Claude Code Hooks System - Complete Reference (January 2026)

Hooks execute custom commands or prompts in response to Claude Code events. Use for automation, validation, formatting, and security.

---

All Hook Events

Event	When Fired	Matcher Applies	Common Uses
PreToolUse	Before tool execution	Yes	Validation, blocking
PermissionRequest	When user shown permission dialog	Yes	Auto-approval policies
PostToolUse	After successful tool execution	Yes	Formatting, linting
PostToolUseFailure	After tool fails	Yes	Error handling
Notification	When Claude wants attention	Yes	Custom notifications
UserPromptSubmit	User submits prompt	No	Input validation
Stop	Claude finishes response	No	Cleanup, final checks
SubagentStart	When spawning a subagent	No	Subagent initialization
SubagentStop	Subagent (Task tool) completes	No	Result validation
PreCompact	Before context compaction	Yes	State backup
Setup	Repository setup/maintenance	Yes	One-time operations
SessionStart	Session begins or resumes	Yes	Environment setup
SessionEnd	Session ends	No	Cleanup, persistence

---

Configuration

Configuration Locations (Precedence highest to lowest)

1. Managed - managed-settings.json (enterprise)
2. Local - .claude/settings.local.json (gitignored)
3. Project - .claude/settings.json (shared via git)
4. User - ~/.claude/settings.json (personal)
5. Plugin - hooks/hooks.json or frontmatter
6. Capability - Skill/Command/Agent frontmatter

Note: Enterprise administrators can use allowManagedHooksOnly to block user, project, and plugin hooks.

Structure

Hooks are organized by matchers, where each matcher can have multiple hooks:

{
  "hooks": {
    "EventName": [
      {
        "matcher": "ToolPattern",
        "hooks": [
          {
            "type": "command",
            "command": "your-command-here"
          }
        ]
      }
    ]
  }
}

Fields:

• matcher: Pattern to match tool names, case-sensitive (only for PreToolUse, PermissionRequest, PostToolUse)
  • Simple strings match exactly: Write matches only the Write tool
  • Supports regex: Edit|Write or Notebook.*
  • Use * to match all tools. Also accepts empty string ("") or omit matcher
• hooks: Array of hooks to execute when pattern matches
  • type: "command" for bash commands or "prompt" for LLM evaluation
  • command: (For type: "command") The bash command to execute
  • prompt: (For type: "prompt") The prompt to send to the LLM
  • timeout: (Optional) Seconds before canceling (default: 60 for commands, 30 for prompts)

Project-Specific Hook Scripts

Use $CLAUDE_PROJECT_DIR to reference scripts in your project:

{
  "hooks": {
    "PostToolUse": [
      {
        "matcher": "Write|Edit",
        "hooks": [
          {
            "type": "command",
            "command": "\"$CLAUDE_PROJECT_DIR\"/.claude/hooks/check-style.sh"
          }
        ]
      }
    ]
  }
}

Events Without Matchers

For UserPromptSubmit, Stop, SubagentStop, and SessionEnd, omit the matcher:

{
  "hooks": {
    "UserPromptSubmit": [
      {
        "hooks": [
          {
            "type": "command",
            "command": "/path/to/prompt-validator.py"
          }
        ]
      }
    ]
  }
}

---

Plugin Hooks

Plugins can provide hooks that integrate with user and project hooks. For complete plugin documentation including plugin.json schema, directory structure, and component integration, see ./claude-plugins-reference-2026/SKILL.md.

How Plugin Hooks Work

• Plugin hooks defined in hooks/hooks.json or custom path via hooks field in plugin.json
• When plugin enabled, its hooks merge with user and project hooks
• Multiple hooks from different sources can respond to same event
• Plugin hooks run alongside custom hooks in parallel

Plugin Hook Configuration

Hooks can be configured in hooks/hooks.json or inline in plugin.json:

{
  "description": "Automatic code formatting",
  "hooks": {
    "PostToolUse": [
      {
        "matcher": "Write|Edit",
        "hooks": [
          {
            "type": "command",
            "command": "${CLAUDE_PLUGIN_ROOT}/scripts/format.sh",
            "timeout": 30
          }
        ]
      }
    ]
  }
}

