hook-creator

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(skill: "plugin-creator:hooks-io-api") For working code examples and plugin hook configuration patterns, load: Skill(skill: "plugin-creator:hooks-patterns") For all hook events, matchers, and environment variables, load: Skill(skill: "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 + tool name matcher]
    Q1 -->|After a tool succeeds| PostToolUse[PostToolUse + tool name matcher]
    Q1 -->|After a tool fails| PostToolUseFailure[PostToolUseFailure + tool name matcher]
    Q1 -->|Before Claude stops responding| Stop[Stop — no matcher]
    Q1 -->|When subagent spawned| SubagentStart[SubagentStart — matcher: agent type name]
    Q1 -->|When subagent finishes| SubagentStop[SubagentStop — matcher: agent type name]
    Q1 -->|Agent team teammate going idle| TeammateIdle[TeammateIdle — no matcher]
    Q1 -->|Task being marked complete| TaskCompleted[TaskCompleted — no matcher]
    Q1 -->|CLAUDE.md or rules file loaded| InstructionsLoaded[InstructionsLoaded — no matcher]
    Q1 -->|Config file changes| ConfigChange[ConfigChange — matcher: config source]
    Q1 -->|Worktree being created| WorktreeCreate[WorktreeCreate — no matcher]
    Q1 -->|Worktree being removed| WorktreeRemove[WorktreeRemove — no matcher]
    Q1 -->|MCP server requests user input| Elicitation[Elicitation — matcher: MCP server name]
    Q1 -->|User responds to MCP elicitation| ElicitationResult[ElicitationResult — matcher: MCP server name]
    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 — matcher: exit reason]
    Q1 -->|Before context compaction| PreCompact[PreCompact — matcher: manual|auto]
    Q1 -->|After context compaction| PostCompact[PostCompact — matcher: manual|auto]
    Q1 -->|On repo setup or maintenance| Setup[Setup — matcher: init|maintenance]
    Q1 -->|When Claude sends notifications| Notification[Notification — notification matcher]
    PreToolUse --> HookType[Choose hook type]
    PostToolUse --> HookType
    PostToolUseFailure --> HookType
    Stop --> HookType
    SubagentStart --> HookType
    SubagentStop --> HookType
    TeammateIdle --> HookType
    TaskCompleted --> HookType
    InstructionsLoaded --> HookType
    ConfigChange --> HookType
    WorktreeCreate --> HookType
    WorktreeRemove --> HookType
    Elicitation --> HookType
    ElicitationResult --> HookType
    UserPromptSubmit --> HookType
    SessionStart --> HookType
    SessionEnd --> HookType
    PreCompact --> HookType
    PostCompact --> 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]
    Q2 -->|Complex verification with tools| Agent[type: agent — agentic verifier]
    Q2 -->|Send to external service| HTTP[type: http — POST to URL]

Events with matchers (non-tool): SubagentStart/SubagentStop match agent type name; SessionEnd matches exit reason; ConfigChange matches config source; Elicitation/ElicitationResult match MCP server name.

Events without matchers: Stop, UserPromptSubmit, TeammateIdle, TaskCompleted, InstructionsLoaded, WorktreeCreate, WorktreeRemove — 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 (fast model)\nSlower, flexible, context-aware"]
    Q -->|Complex verification needing tool access| Agent["type: agent\nAgentic verifier with tools\nMost flexible, highest latency"]
    Q -->|Send to external service or webhook| HTTP["type: http\nPOST to URL\nFor remote validation services"]
    Command --> Timeout1[Timeout: 3s for local binary checks\nMax 10s for slow operations\nDefault: 600s built-in]
    Prompt --> Timeout2[Timeout: 30s default\nAdjust if reasoning takes longer]
    Agent --> Timeout3[Timeout: 60s default\nAdjust for complex tasks]
    HTTP --> Timeout4[Timeout: configurable\nNon-2xx errors are non-blocking]

Prompt hook response schema (LLM must return):

{"ok": true}

or:

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

---

Timeout Sizing

Operation type	Timeout
Local file check, regex match	3s
External binary (linter, formatter)	5–10s
Network call	15–30s
Prompt hook (LLM evaluation)	30s default

Default: 600s for command hooks, 30s for prompt hooks, 60s for agent hooks (Claude Code built-in). Set explicit timeouts to reflect actual operation time — oversized timeouts mask runaway scripts.

Note: The Agent tool was renamed from Task in Claude Code v2.1.63. Use Agent as the matcher name for PreToolUse/PostToolUse hooks targeting subagent-spawning tool calls.

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, UserPromptSubmit, TeammateIdle, TaskCompleted, InstructionsLoaded, WorktreeCreate, WorktreeRemove

---

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	Behavior
0	Success — stdout JSON processed
2	Blocking error — stderr fed to Claude as error message
Other	Non-blocking error — stderr shown in verbose mode only

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:

Problem	Cause	Fix
Hook not firing	Wrong matcher case	Matchers are case-sensitive — Write not write
Hook fires but no effect	Non-zero exit without exit code 2	Use exit 2 for blocking errors
JSON not parsed	Empty stdout	Always write {} on early-exit paths
Timeout exceeded	Slow operation	Reduce timeout or optimize script
Path not found	Missing ${CLAUDE_PLUGIN_ROOT}	Use variable for all plugin script paths

---

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(skill: "plugin-creator:hooks-core-reference") — event reference, matchers, environment variables (accessed 2026-01-28)
• Skill(skill: "plugin-creator:hooks-io-api") — JSON input/output schemas (accessed 2026-01-28)
• Skill(skill: "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)