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
SessionEnd	Session ends	NO	Cleanup/save state
Stop	Agent finishes	YES	Force continuation
SubagentStop	Subagent finishes	YES	Force continuation
PreCompact	Before compaction	NO	Save state
Notification	Notification sent	NO	Custom alerts

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.

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" }]
      }
    ]
  }
}

Environment Variables

Available to All Hooks

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

SessionStart Only

Variable	Description
`CLAUDE_ENV_FILE`	Path to write `export VAR=value` lines

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']
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

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

Hook Input/Output Schemas

PreToolUse

PostToolUse

UserPromptSubmit

PermissionRequest

SessionStart

SessionEnd

Stop

SubagentStop

PreCompact

Notification

Registration in settings.json

Standard Structure

Matcher Patterns

Events Requiring Matchers

Events Without Matchers

Environment Variables

Available to All Hooks

SessionStart Only

Exit Codes

Exit Code 2 by Hook

Shell Wrapper Pattern

TypeScript Handler Pattern

Testing Hooks

Manual Test Commands

Rebuild After TypeScript Edits

Common Patterns

Block Dangerous Files (PreToolUse)

Auto-Format Files (PostToolUse)

Inject Git Context (UserPromptSubmit)

Force Test Verification (Stop)

Debugging Checklist

Key Learnings from Past Sessions

See Also

hook-developer

Hook Developer

When to Use

Quick Reference