Reference in plugin.json:

{
  "name": "my-plugin",
  "hooks": "./hooks/hooks.json"
}

Or define inline:

{
  "name": "my-plugin",
  "hooks": {
    "PostToolUse": [
      {
        "matcher": "Write|Edit",
        "hooks": [
          {
            "type": "command",
            "command": "${CLAUDE_PLUGIN_ROOT}/scripts/format.sh"
          }
        ]
      }
    ]
  }
}

Plugin Environment Variables

• ${CLAUDE_PLUGIN_ROOT}: Absolute path to the plugin directory
• ${CLAUDE_PROJECT_DIR}: Project root directory
• All standard environment variables available

---

Hooks in Skills, Agents, and Slash Commands

Hooks can be defined in frontmatter. These are scoped to the component's lifecycle. For complete skill documentation, see ./claude-skills-reference-2026/SKILL.md.

Supported events: PreToolUse, PostToolUse, Stop

Skill Example

---
description: Perform operations with security checks
hooks:
  PreToolUse:
    - matcher: "Bash"
      hooks:
        - type: command
          command: "./scripts/security-check.sh"
---

Agent Example

---
description: Review code changes
hooks:
  PostToolUse:
    - matcher: "Edit|Write"
      hooks:
        - type: command
          command: "./scripts/run-linter.sh"
---

once Option

Set once: true to run hook only once per session. After first successful execution, hook is removed.

hooks:
  PreToolUse:
    - matcher: "Bash"
      hooks:
        - type: command
          command: "./scripts/one-time-setup.sh"
          once: true

Note: Only supported for skills and slash commands, not agents.

---

Event-Specific Matchers

PreToolUse / PermissionRequest / PostToolUse Common Matchers

Matcher	Description
Task	Subagent tasks
Bash	Shell commands
Glob	File pattern matching
Grep	Content search
Read	File reading
Edit	File editing
Write	File writing
WebFetch	Web fetching
WebSearch	Web search
Notebook.*	NotebookEdit, NotebookRead
mcp__<server>__.*	All tools from MCP server
mcp__.*__write.*	MCP write tools across servers

SessionStart Matchers

Matcher	Trigger
startup	New session started
resume	--resume, --continue, or /resume
clear	/clear command
compact	Auto or manual compact

PreCompact Matchers

Matcher	Trigger
manual	/compact command
auto	Auto-compact (full context)

Setup Matchers

Matcher	Trigger
init	--init or --init-only flags
maintenance	--maintenance flag

Notification Matchers

Matcher	Trigger
permission_prompt	Permission requests from Claude
idle_prompt	Claude waiting for input (60s+ idle)
auth_success	Authentication success
elicitation_dialog	MCP tool elicitation

---

Hook Input (JSON via stdin)

Common Fields (All Events)

{
  "session_id": "abc123",
  "transcript_path": "/path/to/session.jsonl",
  "cwd": "/current/working/directory",
  "permission_mode": "default",
  "hook_event_name": "PreToolUse"
}

permission_mode values: "default", "plan", "acceptEdits", "dontAsk", "bypassPermissions"

PreToolUse Input

{
  "session_id": "abc123",
  "transcript_path": "/path/to/session.jsonl",
  "cwd": "/path/to/project",
  "permission_mode": "default",
  "hook_event_name": "PreToolUse",
  "tool_name": "Bash",
  "tool_input": {
    "command": "psql -c 'SELECT * FROM users'",
    "description": "Query the users table",
    "timeout": 120000
  },
  "tool_use_id": "toolu_01ABC123"
}

Tool-specific tool_input fields:

Tool	Fields
Bash	command, description, timeout, run_in_background
Write	file_path, content
Edit	file_path, old_string, new_string, replace_all
Read	file_path, offset, limit

