Hook Development

AnthropicOfficialPublished by Anthropic

Skill
Tool integration
Agent tooling
Apache-2.0
Low risk
Has scripts

From the author

This skill should be used when the user asks to "create a hook", "add a PreToolUse/PostToolUse/Stop hook", "validate tool use", "implement prompt-based hooks", "use ${CLAUDE_PLUGIN_ROOT}", "set up event-driven automation", "block dangerous commands", or mentions hook events (PreToolUse, PostToolUse, Stop, SubagentStop, SessionStart, SessionEnd, UserPromptSubmit, PreCompact, Notification). Provides comprehensive guidance for creating and implementing Claude Code plugin hooks with focus on advanced prompt-based hooks API.

— the description by Anthropic

AutoGineer summary

Written by AutoGineer from the skill’s own files, not by the author

This skill provides a framework for developing event-driven hooks for Claude Code. It covers both prompt-based hooks, which leverage LLM reasoning for context-aware validation, and command-based hooks for deterministic bash-scripted checks. The guide details configuration formats for plugins and user settings, event types, input/output structures, and security best practices for building robust, portable automation.

What it does

Defines event-driven hooks for Claude Code lifecycle events like PreToolUse and SessionStart.
Supports prompt-based hooks for intelligent, context-aware decision making.
Supports command-based hooks for fast, deterministic bash-scripted validations.
Provides configuration patterns for both plugin-specific and user-level settings.
Outlines security practices for input validation and path safety in hooks.
Offers debugging techniques and lifecycle management for hook development.

When to use it

When you need to validate or modify tool calls before they execute.
When you want to enforce project-specific policies or security constraints.
When you need to automate environment setup or cleanup at session start/end.
When you want to add context-aware reasoning to agent workflows.
When you need to block dangerous commands or prevent unauthorized file access.

Advantages

Enables flexible, context-aware validation using LLM reasoning.
Supports portable configurations using environment variables like CLAUDE_PLUGIN_ROOT.
Allows for layered validation by combining command and prompt hooks.
Provides structured output formats for clear agent-hook communication.
Facilitates robust security enforcement through customizable hook logic.

Usage examples

“How do I create a PreToolUse hook to block dangerous bash commands?”: The agent will guide you to define a PreToolUse hook in your hooks.json, using either a prompt-based hook to analyze command intent or a command-based hook to run a validation script.
“How can I load project-specific environment variables when a session starts?”: The agent will explain how to implement a SessionStart hook using a command script that writes to the $CLAUDE_ENV_FILE.
“What is the difference between the plugin hooks.json and user settings format?”: The agent will clarify that plugin hooks require a 'hooks' wrapper field, while user settings in .claude/settings.json use a direct event-based structure.
“How do I test my hook configuration before deploying it?”: The agent will instruct you to use the provided test scripts and run Claude Code with the --debug flag to inspect hook execution logs.

Install

Download the skill as a ZIP:
https://autogineer.com/agent-skills/anthropics/hook-development/download

Unzip it into the cross-client skills folder of your project:

mkdir -p .agents/skills && unzip hook-development.zip -d .agents/skills

Documentation by Anthropic

Hook Development for Claude Code Plugins

Overview

Hooks are event-driven automation scripts that execute in response to Claude Code events. Use hooks to validate operations, enforce policies, add context, and integrate external tools into workflows.

Key capabilities:

Validate tool calls before execution (PreToolUse)
React to tool results (PostToolUse)
Enforce completion standards (Stop, SubagentStop)
Load project context (SessionStart)
Automate workflows across the development lifecycle

Hook Types

Prompt-Based Hooks (Recommended)

Use LLM-driven decision making for context-aware validation:

{
  "type": "prompt",
  "prompt": "Evaluate if this tool use is appropriate: $TOOL_INPUT",
  "timeout": 30
}

Supported events: Stop, SubagentStop, UserPromptSubmit, PreToolUse

Benefits:

Context-aware decisions based on natural language reasoning
Flexible evaluation logic without bash scripting
Better edge case handling
Easier to maintain and extend

Command Hooks

Execute bash commands for deterministic checks:

{
  "type": "command",
  "command": "bash ${CLAUDE_PLUGIN_ROOT}/scripts/validate.sh",
  "timeout": 60
}

Use for:

Fast deterministic validations
File system operations
External tool integrations
Performance-critical checks

