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Hooks are shell commands that receive tool input as JSON on stdin and must output JSON on stdout. You can create custom hooks to enforce project-specific rules and automate repetitive checks.

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Basic Hook Structure

my-hook.js
let data = '';
process.stdin.on('data', chunk => data += chunk);
process.stdin.on('end', () => {
  const input = JSON.parse(data);

  // Access tool info
  const toolName = input.tool_name;        // "Edit", "Bash", "Write", etc.
  const toolInput = input.tool_input;      // Tool-specific parameters
  const toolOutput = input.tool_output;    // Only available in PostToolUse

  // Warn (non-blocking): write to stderr
  console.error('[Hook] Warning message shown to Claude');

  // Block (PreToolUse only): exit with code 2
  // process.exit(2);

  // Always output the original data to stdout
  console.log(data);
});

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Exit Codes

​
0
success
Success — continue execution
​
2
block
Block the tool call (PreToolUse only)
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other
error
Error — logged but does not block

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Hook Input Schema

interface HookInput {
  tool_name: string;          // "Bash", "Edit", "Write", "Read", etc.
  tool_input: {
    command?: string;         // Bash: the command being run
    file_path?: string;       // Edit/Write/Read: target file
    old_string?: string;      // Edit: text being replaced
    new_string?: string;      // Edit: replacement text
    content?: string;         // Write: file content
  };
  tool_output?: {             // PostToolUse only
    output?: string;          // Command/tool output
  };
}

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Common Hook Recipes

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Warn About TODO Comments

{
  "matcher": "Edit",
  "hooks": [{
    "type": "command",
    "command": "node -e \"let d='';process.stdin.on('data',c=>d+=c);process.stdin.on('end',()=>{const i=JSON.parse(d);const ns=i.tool_input?.new_string||'';if(/TODO|FIXME|HACK/.test(ns)){console.error('[Hook] New TODO/FIXME added - consider creating an issue')}console.log(d)})\""
  }],
  "description": "Warn when adding TODO/FIXME comments"
}

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Block Large File Creation

{
  "matcher": "Write",
  "hooks": [{
    "type": "command",
    "command": "node block-large-files.js"
  }],
  "description": "Block creation of files larger than 800 lines"
}

​
Auto-Format Python Files with Ruff

{
  "matcher": "Edit",
  "hooks": [{
    "type": "command",
    "command": "node format-python.js"
  }],
  "description": "Auto-format Python files with ruff after edits"
}

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Require Test Files Alongside New Source Files

{
  "matcher": "Write",
  "hooks": [{
    "type": "command",
    "command": "node require-tests.js"
  }],
  "description": "Remind to create tests when adding new source files"
}

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Async Hooks

For hooks that should not block the main flow (e.g., background analysis): 
{
  "type": "command",
  "command": "node my-slow-hook.js",
  "async": true,
  "timeout": 30
}
Async hooks run in the background. They cannot block tool execution.

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Example: Background Build Analysis

build-analysis.js
const { exec } = require('child_process');

let data = '';
process.stdin.on('data', chunk => data += chunk);
process.stdin.on('end', () => {
  const input = JSON.parse(data);
  const command = input.tool_input?.command || '';
  
  // Check if it's a build command
  if (/(npm run build|pnpm build|yarn build)/.test(command)) {
    console.error('[Hook] Build completed - async analysis running in background');
    
    // Run analysis in background (async hook)
    exec('node analyze-bundle.js', (error, stdout, stderr) => {
      if (error) {
        console.error('[Hook] Analysis failed:', error.message);
      } else {
        console.error('[Hook] Bundle analysis complete');
      }
    });
  }
  
  console.log(data);
});

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Customizing Hooks

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Disabling a Hook

Remove or comment out the hook entry in hooks.json. If installed as a plugin, override in your ~/.claude/settings.json:
~/.claude/settings.json
{
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "Write",
        "hooks": [],
        "description": "Override: allow all .md file creation"
      }
    ]
  }
}

​
Environment Variables

Use environment variables for hook configuration: 
const threshold = parseInt(process.env.MAX_FILE_LINES || '800', 10);
const enableCheck = process.env.ENABLE_TEST_CHECK === 'true';

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Cross-Platform Notes

All hooks should use Node.js (node -e or node script.js) for maximum compatibility across Windows, macOS, and Linux. Avoid bash-specific syntax in hooks.

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File Path Handling

const path = require('path');

// WRONG: Unix-specific paths
if (filePath.includes('src/')) { ... }

// CORRECT: Cross-platform path handling
if (filePath.includes(path.join('src', ''))) { ... }
// OR use regex that matches both separators
if (/[\\/]src[\\/]/.test(filePath)) { ... }