PostToolUse Input

{
  "session_id": "abc123",
  "transcript_path": "/path/to/session.jsonl",
  "cwd": "/path/to/project",
  "permission_mode": "default",
  "hook_event_name": "PostToolUse",
  "tool_name": "Write",
  "tool_input": {
    "file_path": "/path/to/file.txt",
    "content": "file content"
  },
  "tool_response": {
    "filePath": "/path/to/file.txt",
    "success": true
  },
  "tool_use_id": "toolu_01ABC123"
}

Notification Input

{
  "session_id": "abc123",
  "transcript_path": "/path/to/session.jsonl",
  "cwd": "/path/to/project",
  "permission_mode": "default",
  "hook_event_name": "Notification",
  "message": "Claude needs your permission to use Bash",
  "notification_type": "permission_prompt"
}

UserPromptSubmit Input

{
  "session_id": "abc123",
  "transcript_path": "/path/to/session.jsonl",
  "cwd": "/path/to/project",
  "permission_mode": "default",
  "hook_event_name": "UserPromptSubmit",
  "prompt": "Write a function to calculate factorial"
}

Stop Input

{
  "session_id": "abc123",
  "transcript_path": "/path/to/session.jsonl",
  "cwd": "/path/to/project",
  "permission_mode": "default",
  "hook_event_name": "Stop",
  "stop_hook_active": true
}

stop_hook_active: True when Claude is already continuing due to a stop hook. Check this to prevent infinite loops.

SubagentStart Input

{
  "session_id": "abc123",
  "transcript_path": "/path/to/session.jsonl",
  "cwd": "/path/to/project",
  "permission_mode": "default",
  "hook_event_name": "SubagentStart",
  "agent_id": "agent-abc123",
  "agent_type": "Explore"
}

Fields:

• agent_id: Unique identifier for the subagent
• agent_type: Agent name (built-in like "Bash", "Explore", "Plan", or custom agent names)

SubagentStop Input

{
  "session_id": "abc123",
  "transcript_path": "/path/to/session.jsonl",
  "cwd": "/path/to/project",
  "permission_mode": "default",
  "hook_event_name": "SubagentStop",
  "stop_hook_active": false,
  "agent_id": "def456",
  "agent_transcript_path": "/path/to/subagents/agent-def456.jsonl"
}

Fields:

• agent_id: Unique identifier for the subagent
• agent_transcript_path: Path to the subagent's own transcript in nested subagents/ folder
• stop_hook_active: True when already continuing due to a stop hook

PreCompact Input

{
  "session_id": "abc123",
  "transcript_path": "/path/to/session.jsonl",
  "cwd": "/path/to/project",
  "permission_mode": "default",
  "hook_event_name": "PreCompact",
  "trigger": "manual",
  "custom_instructions": ""
}

Setup Input

{
  "session_id": "abc123",
  "transcript_path": "/path/to/session.jsonl",
  "cwd": "/path/to/project",
  "permission_mode": "default",
  "hook_event_name": "Setup",
  "trigger": "init"
}

Fields:

• trigger: Either "init" (from --init or --init-only) or "maintenance" (from --maintenance)
• Setup hooks have access to CLAUDE_ENV_FILE for persisting environment variables

SessionStart Input

{
  "session_id": "abc123",
  "transcript_path": "/path/to/session.jsonl",
  "cwd": "/path/to/project",
  "permission_mode": "default",
  "hook_event_name": "SessionStart",
  "source": "startup",
  "model": "claude-sonnet-4-20250514"
}

Fields:

• source: Indicates how session started: "startup" (new), "resume" (resumed), "clear" (after /clear), or "compact" (after compaction)
• model: Model identifier when available
• agent_type: (Optional) Present when Claude Code started with claude --agent <name>

SessionEnd Input

{
  "session_id": "abc123",
  "transcript_path": "/path/to/session.jsonl",
  "cwd": "/path/to/project",
  "permission_mode": "default",
  "hook_event_name": "SessionEnd",
  "reason": "exit"
}

reason values: clear, logout, prompt_input_exit, other

---

Hook Output

Exit Codes

Code	Behavior
0	Success. stdout processed (JSON or plain text)
2	Blocking error. stderr used as error message, fed back to Claude
Other	Non-blocking error. stderr shown in verbose mode (Ctrl+O)

Important: Claude Code does not see stdout if exit code is 0, except for UserPromptSubmit and SessionStart where stdout is added to context.