Hook Configuration Formats

Plugin hooks.json Format

For plugin hooks in hooks/hooks.json, use wrapper format:

{
  "description": "Brief explanation of hooks (optional)",
  "hooks": {
    "PreToolUse": [...],
    "Stop": [...],
    "SessionStart": [...]
  }
}

Key points:

description field is optional
hooks field is required wrapper containing actual hook events
This is the plugin-specific format

Example:

{
  "description": "Validation hooks for code quality",
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "Write",
        "hooks": [
          {
            "type": "command",
            "command": "${CLAUDE_PLUGIN_ROOT}/hooks/validate.sh"
          }
        ]
      }
    ]
  }
}

Settings Format (Direct)

For user settings in .claude/settings.json, use direct format:

{
  "PreToolUse": [...],
  "Stop": [...],
  "SessionStart": [...]
}

Key points:

No wrapper - events directly at top level
No description field
This is the settings format

Important: The examples below show the hook event structure that goes inside either format. For plugin hooks.json, wrap these in {"hooks": {...}}.

Hook Events

PreToolUse

Execute before any tool runs. Use to approve, deny, or modify tool calls.

Example (prompt-based):

{
  "PreToolUse": [
    {
      "matcher": "Write|Edit",
      "hooks": [
        {
          "type": "prompt",
          "prompt": "Validate file write safety. Check: system paths, credentials, path traversal, sensitive content. Return 'approve' or 'deny'."
        }
      ]
    }
  ]
}

Output for PreToolUse:

{
  "hookSpecificOutput": {
    "permissionDecision": "allow|deny|ask",
    "updatedInput": {"field": "modified_value"}
  },
  "systemMessage": "Explanation for Claude"
}

PostToolUse

Execute after tool completes. Use to react to results, provide feedback, or log.

Example:

{
  "PostToolUse": [
    {
      "matcher": "Edit",
      "hooks": [
        {
          "type": "prompt",
          "prompt": "Analyze edit result for potential issues: syntax errors, security vulnerabilities, breaking changes. Provide feedback."
        }
      ]
    }
  ]
}

Output behavior:

Exit 0: stdout shown in transcript
Exit 2: stderr fed back to Claude
systemMessage included in context

Stop

Execute when main agent considers stopping. Use to validate completeness.

Example:

{
  "Stop": [
    {
      "matcher": "*",
      "hooks": [
        {
          "type": "prompt",
          "prompt": "Verify task completion: tests run, build succeeded, questions answered. Return 'approve' to stop or 'block' with reason to continue."
        }
      ]
    }
  ]
}

Decision output:

{
  "decision": "approve|block",
  "reason": "Explanation",
  "systemMessage": "Additional context"
}

SubagentStop

Execute when subagent considers stopping. Use to ensure subagent completed its task.

Similar to Stop hook, but for subagents.

UserPromptSubmit

Execute when user submits a prompt. Use to add context, validate, or block prompts.

Example:

{
  "UserPromptSubmit": [
    {
      "matcher": "*",
      "hooks": [
        {
          "type": "prompt",
          "prompt": "Check if prompt requires security guidance. If discussing auth, permissions, or API security, return relevant warnings."
        }
      ]
    }
  ]
}

SessionStart

Execute when Claude Code session begins. Use to load context and set environment.

Example:

{
  "SessionStart": [
    {
      "matcher": "*",
      "hooks": [
        {
          "type": "command",
          "command": "bash ${CLAUDE_PLUGIN_ROOT}/scripts/load-context.sh"
        }
      ]
    }
  ]
}

Special capability: Persist environment variables using $CLAUDE_ENV_FILE:

echo "export PROJECT_TYPE=nodejs" >> "$CLAUDE_ENV_FILE"

See examples/load-context.sh for complete example.

SessionEnd

Execute when session ends. Use for cleanup, logging, and state preservation.

PreCompact

Execute before context compaction. Use to add critical information to preserve.

Notification

Execute when Claude sends notifications. Use to react to user notifications.

