Hook Development

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

Response format:

Prompt hooks must return:

{ "ok": true, "reason": "Explanation of decision" }

• ok: true -- approve/allow the action
• ok: false -- block/deny the action
• reason -- required when ok: false, fed back to Claude
• Default model: Haiku

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

Agent Hooks

Use an LLM agent for complex, multi-step verification that requires tool access:

{
  "type": "agent",
  "prompt": "Verify all generated code has tests and passes linting",
  "timeout": 120
}

Supported events: Stop, SubagentStop

Agent hooks spawn a subagent that can use tools (Read, Bash, etc.) for thorough verification — useful when prompt hooks lack sufficient context or tool access. See references/advanced.md for patterns.

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"
}

PermissionRequest

Execute when user is shown a permission dialog. Use to automatically allow or deny permissions.

Example:

{
  "PermissionRequest": [
    {
      "matcher": "Bash",
      "hooks": [
        {
          "type": "command",
          "command": "bash ${CLAUDE_PLUGIN_ROOT}/scripts/check-permission.sh"
        }
      ]
    }
  ]
}

Output for PermissionRequest:

{
  "hookSpecificOutput": {
    "decision": {
      "behavior": "allow|deny",
      "updatedInput": { "command": "modified command" },
      "message": "Reason for denial",
      "interrupt": false
    }
  }
}

• behavior: "allow" to approve, "deny" to reject
• updatedInput: Optional modified tool parameters (only with "allow")
• message: Explanation shown to user (only with "deny")
• interrupt: If true with "deny", stops the current operation

Use cases:

• Auto-approve safe commands matching patterns
• Block dangerous operations with explanations
• Modify tool inputs before execution

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

PostToolUseFailure

Execute when a tool fails after PostToolUse hooks have run. Use to handle errors or provide fallback actions.

Example:

{
  "PostToolUseFailure": [
    {
      "matcher": "Edit",
      "hooks": [
        {
          "type": "prompt",
          "prompt": "Error occurred during edit. Provide fallback action or ask for user input."
        }
      ]
    }
  ]
}

Output behavior:

• 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.

SubagentStart

Execute when a subagent is started. Use to initialize subagent state or perform setup.

Example:

{
  "SubagentStart": [
    {
      "matcher": "mcp__subagent_name",
      "hooks": [
        {
          "type": "command",
          "command": "bash ${CLAUDE_PLUGIN_ROOT}/scripts/subagent-init.sh"
        }
      ]
    }
  ]
}

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.

Supported matchers: startup (first launch), resume (resuming session), clear (after /clear), compact (after context compaction).

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/PermissionRequest/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.

For comprehensive per-tool and per-event input schemas, see Hook Input Schemas.

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 using the plugin wrapper format described in Hook Configuration Formats:

{
  "hooks": {
    "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
          }
        ]
      }
    ]
  }
}

Note: Plugin hooks use the {"hooks": {...}} wrapper format, not the direct settings format. Plugin hooks merge with user's hooks and run in parallel.

Scoped Hooks in Skill/Agent Frontmatter

Beyond hooks.json (global) and settings (user-level), hooks can be defined directly in skill or agent YAML frontmatter. Scoped hooks activate only when that component is in use:

---
name: validated-writer
description: Write files with safety checks...
hooks:
  PreToolUse:
    - matcher: Write
      hooks:
        - type: command
          command: "${CLAUDE_PLUGIN_ROOT}/scripts/validate-write.sh"
---

Supported events in frontmatter: PreToolUse, PostToolUse, Stop

Scoped hooks use the same event/matcher/hook structure but are lifecycle-bound — they activate when the skill loads and deactivate when it completes. This is ideal for skill-specific validation without affecting other workflows.

See references/advanced.md for detailed syntax and comparison with hooks.json.

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

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

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:

1. Edit hook configuration or scripts
2. Exit Claude Code session
3. Restart: claude
4. New hook configuration loads
5. 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.

For additional hook debugging output, use --verbose:

claude --verbose

This shows hook registration, event matching, and execution timing without the full debug output. Combine with --debug for maximum detail.

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

Handler Configuration Fields

Beyond type, timeout, and matcher, hook handlers support:

• once (boolean): Run only once per session, then auto-removed. Useful for one-time initialization in scoped hooks.
• statusMessage (string): Display text shown in the UI while the hook runs.

See references/advanced.md for detailed decision control output schemas and event-specific matchers.

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 - 10 proven patterns including temporarily active and configuration-driven hooks
• references/migration.md - Migrating from basic to advanced hooks
• references/advanced.md - Advanced use cases and techniques

Example Hook Scripts

Working examples in examples/:

Note: After copying example scripts, make them executable: chmod +x script.sh

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

Utility Scripts

Prerequisites: These scripts require jq for JSON validation. See the main README for setup.

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:

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

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

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