Exit Code 2 Behavior Per Event

Event	Exit Code 2 Behavior
PreToolUse	Blocks tool call, shows stderr to Claude
PermissionRequest	Denies permission, shows stderr to Claude
PostToolUse	Shows stderr to Claude (tool already ran)
PostToolUseFailure	Shows stderr to Claude (tool already failed)
Notification	Shows stderr to user only
UserPromptSubmit	Blocks prompt, erases it, shows stderr to user
Stop	Blocks stoppage, shows stderr to Claude
SubagentStart	Shows stderr to user only
SubagentStop	Blocks stoppage, shows stderr to Claude subagent
PreCompact	Shows stderr to user only
Setup	Shows stderr to user only
SessionStart	Shows stderr to user only
SessionEnd	Shows stderr to user only

---

JSON Output Control

Important: JSON output only processed with exit code 0. Exit code 2 uses stderr only.

Common JSON Fields (All Events)

{
  "continue": true,
  "stopReason": "Message shown when continue is false",
  "suppressOutput": false,
  "systemMessage": "Optional warning message shown to user"
}

Field	Type	Effect
continue	boolean	false stops Claude (takes precedence over all)
stopReason	string	Shown to user when continue is false
suppressOutput	boolean	Hide stdout from transcript mode
systemMessage	string	Warning message shown to user

Precedence: continue: false takes precedence over any decision: "block" output.

PreToolUse JSON Output

{
  "hookSpecificOutput": {
    "hookEventName": "PreToolUse",
    "permissionDecision": "allow",
    "permissionDecisionReason": "Auto-approved documentation file",
    "updatedInput": {
      "field_to_modify": "new value"
    },
    "additionalContext": "Current environment: production. Proceed with caution."
  }
}

Field	Values	Effect
permissionDecision	allow, deny, ask	Controls tool execution
permissionDecisionReason	string	Shown to user (allow/ask) or Claude (deny)
updatedInput	object	Modifies tool input before execution
additionalContext	string	Added to Claude's context

Note: decision and reason fields are deprecated. Use hookSpecificOutput.permissionDecision and hookSpecificOutput.permissionDecisionReason. Deprecated "approve" and "block" map to "allow" and "deny".

PermissionRequest JSON Output

Allow with modified input:

{
  "hookSpecificOutput": {
    "hookEventName": "PermissionRequest",
    "decision": {
      "behavior": "allow",
      "updatedInput": {
        "command": "npm run lint"
      }
    }
  }
}

Deny with message:

{
  "hookSpecificOutput": {
    "hookEventName": "PermissionRequest",
    "decision": {
      "behavior": "deny",
      "message": "Command not allowed by policy",
      "interrupt": true
    }
  }
}

PostToolUse JSON Output

{
  "decision": "block",
  "reason": "Explanation for decision",
  "hookSpecificOutput": {
    "hookEventName": "PostToolUse",
    "additionalContext": "Additional information for Claude"
  }
}

• "block" automatically prompts Claude with reason
• undefined does nothing, reason is ignored

UserPromptSubmit JSON Output

{
  "decision": "block",
  "reason": "Prompt contains sensitive data",
  "hookSpecificOutput": {
    "hookEventName": "UserPromptSubmit",
    "additionalContext": "My additional context here"
  }
}

Two ways to add context (exit code 0):

1. Plain text stdout: Any non-JSON text is added as context
2. JSON with additionalContext: More structured control

Blocking prompts:

• "decision": "block" prevents prompt processing, erases prompt from context
• "reason" shown to user but not added to context

Stop / SubagentStop JSON Output

{
  "decision": "block",
  "reason": "Must be provided when Claude is blocked from stopping"
}

• "block" prevents Claude from stopping. Must populate reason
• undefined allows Claude to stop. reason is ignored

Setup JSON Output

{
  "hookSpecificOutput": {
    "hookEventName": "Setup",
    "additionalContext": "Repository initialized with custom configuration"
  }
}

Note: Multiple hooks' additionalContext values are concatenated. Setup hooks have access to CLAUDE_ENV_FILE for persisting environment variables.

