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:

json

{
  "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:

json

{
  "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:

json

{
  "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:

json

{
  "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:

json

{
  "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):

json

{
  "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:

json

{
  "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:

json

{
  "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:

json

{
  "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:

json

{
  "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:

json

{
  "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:

json

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

Special capability: Persist environment variables using $CLAUDE_ENV_FILE:

bash

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)

json

{
  "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:

json

{
  "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:

json

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

Plugin Hook Configuration

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

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:

json

"matcher": "Write"

Multiple tools:

json

"matcher": "Read|Write|Edit"

Wildcard (all tools):

json

"matcher": "*"

Regex patterns:

json

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

Note: Matchers are case-sensitive.

Common Patterns

json

// 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:

bash

#!/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:

bash

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

bash

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

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

Set Appropriate Timeouts

json

{
  "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:

json

{
  "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

bash

#!/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

bash

#!/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

bash

claude --debug

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

Test Hook Scripts

Test command hooks directly:

bash

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:

bash

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.