g., rm -rf, force push)
• Want to auto-format code whenever Claude edits files
• Need to log or audit all Claude Code actions
• Want to enforce quality checks before Claude finishes a task

Core Jobs

1. Hook Types

2. Configuring Hooks (.claude/settings.json)

{
  "hooks": {
    "PreToolUse": [{
      "matcher": "Bash",
      "hooks": [{
        "type": "command",
        "command": "bash .claude/hooks/validate-bash.sh '$CLAUDE_TOOL_INPUT'"
      }]
    }],
    "PostToolUse": [{
      "matcher": "Edit|Write",
      "hooks": [{
        "type": "command",
        "command": "bash .claude/hooks/post-edit.sh",
        "async": true
      }]
    }],
    "Stop": [{
      "hooks": [{
        "type": "command",
        "command": "bash .claude/hooks/enforce-done.sh"
      }]
    }]
  }
}

3. Practical Hook Examples

Block dangerous bash commands (PreToolUse):

#!/bin/bash
# .claude/hooks/validate-bash.sh
INPUT="$1"

# Block destructive operations
if echo "$INPUT" | grep -qE "rm -rf|DROP TABLE|git push --force"; then
    echo "BLOCKED: Destructive operation requires explicit confirmation"
    exit 1  # Exit code 1 = block the tool, show error to Claude
fi

exit 0  # Exit code 0 = allow

Auto-run tests after file edits (PostToolUse):

#!/bin/bash
# .claude/hooks/post-edit.sh - runs async (doesn't block Claude)
CHANGED_FILE="$CLAUDE_TOOL_RESULT"

if [[ "$CHANGED_FILE" == *.py ]]; then
    cd "$(git rev-parse --show-toplevel)"
    python -m pytest tests/ -x -q --tb=short 2>&1 | tail -20
fi

Enforce test passing before finishing (Stop):

#!/bin/bash
# .claude/hooks/enforce-done.sh
if git diff --name-only | grep -q "\.py$"; then
    echo "Running tests before declaring done..."
    if ! python -m pytest tests/ -x -q; then
        echo "Tests failing — cannot mark task complete"
        exit 1  # Blocks Claude from stopping, forces it to fix tests
    fi
fi
exit 0

4. Hook Environment Variables

Available in hook scripts:

CLAUDE_TOOL_NAME        # Name of tool being called (Bash, Edit, Write, etc.)
CLAUDE_TOOL_INPUT       # Input to the tool (command being run, file path, etc.)
CLAUDE_TOOL_RESULT      # Result of tool execution (PostToolUse only)
CLAUDE_SESSION_ID       # Current session ID
CLAUDE_PLUGIN_ROOT      # Path to magic-powers plugin root

Key Concepts

• PreToolUse hook — intercepts tool calls before execution; exit 1 = block, exit 0 = allow
• PostToolUse hook — runs after tool completion; async: true means non-blocking
• Stop hook — runs when Claude finishes; exit 1 = force Claude to continue working
• Matcher — regex or tool name that determines when hook fires
• Hook chaining — multiple hooks can fire for the same event in sequence

Checklist

• .claude/settings.json exists in project?
• Dangerous commands blocked with PreToolUse hook?
• Tests run automatically after code edits (PostToolUse)?
• Stop hook enforces done-criteria (tests pass, lint clean)?
• Hooks committed to repo so team benefits too?
• Async hooks used for slow operations (don't block Claude)?

Key Outputs

• .claude/settings.json: hooks configuration
• .claude/hooks/ directory: shell scripts for each hook behavior
• Documented hook behaviors in CLAUDE.md

Output Format

• 🔴 Critical — no hooks configured (no safety net, no automation), hooks blocking Claude unnecessarily (overly strict PreToolUse that never allows anything)
• 🟡 Warning — hooks not committed to repo (team doesn't benefit), synchronous hooks for slow operations (blocks Claude mid-task)
• 🟢 Suggestion — add auto-format hook (PostToolUse on Edit), add Stop hook enforcing tests pass, block common accidents (force push, DROP TABLE)

Anti-Patterns

• Hooks that always exit 1 (defeats the purpose — Claude can't do any work)
• Slow operations in synchronous hooks (use async: true for anything taking >1s)
• Not committing hooks to repo (only you benefit, not the team)
• Complex hooks with no error handling (hooks that crash silently, allowing everything)

Integration

• Use with claude-md-authoring (document hook behaviors in CLAUDE.md so team understands them)
• Use with claude-project-settings (hooks configured in same settings.json)
• Agent: @debugger can help debug why hooks are firing unexpectedly