SessionStart JSON Output

{
  "hookSpecificOutput": {
    "hookEventName": "SessionStart",
    "additionalContext": "Project context loaded successfully"
  }
}

Note: Multiple hooks' additionalContext values are concatenated.

---

Prompt-Based Hooks

LLM-evaluated decisions using a fast model (Haiku). Also known as "agent hooks" for complex verification tasks.

How Prompt-Based Hooks Work

1. Send the hook input and your prompt to Haiku
2. The LLM responds with structured JSON containing a decision
3. Claude Code processes the decision automatically

Configuration

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

Alternatively, use "type": "agent" for complex verification tasks that require tool access.

Field	Required	Description
type	Yes	"prompt" for LLM evaluation, "agent" for tools
prompt	Yes	Prompt text sent to LLM
timeout	No	Seconds (default: 30 for prompt, 60 for agent)

Response Schema

The LLM must respond with JSON:

{
  "ok": true,
  "reason": "Explanation for the decision"
}

Field	Type	Description
ok	boolean	true allows the action, false prevents it
reason	string	Required when ok is false. Shown to Claude

$ARGUMENTS Placeholder

Use $ARGUMENTS in prompt to include hook input JSON. If omitted, input is appended to the prompt.

Example: Intelligent Stop Hook

{
  "hooks": {
    "Stop": [
      {
        "hooks": [
          {
            "type": "prompt",
            "prompt": "You are evaluating whether Claude should stop working. Context: $ARGUMENTS\n\nAnalyze the conversation and determine if:\n1. All user-requested tasks are complete\n2. Any errors need to be addressed\n3. Follow-up work is needed\n\nRespond with JSON: {\"ok\": true} to allow stopping, or {\"ok\": false, \"reason\": \"your explanation\"} to continue working.",
            "timeout": 30
          }
        ]
      }
    ]
  }
}

Example: SubagentStop Validation

{
  "hooks": {
    "SubagentStop": [
      {
        "hooks": [
          {
            "type": "prompt",
            "prompt": "Evaluate if this subagent should stop. Input: $ARGUMENTS\n\nCheck if:\n- The subagent completed its assigned task\n- Any errors occurred that need fixing\n- Additional context gathering is needed\n\nReturn: {\"ok\": true} to allow stopping, or {\"ok\": false, \"reason\": \"explanation\"} to continue."
          }
        ]
      }
    ]
  }
}

Best Use Cases

Event	Use Case
Stop	Intelligent task completion detection
SubagentStop	Verify subagent completed task
UserPromptSubmit	Context-aware prompt validation
PreToolUse	Complex permission decisions
PermissionRequest	Intelligent allow/deny dialogs

Comparison with Command Hooks

Feature	Command Hooks	Prompt Hooks
Execution	Runs bash script	Queries LLM
Decision logic	You implement in code	LLM evaluates context
Setup complexity	Requires script file	Configure prompt only
Context awareness	Limited to script	Natural language understanding
Performance	Fast (local)	Slower (API call)
Use case	Deterministic rules	Context-aware decisions

Best Practices for Prompt Hooks

1. Be specific in prompts - Clearly state what you want the LLM to evaluate
2. Include decision criteria - List the factors the LLM should consider
3. Test your prompts - Verify the LLM makes correct decisions for your use cases
4. Set appropriate timeouts - Default is 30 seconds, adjust if needed
5. Use for complex decisions - Bash hooks are better for simple, deterministic rules

---

Important Hook Events

Setup Hook

Runs when Claude Code is invoked with repository setup and maintenance flags (--init, --init-only, or --maintenance).

Use Setup hooks for:

• One-time or occasional operations (dependency installation, migrations, cleanup)
• Operations you don't want on every session start

Matchers:

• init - Invoked from --init or --init-only flags
• maintenance - Invoked from --maintenance flag

Key characteristics:

• Requires explicit flags because running automatically would slow down every session start
• Has access to CLAUDE_ENV_FILE for persisting environment variables
• Output added to Claude's context

SessionStart Hook

Runs when Claude Code starts a new session or resumes an existing session.

