Hook Development — quality + safety report

---
name: Hook Development
description: 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.
version: 0.1.0
---

# 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

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

## 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 Limi

… (truncated)