Hook Output Format

Standard Output (All Hooks)

{
  "continue": true,
  "suppressOutput": false,
  "systemMessage": "Message for Claude"
}

continue: If false, halt processing (default true)
suppressOutput: Hide output from transcript (default false)
systemMessage: Message shown to Claude

Exit Codes

0 - Success (stdout shown in transcript)
2 - Blocking error (stderr fed back to Claude)
Other - Non-blocking error

Hook Input Format

All hooks receive JSON via stdin with common fields:

{
  "session_id": "abc123",
  "transcript_path": "/path/to/transcript.txt",
  "cwd": "/current/working/dir",
  "permission_mode": "ask|allow",
  "hook_event_name": "PreToolUse"
}

Event-specific fields:

PreToolUse/PostToolUse: tool_name, tool_input, tool_result
UserPromptSubmit: user_prompt
Stop/SubagentStop: reason

Access fields in prompts using $TOOL_INPUT, $TOOL_RESULT, $USER_PROMPT, etc.

Environment Variables

Available in all command hooks:

$CLAUDE_PROJECT_DIR - Project root path
$CLAUDE_PLUGIN_ROOT - Plugin directory (use for portable paths)
$CLAUDE_ENV_FILE - SessionStart only: persist env vars here
$CLAUDE_CODE_REMOTE - Set if running in remote context

Always use ${CLAUDE_PLUGIN_ROOT} in hook commands for portability:

{
  "type": "command",
  "command": "bash ${CLAUDE_PLUGIN_ROOT}/scripts/validate.sh"
}

Plugin Hook Configuration

In plugins, define hooks in hooks/hooks.json:

{
  "PreToolUse": [
    {
      "matcher": "Write|Edit",
      "hooks": [
        {
          "type": "prompt",
          "prompt": "Validate file write safety"
        }
      ]
    }
  ],
  "Stop": [
    {
      "matcher": "*",
      "hooks": [
        {
          "type": "prompt",
          "prompt": "Verify task completion"
        }
      ]
    }
  ],
  "SessionStart": [
    {
      "matcher": "*",
      "hooks": [
        {
          "type": "command",
          "command": "bash ${CLAUDE_PLUGIN_ROOT}/scripts/load-context.sh",
          "timeout": 10
        }
      ]
    }
  ]
}

Plugin hooks merge with user's hooks and run in parallel.

Matchers

Tool Name Matching

Exact match:

"matcher": "Write"

Multiple tools:

"matcher": "Read|Write|Edit"

Wildcard (all tools):

"matcher": "*"

Regex patterns:

"matcher": "mcp__.*__delete.*"  // All MCP delete tools

Note: Matchers are case-sensitive.

Common Patterns

// All MCP tools
"matcher": "mcp__.*"

// Specific plugin's MCP tools
"matcher": "mcp__plugin_asana_.*"

// All file operations
"matcher": "Read|Write|Edit"

// Bash commands only
"matcher": "Bash"

Security Best Practices

Input Validation

Always validate inputs in command hooks:

#!/bin/bash
set -euo pipefail

input=$(cat)
tool_name=$(echo "$input" | jq -r '.tool_name')

# Validate tool name format
if [[ ! "$tool_name" =~ ^[a-zA-Z0-9_]+$ ]]; then
  echo '{"decision": "deny", "reason": "Invalid tool name"}' >&2
  exit 2
fi

Path Safety

Check for path traversal and sensitive files:

file_path=$(echo "$input" | jq -r '.tool_input.file_path')

# Deny path traversal
if [[ "$file_path" == *".."* ]]; then
  echo '{"decision": "deny", "reason": "Path traversal detected"}' >&2
  exit 2
fi

# Deny sensitive files
if [[ "$file_path" == *".env"* ]]; then
  echo '{"decision": "deny", "reason": "Sensitive file"}' >&2
  exit 2
fi

See examples/validate-write.sh and examples/validate-bash.sh for complete examples.

Quote All Variables

# GOOD: Quoted
echo "$file_path"
cd "$CLAUDE_PROJECT_DIR"

# BAD: Unquoted (injection risk)
echo $file_path
cd $CLAUDE_PROJECT_DIR

Set Appropriate Timeouts

{
  "type": "command",
  "command": "bash script.sh",
  "timeout": 10
}

Defaults: Command hooks (60s), Prompt hooks (30s)

Performance Considerations

Parallel Execution

All matching hooks run in parallel:

{
  "PreToolUse": [
    {
      "matcher": "Write",
      "hooks": [
        {"type": "command", "command": "check1.sh"},  // Parallel
        {"type": "command", "command": "check2.sh"},  // Parallel
        {"type": "prompt", "prompt": "Validate..."}   // Parallel
      ]
    }
  ]
}

