Hook Creator for Claude Code Plugins

Hook Creator for Claude Code Plugins

Create hooks that integrate with the Claude Code event system. Hooks automate validation, enforcement, and context injection across the session lifecycle.

For the complete hooks I/O API and JSON schemas, load: Skill(command: "plugin-creator:hooks-io-api") For working code examples and plugin hook configuration patterns, load: Skill(command: "plugin-creator:hooks-patterns") For all hook events, matchers, and environment variables, load: Skill(command: "plugin-creator:hooks-core-reference")

Language Constraint — Node.js Only

Hook scripts in this repository use Node.js .cjs exclusively.

flowchart TD
    Start([Need a hook script?]) --> Q1{Language choice}
    Q1 -->|JavaScript .cjs| Correct[Correct — use Node.js CommonJS]
    Q1 -->|Python| Wrong1[Prohibited — use .cjs instead]
    Q1 -->|Bash / shell| Wrong2[Prohibited — use .cjs instead]
    Q1 -->|TypeScript| Wrong3[Compile step required — write .cjs directly]
    Correct --> Pattern[Follow Node.js hook pattern below]

Evidence: orchestrator-discipline plugin hooks (pre-tool-orchestrator-read-warning.cjs, pre-tool-diagnostic-command-gate.cjs), .claude/hooks/session-start-backlog.cjs, .claude/hooks/session-start-rtica.cjs, .claude/hooks/stop-backlog-reminder.cjs.

SOURCE: Repository observation 2026-02-21 — all hooks use .cjs (Node.js CommonJS).

Node.js Hook Template

All hooks follow this structure:

#!/usr/bin/env node
/**
 * Brief description of what this hook does.
 * Fires on: EventName — ToolMatcher
 * Action: blocking | non-blocking — what it does
 */

const { execFileSync } = require('node:child_process');
const fs = require('node:fs');

let input = '';
process.stdin.setEncoding('utf8');
process.stdin.on('data', (chunk) => { input += chunk; });
process.stdin.on('end', () => {
  let data = {};
  try {
    data = JSON.parse(input);
  } catch {
    process.stdout.write(JSON.stringify({}));
    process.exit(0);
  }

  // Extract fields
  const toolName = data.tool_name || '';
  const toolInput = data.tool_input || {};

  // Guard: exit early if this hook does not apply
  if (toolName !== 'Write') {
    process.stdout.write(JSON.stringify({}));
    process.exit(0);
  }

  // Hook logic here
  const output = {
    hookSpecificOutput: {
      hookEventName: 'PreToolUse',
      additionalContext: 'Context injected into Claude.',
    },
  };

  process.stdout.write(JSON.stringify(output));
  process.exit(0);
});

Critical rules:

• Use process.stdin.on('data') / process.stdin.on('end') — not synchronous stdin reads
• Exit with process.exit(0) for success, process.exit(2) for blocking error
• Write JSON to stdout via process.stdout.write(JSON.stringify(output))
• Suppress stderr — use stdio: ['ignore', 'pipe', 'ignore'] when calling child processes via execFileSync
• Output empty {} on early exit paths, not nothing

hooks.json Structure

Plugin hooks live in hooks/hooks.json inside the plugin directory.

Empty state (hooks file exists but no active hooks):

{"hooks": {}}

The file must exist. Remove entries to disable hooks. Remove the file only when decommissioning the plugin.

Active hooks format:

{
  "description": "Brief explanation of what these hooks enforce",
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "Write|Edit",
        "hooks": [
          {
            "type": "command",
            "command": "node \"${CLAUDE_PLUGIN_ROOT}/hooks/my-hook.cjs\"",
            "timeout": 3
          }
        ]
      }
    ],
    "Stop": [
      {
        "hooks": [
          {
            "type": "prompt",
            "prompt": "Verify the task is complete. Return {\"ok\": true} or {\"ok\": false, \"reason\": \"...\"}.",
            "timeout": 30
          }
        ]
      }
    ]
  }
}

Path rule: Always use ${CLAUDE_PLUGIN_ROOT}/hooks/filename.cjs — never hardcoded paths.

