hook-development

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

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

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": {...}}.

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.

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

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.

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"

}

```

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.

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"

```

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)

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

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.

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 or cc
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.

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 .

```

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

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

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.