hook-developer skill

Hook Developer

Complete reference for developing Claude Code hooks. Use this to write hooks with correct input/output schemas.

When to Use

• Creating a new hook
• Debugging hook input/output format
• Understanding what fields are available
• Setting up hook registration in settings.json
• Learning what hooks can block vs inject context

Quick Reference

Hook type options: type: "command" (bash) or type: "prompt" (LLM evaluation)

Hook Input/Output Schemas

PreToolUse

Purpose: Block or modify tool execution before it happens.

Input:

{
  "session_id": "string",
  "transcript_path": "string",
  "cwd": "string",
  "permission_mode": "default|plan|acceptEdits|bypassPermissions",
  "hook_event_name": "PreToolUse",
  "tool_name": "string",
  "tool_input": {
    "file_path": "string",
    "command": "string"
  },
  "tool_use_id": "string"
}

Output (JSON):

{
  "hookSpecificOutput": {
    "hookEventName": "PreToolUse",
    "permissionDecision": "allow|deny|ask",
    "permissionDecisionReason": "string",
    "updatedInput": {}
  },
  "continue": true,
  "stopReason": "string",
  "systemMessage": "string",
  "suppressOutput": true
}

Exit code 2: Blocks tool, stderr shown to Claude.

Common matchers: Bash, Edit|Write, Read, Task, mcp.

PostToolUse

Purpose: React to tool execution results, provide feedback to Claude.

Input:

{
  "session_id": "string",
  "transcript_path": "string",
  "cwd": "string",
  "permission_mode": "string",
  "hook_event_name": "PostToolUse",
  "tool_name": "string",
  "tool_input": {},
  "tool_response": {
    "filePath": "string",
    "success": true,
    "output": "string",
    "exitCode": 0
  },
  "tool_use_id": "string"
}

CRITICAL: The response field is toolresponse, NOT toolresult.

Output (JSON):

{
  "decision": "block",
  "reason": "string",
  "hookSpecificOutput": {
    "hookEventName": "PostToolUse",
    "additionalContext": "string"
  },
  "continue": true,
  "stopReason": "string",
  "suppressOutput": true
}

Blocking: "decision": "block" with "reason" prompts Claude to address the issue.

Common matchers: Edit|Write, Bash

UserPromptSubmit

Purpose: Validate user prompts, inject context before Claude processes.

Input:

{
  "session_id": "string",
  "transcript_path": "string",
  "cwd": "string",
  "permission_mode": "string",
  "hook_event_name": "UserPromptSubmit",
  "prompt": "string"
}

Output (Plain text):

Any stdout text is added to context for Claude.

Output (JSON):

{
  "decision": "block",
  "reason": "string",
  "hookSpecificOutput": {
    "hookEventName": "UserPromptSubmit",
    "additionalContext": "string"
  }
}

Blocking: "decision": "block" erases prompt, shows "reason" to user only (not Claude).

Exit code 2: Blocks prompt, shows stderr to user only.

PermissionRequest

Purpose: Automate permission dialog decisions.

Input:

{
  "session_id": "string",
  "transcript_path": "string",
  "cwd": "string",
  "permission_mode": "string",
  "hook_event_name": "PermissionRequest",
  "tool_name": "string",
  "tool_input": {}
}

Output:

{
  "hookSpecificOutput": {
    "hookEventName": "PermissionRequest",
    "decision": {
      "behavior": "allow|deny",
      "updatedInput": {},
      "message": "string",
      "interrupt": false
    }
  }
}

SessionStart

Purpose: Initialize session, load context, set environment variables.

Input:

{
  "session_id": "string",
  "transcript_path": "string",
  "cwd": "string",
  "permission_mode": "string",
  "hook_event_name": "SessionStart",
  "source": "startup|resume|clear|compact"
}

Environment variable: CLAUDEENVFILE - write export VAR=value to persist env vars.

Output (Plain text or JSON):

{
  "hookSpecificOutput": {
    "hookEventName": "SessionStart",
    "additionalContext": "string"
  },
  "suppressOutput": true
}

Plain text stdout is added as context.

SessionEnd

Purpose: Cleanup, save state, log session.

Input:

{
  "session_id": "string",
  "transcript_path": "string",
  "cwd": "string",
  "permission_mode": "string",
  "hook_event_name": "SessionEnd",
  "reason": "clear|logout|prompt_input_exit|other"
}

Output: Cannot affect session (already ending). Use for cleanup only.

Stop

Purpose: Control when Claude stops, force continuation.

Input:

{
  "session_id": "string",
  "transcript_path": "string",
  "cwd": "string",
  "permission_mode": "string",
  "hook_event_name": "Stop",
  "stop_hook_active": false
}

CRITICAL: Check stophookactive: true to prevent infinite loops!

Output:

{
  "decision": "block",
  "reason": "string"
}