SOURCE: plugins/orchestrator-discipline/hooks.json (lines 1-25), verified 2026-02-19.

Event Selection

flowchart TD
    Start([What should the hook do?]) --> Q1{When does it act?}
    Q1 -->|Before a tool runs — can block| PreToolUse[PreToolUse + matcher]
    Q1 -->|After a tool succeeds| PostToolUse[PostToolUse + matcher]
    Q1 -->|After a tool fails| PostToolUseFailure[PostToolUseFailure + matcher]
    Q1 -->|Before Claude stops responding| Stop[Stop — no matcher]
    Q1 -->|Before a subagent stops| SubagentStop[SubagentStop — no matcher]
    Q1 -->|When user submits a prompt| UserPromptSubmit[UserPromptSubmit — no matcher]
    Q1 -->|When session starts| SessionStart[SessionStart — matcher: startup|resume|clear|compact]
    Q1 -->|When session ends| SessionEnd[SessionEnd — no matcher]
    Q1 -->|Before context compaction| PreCompact[PreCompact — matcher: manual|auto]
    Q1 -->|On repo setup or maintenance| Setup[Setup — matcher: init|maintenance]
    Q1 -->|When Claude sends notifications| Notification[Notification + notification matcher]
    PreToolUse --> Matcher[Choose matcher: exact tool name, regex, or wildcard]
    PostToolUse --> Matcher
    PostToolUseFailure --> Matcher
    Matcher --> HookType[Choose hook type]
    Stop --> HookType
    SubagentStop --> HookType
    UserPromptSubmit --> HookType
    SessionStart --> HookType
    SessionEnd --> HookType
    PreCompact --> HookType
    Setup --> HookType
    Notification --> HookType
    HookType --> Q2{Decision logic complexity?}
    Q2 -->|Deterministic rule — fast| Command[type: command — Node.js .cjs]
    Q2 -->|Context-aware — needs reasoning| Prompt[type: prompt — LLM-evaluated]

Events without matchers: Stop, SubagentStop, UserPromptSubmit, SessionEnd — omit the matcher field entirely.

Hook Type Selection

flowchart TD
    Start([Choose hook type]) --> Q{What kind of logic?}
    Q -->|Regex/pattern match, path check, fast rule| Command["type: command\nNode.js .cjs script\nFast, deterministic, no API call"]
    Q -->|Understand intent, evaluate context, reasoning| Prompt["type: prompt\nLLM evaluated (Haiku)\nSlower, flexible, context-aware"]
    Command --> Timeout1[Timeout: 3s for local binary checks\nMax 10s for slow operations]
    Prompt --> Timeout2[Timeout: 30s default\nAdjust if reasoning takes longer]

Prompt hook response schema (LLM must return):

{"ok": true}

or:

{"ok": false, "reason": "Explanation shown to Claude"}

Timeout Sizing

Default: 60s for command hooks, 30s for prompt hooks (Claude Code built-in). Set explicit timeouts to reflect actual operation time — oversized timeouts mask runaway scripts.

SOURCE: hooks-core-reference — timeout defaults verified from official docs.

Matchers

Tool name matchers (case-sensitive):

"matcher": "Write"
"matcher": "Write|Edit"
"matcher": "Bash"
"matcher": "mcp__.*__write.*"
"matcher": "*"

SessionStart matchers: startup, resume, clear, compact

PreCompact matchers: manual, auto

Notification matchers: permission_prompt, idle_prompt, auth_success, elicitation_dialog

Events without matchers — omit the field: Stop, SubagentStop, UserPromptSubmit, SessionEnd, SubagentStart

Stdio Suppression

When calling external processes from a hook, suppress stderr to prevent noise in the transcript:

const { execFileSync } = require('node:child_process');

// Correct — stderr suppressed
try {
  const result = execFileSync('/usr/bin/node', ['--version'], {
    stdio: ['ignore', 'pipe', 'ignore'],
    timeout: 3000,
  });
} catch (err) {
  // Handle failure without leaking stderr
}

Use execFileSync (not execSync) for binary calls — prevents shell injection.

