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	Fires When	Can Block?	Primary Use
PreToolUse	Before tool executes	YES	Block/modify tool calls
PostToolUse	After tool completes	Partial	React to tool results
UserPromptSubmit	User sends prompt	YES	Validate/inject context
PermissionRequest	Permission dialog shows	YES	Auto-approve/deny
SessionStart	Session begins	NO	Load context, set env vars
SessionEnd	Session ends	NO	Cleanup/save state
Stop	Agent finishes	YES	Force continuation
SubagentStart	Subagent spawns	NO	Pattern coordination
SubagentStop	Subagent finishes	YES	Force continuation
PreCompact	Before compaction	NO	Save state
Notification	Notification sent	NO	Custom alerts

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 tool_response, NOT tool_result.

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: CLAUDE_ENV_FILE - 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 stop_hook_active: true to prevent infinite loops!

Output:

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

Blocking: "decision": "block" forces Claude to continue with "reason" as prompt.

---

SubagentStart

Purpose: Run when a subagent (Task tool) is spawned.

Input:

{
  "session_id": "string",
  "transcript_path": "string",
  "cwd": "string",
  "permission_mode": "string",
  "hook_event_name": "SubagentStart",
  "agent_id": "string"
}

Output: Context injection only (cannot block).

---

SubagentStop

Purpose: Control when subagents (Task tool) stop.

Input:

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

Output: Same as Stop.

---

PreCompact

Purpose: Save state before context compaction.

Input:

{
  "session_id": "string",
  "transcript_path": "string",
  "cwd": "string",
  "permission_mode": "string",
  "hook_event_name": "PreCompact",
  "trigger": "manual|auto",
  "custom_instructions": "string"
}

Matchers: manual, auto

Output:

{
  "continue": true,
  "systemMessage": "string"
}

---

Notification

Purpose: Custom notification handling.

Input:

{
  "session_id": "string",
  "transcript_path": "string",
  "cwd": "string",
  "permission_mode": "string",
  "hook_event_name": "Notification",
  "message": "string",
  "notification_type": "permission_prompt|idle_prompt|auth_success|elicitation_dialog"
}

Matchers: permission_prompt, idle_prompt, auth_success, elicitation_dialog, *

Output:

{
  "continue": true,
  "suppressOutput": true,
  "systemMessage": "string"
}

---

Registration in settings.json

Standard Structure

{
  "hooks": {
    "EventName": [
      {
        "matcher": "ToolPattern",
        "hooks": [
          {
            "type": "command",
            "command": "$CLAUDE_PROJECT_DIR/.claude/hooks/my-hook.sh",
            "timeout": 60
          }
        ]
      }
    ]
  }
}

Matcher Patterns

Pattern	Matches
Bash	Exactly Bash tool
Edit|Write	Edit OR Write
Read.*	Regex: Read*
mcp__.*__write.*	MCP write tools
*	All tools

Case-sensitive: Bash ≠ bash

Events Requiring Matchers

• PreToolUse - YES (required)
• PostToolUse - YES (required)
• PermissionRequest - YES (required)
• Notification - YES (optional)
• SessionStart - YES (startup|resume|clear|compact)
• PreCompact - YES (manual|auto)

Events Without Matchers

{
  "hooks": {
    "UserPromptSubmit": [
      {
        "hooks": [{ "type": "command", "command": "/path/to/hook.sh" }]
      }
    ]
  }
}

---

Hook Types

Command Hooks (type: "command")

Default type. Executes bash commands or scripts.

{
  "type": "command",
  "command": "$CLAUDE_PROJECT_DIR/.claude/hooks/my-hook.sh",
  "timeout": 60
}

Prompt-Based Hooks (type: "prompt")

Uses LLM (Haiku) for context-aware decisions. Best for Stop/SubagentStop.

{
  "type": "prompt",
  "prompt": "Evaluate if Claude should stop. Context: $ARGUMENTS. Check if all tasks are complete.",
  "timeout": 30
}

Response schema:

{
  "decision": "approve" | "block",
  "reason": "Explanation",
  "continue": false,
  "stopReason": "Message to user",
  "systemMessage": "Warning"
}

MCP Tool Naming

MCP tools use pattern mcp__<server>__<tool>:

Pattern	Matches
mcp__memory__.*	All memory server tools
mcp__.*__write.*	All MCP write tools
mcp__github__.*	All GitHub tools

---

Environment Variables

Available to All Hooks

Variable	Description
CLAUDE_PROJECT_DIR	Absolute path to project root
CLAUDE_CODE_REMOTE	"true" if remote/web, empty if local CLI

SessionStart Only

Variable	Description
CLAUDE_ENV_FILE	Path to write export VAR=value lines

Plugin Hooks Only

Variable	Description
CLAUDE_PLUGIN_ROOT	Absolute path to plugin directory

---

Exit Codes

Exit Code	Behavior	stdout	stderr
0	Success	JSON processed	Ignored
2	Blocking error	IGNORED	Error message
Other	Non-blocking error	Ignored	Verbose mode