Design implications:

Hooks don't see each other's output
Non-deterministic ordering
Design for independence

Optimization

Use command hooks for quick deterministic checks
Use prompt hooks for complex reasoning
Cache validation results in temp files
Minimize I/O in hot paths

Temporarily Active Hooks

Create hooks that activate conditionally by checking for a flag file or configuration:

Pattern: Flag file activation

#!/bin/bash
# Only active when flag file exists
FLAG_FILE="$CLAUDE_PROJECT_DIR/.enable-strict-validation"

if [ ! -f "$FLAG_FILE" ]; then
  # Flag not present, skip validation
  exit 0
fi

# Flag present, run validation
input=$(cat)
# ... validation logic ...

Pattern: Configuration-based activation

#!/bin/bash
# Check configuration for activation
CONFIG_FILE="$CLAUDE_PROJECT_DIR/.claude/plugin-config.json"

if [ -f "$CONFIG_FILE" ]; then
  enabled=$(jq -r '.strictMode // false' "$CONFIG_FILE")
  if [ "$enabled" != "true" ]; then
    exit 0  # Not enabled, skip
  fi
fi

# Enabled, run hook logic
input=$(cat)
# ... hook logic ...

Use cases:

Enable strict validation only when needed
Temporary debugging hooks
Project-specific hook behavior
Feature flags for hooks

Best practice: Document activation mechanism in plugin README so users know how to enable/disable temporary hooks.

Hook Lifecycle and Limitations

Hooks Load at Session Start

Important: Hooks are loaded when Claude Code session starts. Changes to hook configuration require restarting Claude Code.

Cannot hot-swap hooks:

Editing hooks/hooks.json won't affect current session
Adding new hook scripts won't be recognized
Changing hook commands/prompts won't update
Must restart Claude Code: exit and run claude again

To test hook changes:

Edit hook configuration or scripts
Exit Claude Code session
Restart: claude or cc
New hook configuration loads
Test hooks with claude --debug

Hook Validation at Startup

Hooks are validated when Claude Code starts:

Invalid JSON in hooks.json causes loading failure
Missing scripts cause warnings
Syntax errors reported in debug mode

Use /hooks command to review loaded hooks in current session.

Debugging Hooks

Enable Debug Mode

claude --debug

Look for hook registration, execution logs, input/output JSON, and timing information.

Test Hook Scripts

Test command hooks directly:

echo '{"tool_name": "Write", "tool_input": {"file_path": "/test"}}' | \
  bash ${CLAUDE_PLUGIN_ROOT}/scripts/validate.sh

echo "Exit code: $?"

Validate JSON Output

Ensure hooks output valid JSON:

output=$(./your-hook.sh < test-input.json)
echo "$output" | jq .

Quick Reference

Hook Events Summary

Event	When	Use For
PreToolUse	Before tool	Validation, modification
PostToolUse	After tool	Feedback, logging
UserPromptSubmit	User input	Context, validation
Stop	Agent stopping	Completeness check
SubagentStop	Subagent done	Task validation
SessionStart	Session begins	Context loading
SessionEnd	Session ends	Cleanup, logging
PreCompact	Before compact	Preserve context
Notification	User notified	Logging, reactions

Best Practices

DO:

✅ Use prompt-based hooks for complex logic
✅ Use ${CLAUDE_PLUGIN_ROOT} for portability
✅ Validate all inputs in command hooks
✅ Quote all bash variables
✅ Set appropriate timeouts
✅ Return structured JSON output
✅ Test hooks thoroughly

DON'T:

❌ Use hardcoded paths
❌ Trust user input without validation
❌ Create long-running hooks
❌ Rely on hook execution order
❌ Modify global state unpredictably
❌ Log sensitive information

Additional Resources

Reference Files

For detailed patterns and advanced techniques, consult:

references/patterns.md - Common hook patterns (8+ proven patterns)
references/migration.md - Migrating from basic to advanced hooks
references/advanced.md - Advanced use cases and techniques

Example Hook Scripts

Working examples in examples/:

validate-write.sh - File write validation example
validate-bash.sh - Bash command validation example
load-context.sh - SessionStart context loading example