Use SessionStart hooks for:

• Loading development context (existing issues, recent changes)
• Setting up environment variables

Important: For one-time operations like installing dependencies or running migrations, use Setup hooks instead. SessionStart runs on every session, so keep these hooks fast.

Matchers:

• startup - New sessions
• resume - Resumed sessions (from --resume, --continue, or /resume)
• clear - After /clear command
• compact - After auto or manual compact

PostToolUseFailure Hook

Runs immediately after a tool fails (returns an error). This complements PostToolUse, which only runs on successful tool execution.

Use PostToolUseFailure hooks for:

• Error recovery actions
• Logging tool failures
• Custom error handling and reporting

Recognizes the same matcher values as PreToolUse and PostToolUse.

SubagentStart Hook

Runs when a Claude Code subagent (Task tool call) is spawned.

Use SubagentStart hooks for:

• Subagent initialization
• Logging subagent creation
• Context injection for specific agent types

Input includes:

• agent_id: Unique identifier for the subagent
• agent_type: Agent name (built-in like "Bash", "Explore", "Plan", or custom agent names)

---

Environment Variables

Variable	Description	Available In
CLAUDE_PROJECT_DIR	Project root (absolute path)	All hooks
CLAUDE_CODE_REMOTE	"true" if remote, empty if local	All hooks
CLAUDE_ENV_FILE	Path for persisting env vars	SessionStart, Setup
CLAUDE_PLUGIN_ROOT	Plugin directory (absolute)	Plugin hooks

SessionStart Environment Persistence

Example: Setting individual environment variables

#!/bin/bash
if [ -n "$CLAUDE_ENV_FILE" ]; then
  echo 'export NODE_ENV=production' >> "$CLAUDE_ENV_FILE"
  echo 'export API_KEY=your-api-key' >> "$CLAUDE_ENV_FILE"
  echo 'export PATH="$PATH:./node_modules/.bin"' >> "$CLAUDE_ENV_FILE"
fi
exit 0

Example: Persisting all environment changes (e.g., nvm use)

#!/bin/bash
ENV_BEFORE=$(export -p | sort)

# Run setup commands that modify environment
source ~/.nvm/nvm.sh
nvm use 20

if [ -n "$CLAUDE_ENV_FILE" ]; then
  ENV_AFTER=$(export -p | sort)
  comm -13 <(echo "$ENV_BEFORE") <(echo "$ENV_AFTER") >> "$CLAUDE_ENV_FILE"
fi
exit 0

Variables in $CLAUDE_ENV_FILE are sourced before each Bash command.

---

Working with MCP Tools

Claude Code hooks work with Model Context Protocol (MCP) tools.

MCP Tool Naming

MCP tools follow the pattern mcp__<server>__<tool>:

• mcp__memory__create_entities - Memory server's create entities tool
• mcp__filesystem__read_file - Filesystem server's read file tool
• mcp__github__search_repositories - GitHub server's search tool

Configuring Hooks for MCP Tools

{
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "mcp__memory__.*",
        "hooks": [
          {
            "type": "command",
            "command": "echo 'Memory operation initiated' >> ~/mcp-operations.log"
          }
        ]
      },
      {
        "matcher": "mcp__.*__write.*",
        "hooks": [
          {
            "type": "command",
            "command": "/home/user/scripts/validate-mcp-write.py"
          }
        ]
      }
    ]
  }
}

---

Execution Details

Aspect	Behavior
Timeout	60 seconds default, configurable
Parallelization	All matching hooks run in parallel
Deduplication	Identical commands deduplicated
Environment	Runs in cwd with Claude Code's env
Hook order	Hooks from all sources execute together

Output Handling by Event

Event	stdout Handling
UserPromptSubmit, SessionStart, Setup	Added to Claude's context
PreToolUse, PostToolUse, Stop	Shown in verbose mode (Ctrl+O)
Notification, SessionEnd, SubagentStart	Logged to debug only (--debug)