Exit Codes

Exit code 2 blocks the triggering action. Exit code 0 with {"ok": false, "reason": "..."} blocks via prompt hook protocol.

JSON Output by Event

All events — common fields:

{
  "continue": true,
  "suppressOutput": false,
  "systemMessage": "Optional message shown to user"
}

PreToolUse — allow or deny:

{
  "hookSpecificOutput": {
    "hookEventName": "PreToolUse",
    "permissionDecision": "allow",
    "permissionDecisionReason": "Auto-approved read-only file",
    "additionalContext": "Context for Claude"
  }
}

Stop / SubagentStop — block or allow:

{"decision": "block", "reason": "Task not complete — tests not run"}

UserPromptSubmit — add context or block:

{
  "hookSpecificOutput": {
    "hookEventName": "UserPromptSubmit",
    "additionalContext": "Additional context prepended to prompt"
  }
}

SessionStart — inject context:

{
  "hookSpecificOutput": {
    "hookEventName": "SessionStart",
    "additionalContext": "Project state loaded"
  }
}

Implementation Workflow

flowchart TD
    A([Start: add a hook to a plugin]) --> B[Identify event and behavior]
    B --> C{Hook type?}
    C -->|Command — deterministic| D[Create hooks/myhook.cjs]
    C -->|Prompt — LLM-evaluated| E[Write prompt string directly in hooks.json]
    D --> F[Test script directly: echo input-json | node hooks/myhook.cjs]
    F --> G{Output valid JSON?}
    G -->|No| D
    G -->|Yes| H[Add entry to hooks/hooks.json using correct event + matcher]
    E --> H
    H --> I[Verify hooks.json is valid JSON: python3 -m json.tool hooks/hooks.json]
    I --> J[Restart Claude Code session — hooks snapshot at startup]
    J --> K[Test in session with claude --debug]
    K --> L{Hook fires correctly?}
    L -->|No| M[Check matcher case-sensitivity, event name, JSON syntax]
    M --> H
    L -->|Yes| N[Done]

Testing directly before wiring:

# Test with minimal input
echo '{"tool_name":"Write","tool_input":{"file_path":"/tmp/test.txt"}}' | node ./hooks/my-hook.cjs

# Validate JSON output
echo '{"tool_name":"Write","tool_input":{"file_path":"/tmp/test.txt"}}' | node ./hooks/my-hook.cjs | python3 -m json.tool

Plugin hooks.json Registration

After writing a hook script, register it in hooks/hooks.json:

{
  "description": "What these hooks enforce",
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "Write",
        "hooks": [
          {
            "type": "command",
            "command": "node \"${CLAUDE_PLUGIN_ROOT}/hooks/validate-write.cjs\"",
            "timeout": 3
          }
        ]
      }
    ]
  }
}

All parallel hooks within one event run simultaneously — each is independent.

Security Rules

• Quote all variable expansions: toolInput.file_path || ''
• Check for path traversal before acting on file paths: reject paths containing ..
• Use execFileSync with explicit argv arrays — never execSync with shell string interpolation
• Skip sensitive file paths: .env, .git/, *.pem, *credentials*

Debugging

# Enable hook debug output
claude --debug
claude --debug "hooks"

# Check loaded hooks in session
/hooks

Common issues:

Sources

• plugins/orchestrator-discipline/hooks.json — verified plugin hooks.json format (lines 1-25, 2026-02-19)
• .claude/hooks/session-start-backlog.cjs — verified Node.js hook pattern (lines 1-69, 2026-02-19)
• plugins/orchestrator-discipline/hooks/pre-tool-orchestrator-read-warning.cjs — verified stdin handling pattern (lines 28-85, 2026-02-19)
• Skill(command: "plugin-creator:hooks-core-reference") — event reference, matchers, environment variables (accessed 2026-01-28)
• Skill(command: "plugin-creator:hooks-io-api") — JSON input/output schemas (accessed 2026-01-28)
• Skill(command: "plugin-creator:hooks-patterns") — prompt-based hooks, code examples (accessed 2026-01-28)
• Official hooks docs: https://code.claude.com/docs/en/hooks.md (accessed 2026-01-28)