Utility Scripts

Development tools in scripts/:

validate-hook-schema.sh - Validate hooks.json structure and syntax
test-hook.sh - Test hooks with sample input before deployment
hook-linter.sh - Check hook scripts for common issues and best practices

External Resources

Official Docs: https://docs.claude.com/en/docs/claude-code/hooks
Examples: See security-guidance plugin in marketplace
Testing: Use claude --debug for detailed logs
Validation: Use jq to validate hook JSON output

Implementation Workflow

To implement hooks in a plugin:

Identify events to hook into (PreToolUse, Stop, SessionStart, etc.)
Decide between prompt-based (flexible) or command (deterministic) hooks
Write hook configuration in hooks/hooks.json
For command hooks, create hook scripts
Use ${CLAUDE_PLUGIN_ROOT} for all file references
Validate configuration with scripts/validate-hook-schema.sh hooks/hooks.json
Test hooks with scripts/test-hook.sh before deployment
Test in Claude Code with claude --debug
Document hooks in plugin README

Focus on prompt-based hooks for most use cases. Reserve command hooks for performance-critical or deterministic checks.

Files

ATTRIBUTION.mdAdded by AutoGineer970 B
LICENSELicence file added by AutoGineer11.1 KiB
SKILL.md15.9 KiB
examples/load-context.shScript1.7 KiB
examples/validate-bash.shScript1.3 KiB
examples/validate-write.shScript1.2 KiB
references/advanced.md9.9 KiB
references/migration.md8.1 KiB
references/patterns.md7.0 KiB
scripts/README.md3.6 KiB
scripts/hook-linter.shScript4.1 KiB
scripts/test-hook.shScript5.2 KiB
scripts/validate-hook-schema.shScript5.0 KiB

LICENSE


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Requirements and compatibility

Written for: Claude Code, Claude apps

Works with: Claude Code, Claude apps, OpenAI Codex, GitHub Copilot, Cursor, Gemini CLI, Kiro, OpenCode, Goose, Amp, Roo Code, JetBrains Junie, Factory Droid, Devin (ex-Windsurf), Mistral Vibe, OpenHands, Letta

“Works with” means the tool reads the open Agent Skills format; a skill written for one tool may use features others lack.

Security

What our scanner noticed· Low risk

Scripts that your agent may run

This skill includes scripts that your agent may run. Read them before you use it.

A static scan is not a guarantee. Skills are third-party content: review a skill before you let an agent use it.

Attribution and licence

Published by

Anthropic

Publisher

Anthropic (Official)

Source

anthropics/claude-plugins-official · plugins/plugin-dev/skills/hook-development · commit 0412be7

Licence

Apache License 2.0 (redistribution allowed)

LICENSE (added by AutoGineer from plugins/plugin-dev/LICENSE)
ATTRIBUTION.md (written by AutoGineer)

Synced

2026-11-09

AutoGineer is not affiliated with or endorsed by Anthropic or Anthropic.

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The ZIP holds the skill’s files, its licence files and an ATTRIBUTION.md. Download hook-development.zip

From the author

AutoGineer summary

What it does

When to use it

Advantages

Usage examples

Install

Documentation by Anthropic

Hook Development for Claude Code Plugins

Overview

Hook Types

Prompt-Based Hooks (Recommended)

Command Hooks

Hook Configuration Formats

Plugin hooks.json Format

Settings Format (Direct)

Hook Events

PreToolUse

PostToolUse

Stop

SubagentStop

UserPromptSubmit

SessionStart

SessionEnd

PreCompact

Notification

Hook Output Format

Standard Output (All Hooks)

Exit Codes

Hook Input Format

Environment Variables

Plugin Hook Configuration

Matchers

Tool Name Matching

Common Patterns

Security Best Practices

Input Validation

Path Safety

Quote All Variables

Set Appropriate Timeouts

Performance Considerations

Parallel Execution

Optimization

Temporarily Active Hooks

Hook Lifecycle and Limitations

Hooks Load at Session Start

Hook Validation at Startup

Debugging Hooks

Enable Debug Mode

Test Hook Scripts

Validate JSON Output

Quick Reference

Hook Events Summary

Best Practices

Additional Resources

Reference Files

Example Hook Scripts

Utility Scripts

External Resources

Implementation Workflow

Files

LICENSE

Requirements and compatibility

Security

Attribution and licence

Related skills

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Skill Creator

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