---

Security Considerations

Disclaimer

USE AT YOUR OWN RISK: Claude Code hooks execute arbitrary shell commands on your system automatically. By using hooks, you acknowledge that:

• You are solely responsible for the commands you configure
• Hooks can modify, delete, or access any files your user account can access
• Malicious or poorly written hooks can cause data loss or system damage
• Anthropic provides no warranty and assumes no liability for any damages
• You should thoroughly test hooks in a safe environment before production use

Security Best Practices

1. Validate and sanitize inputs - Never trust input data blindly
2. Always quote shell variables - Use "$VAR" not $VAR
3. Block path traversal - Check for .. in file paths
4. Use absolute paths via the $CLAUDE_PROJECT_DIR variable - Specify full paths for scripts (use $CLAUDE_PROJECT_DIR)
5. Skip sensitive files - Avoid .env, .git/, keys, etc.

Configuration Safety

Direct edits to hooks in settings files don't take effect immediately:

1. Hooks snapshot captured at startup
2. Snapshot used throughout the session
3. Warns if hooks are modified externally
4. Requires review in /hooks menu for changes to apply

This prevents malicious hook modifications from affecting your current session.

---

Debugging

Enable Debug Mode

claude --debug
claude --debug "hooks"  # Filter to hooks only

Debug Output Example

[DEBUG] Executing hooks for PostToolUse:Write
[DEBUG] Getting matching hook commands for PostToolUse with query: Write
[DEBUG] Found 1 hook matchers in settings
[DEBUG] Matched 1 hooks for query "Write"
[DEBUG] Found 1 hook commands to execute
[DEBUG] Executing hook command: <Your command> with timeout 60000ms
[DEBUG] Hook command completed with status 0: <Your stdout>

Basic Troubleshooting

1. Check configuration - Run /hooks to see if your hook is registered
2. Verify syntax - Ensure your JSON settings are valid
3. Test commands - Run hook commands manually first
4. Check permissions - Make sure scripts are executable
5. Review logs - Use claude --debug to see hook execution details
6. Validate plugin hooks - Use claude plugin validate or /plugin validate for plugin-level hooks

Common Issues

Problem	Cause	Fix
Hook not running	Wrong matcher pattern	Check case-sensitivity, regex
Command not found	Relative path	Use $CLAUDE_PROJECT_DIR
JSON not processed	Non-zero exit code	Exit 0 for JSON processing
Hook times out	Slow script	Optimize or increase timeout
Quotes breaking	Unescaped in JSON	Use \" inside JSON strings
Plugin hook not load	Invalid plugin.json hooks config	Validate with claude plugin validate .
Path not found	Missing ${CLAUDE_PLUGIN_ROOT}	Use variable for plugin scripts

Validation Commands

For plugin hooks:

# CLI (from terminal)
claude plugin validate .
claude plugin validate ./path/to/plugin

# In Claude Code session
/plugin validate .
/plugin validate ./path/to/plugin

For settings hooks: JSON validation with:

python3 -m json.tool .claude/settings.json

Test Commands Manually

echo '{"tool_name":"Write","tool_input":{"file_path":"test.txt"}}' | ./your-hook.sh

---

Code Examples

Python: Bash Command Validation (Exit Code)

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

# Define validation rules as (regex pattern, message) tuples
VALIDATION_RULES = [
    (
        r"\bgrep\b(?!.*\|)",
        "Use 'rg' (ripgrep) instead of 'grep' for better performance",
    ),
    (
        r"\bfind\s+\S+\s+-name\b",
        "Use 'rg --files -g pattern' instead of 'find -name'",
    ),
]

def validate_command(command: str) -> list[str]:
    issues = []
    for pattern, message in VALIDATION_RULES:
        if re.search(pattern, command):
            issues.append(message)
    return issues

try:
    input_data = json.load(sys.stdin)
except json.JSONDecodeError as e:
    print(f"Error: Invalid JSON input: {e}", file=sys.stderr)
    sys.exit(1)

tool_name = input_data.get("tool_name", "")
tool_input = input_data.get("tool_input", {})
command = tool_input.get("command", "")

if tool_name != "Bash" or not command:
    sys.exit(0)

issues = validate_command(command)

if issues:
    for message in issues:
        print(f"• {message}", file=sys.stderr)
    # Exit code 2 blocks tool call and shows stderr to Claude
    sys.exit(2)