Exit Code 2 by Hook

Hook	Effect
PreToolUse	Blocks tool, stderr to Claude
PostToolUse	stderr to Claude (tool already ran)
UserPromptSubmit	Blocks prompt, stderr to user only
Stop	Blocks stop, stderr to Claude

---

Shell Wrapper Pattern

#!/bin/bash
set -e
cd "$CLAUDE_PROJECT_DIR/.claude/hooks"
cat | npx tsx src/my-hook.ts

Or for bundled:

#!/bin/bash
set -e
cd "$HOME/.claude/hooks"
cat | node dist/my-hook.mjs

---

TypeScript Handler Pattern

import { readFileSync } from 'fs';

interface HookInput {
  session_id: string;
  hook_event_name: string;
  tool_name?: string;
  tool_input?: Record<string, unknown>;
  tool_response?: Record<string, unknown>;
  // ... other fields per hook type
}

function readStdin(): string {
  return readFileSync(0, 'utf-8');
}

async function main() {
  const input: HookInput = JSON.parse(readStdin());

  // Process input

  const output = {
    decision: 'block',  // or undefined to allow
    reason: 'Why blocking'
  };

  console.log(JSON.stringify(output));
}

main().catch(console.error);

---

Testing Hooks

Manual Test Commands

# PostToolUse (Write)
echo '{"tool_name":"Write","tool_input":{"file_path":"test.md"},"tool_response":{"success":true},"session_id":"test"}' | \
  .claude/hooks/my-hook.sh

# PreToolUse (Bash)
echo '{"tool_name":"Bash","tool_input":{"command":"ls"},"session_id":"test"}' | \
  .claude/hooks/my-hook.sh

# SessionStart
echo '{"hook_event_name":"SessionStart","source":"startup","session_id":"test"}' | \
  .claude/hooks/session-start.sh

# SessionEnd
echo '{"hook_event_name":"SessionEnd","reason":"clear","session_id":"test"}' | \
  .claude/hooks/session-end.sh

# UserPromptSubmit
echo '{"prompt":"test prompt","session_id":"test"}' | \
  .claude/hooks/prompt-submit.sh

Rebuild After TypeScript Edits

cd .claude/hooks
npx esbuild src/my-hook.ts \
  --bundle --platform=node --format=esm \
  --outfile=dist/my-hook.mjs

---

Common Patterns

Block Dangerous Files (PreToolUse)

#!/usr/bin/env python3
import json, sys

data = json.load(sys.stdin)
path = data.get('tool_input', {}).get('file_path', '')

BLOCKED = ['.env', 'secrets.json', '.git/']
if any(b in path for b in BLOCKED):
    print(json.dumps({
        "hookSpecificOutput": {
            "hookEventName": "PreToolUse",
            "permissionDecision": "deny",
            "permissionDecisionReason": f"Blocked: {path} is protected"
        }
    }))
else:
    print('{}')

Auto-Format Files (PostToolUse)

#!/bin/bash
INPUT=$(cat)
FILE=$(echo "$INPUT" | jq -r '.tool_input.file_path // empty')

if [[ "$FILE" == *.ts ]] || [[ "$FILE" == *.tsx ]]; then
  npx prettier --write "$FILE" 2>/dev/null
fi

echo '{}'

Inject Git Context (UserPromptSubmit)

#!/bin/bash
echo "Git status:"
git status --short 2>/dev/null || echo "(not a git repo)"
echo ""
echo "Recent commits:"
git log --oneline -5 2>/dev/null || echo "(no commits)"

Force Test Verification (Stop)

#!/usr/bin/env python3
import json, sys, subprocess

data = json.load(sys.stdin)

# Prevent infinite loops
if data.get('stop_hook_active'):
    print('{}')
    sys.exit(0)

# Check if tests pass
result = subprocess.run(['npm', 'test'], capture_output=True)
if result.returncode != 0:
    print(json.dumps({
        "decision": "block",
        "reason": "Tests are failing. Please fix before stopping."
    }))
else:
    print('{}')

---

Debugging Checklist

• Hook registered in settings.json?
• Shell script has +x permission?
• Bundle rebuilt after TS changes?
• Using tool_response not tool_result?
• Output is valid JSON (or plain text)?
• Checking stop_hook_active in Stop hooks?
• Using $CLAUDE_PROJECT_DIR for paths?

---

Key Learnings from Past Sessions

1. Field names matter - tool_response not tool_result
2. Output format - decision: "block" + reason for blocking
3. Exit code 2 - stderr goes to Claude/user, stdout IGNORED
4. Rebuild bundles - TypeScript source edits don't auto-apply
5. Test manually - echo '{}' | ./hook.sh before relying on it
6. Check outputs first - ls .claude/cache/ before editing code
7. Detached spawn hides errors - add logging to debug

See Also

• /debug-hooks - Systematic debugging workflow
• .claude/rules/hooks.md - Hook development rules