Python: UserPromptSubmit Context and Validation (JSON Output)

#!/usr/bin/env python3
import json
import sys
import re
import datetime

try:
    input_data = json.load(sys.stdin)
except json.JSONDecodeError as e:
    print(f"Error: Invalid JSON input: {e}", file=sys.stderr)
    sys.exit(1)

prompt = input_data.get("prompt", "")

# Check for sensitive patterns
sensitive_patterns = [
    (r"(?i)\b(password|secret|key|token)\s*[:=]", "Prompt contains potential secrets"),
]

for pattern, message in sensitive_patterns:
    if re.search(pattern, prompt):
        # Use JSON output to block with a specific reason
        output = {
            "decision": "block",
            "reason": f"Security policy violation: {message}. Please rephrase without sensitive information."
        }
        print(json.dumps(output))
        sys.exit(0)

# Add current time to context
context = f"Current time: {datetime.datetime.now()}"
print(context)

# Equivalent JSON approach:
# print(json.dumps({
#   "hookSpecificOutput": {
#     "hookEventName": "UserPromptSubmit",
#     "additionalContext": context,
#   },
# }))

sys.exit(0)

Python: PreToolUse Auto-Approval (JSON Output)

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

try:
    input_data = json.load(sys.stdin)
except json.JSONDecodeError as e:
    print(f"Error: Invalid JSON input: {e}", file=sys.stderr)
    sys.exit(1)

tool_name = input_data.get("tool_name", "")
tool_input = input_data.get("tool_input", {})

# Auto-approve file reads for documentation files
if tool_name == "Read":
    file_path = tool_input.get("file_path", "")
    if file_path.endswith((".md", ".mdx", ".txt", ".json")):
        output = {
            "hookSpecificOutput": {
                "hookEventName": "PreToolUse",
                "permissionDecision": "allow",
                "permissionDecisionReason": "Documentation file auto-approved"
            },
            "suppressOutput": True
        }
        print(json.dumps(output))
        sys.exit(0)

# Let normal permission flow proceed
sys.exit(0)

Node.js: SessionStart Context Injection

#!/usr/bin/env node

const output = {
  hookSpecificOutput: {
    hookEventName: "SessionStart",
    additionalContext: `<project-context>
Environment: ${process.env.NODE_ENV || "development"}
Node version: ${process.version}
Working directory: ${process.cwd()}
</project-context>`,
  },
};

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

---

Examples

Code Formatting (PostToolUse)

{
  "hooks": {
    "PostToolUse": [
      {
        "matcher": "Write|Edit",
        "hooks": [
          {
            "type": "command",
            "command": "prettier --write \"$CLAUDE_PROJECT_DIR\"/**/*.{js,ts,json}"
          }
        ]
      }
    ]
  }
}

File Protection (PreToolUse)

{
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "Write|Edit",
        "hooks": [
          {
            "type": "command",
            "command": "./scripts/check-protected-files.sh"
          }
        ]
      }
    ]
  }
}

Custom Notifications

{
  "hooks": {
    "Notification": [
      {
        "matcher": "permission_prompt",
        "hooks": [
          {
            "type": "command",
            "command": "/path/to/permission-alert.sh"
          }
        ]
      },
      {
        "matcher": "idle_prompt",
        "hooks": [
          {
            "type": "command",
            "command": "/path/to/idle-notification.sh"
          }
        ]
      }
    ]
  }
}

Task Verification (Stop)

{
  "hooks": {
    "Stop": [
      {
        "hooks": [
          {
            "type": "prompt",
            "prompt": "Verify task completion. Check edge cases. Return {\"ok\": true} or {\"ok\": false, \"reason\": \"...\"}."
          }
        ]
      }
    ]
  }
}

---

Sources

• Hooks Reference (accessed 2026-01-28)
• Hooks Guide
• Settings Reference
• Plugin Components Reference