create-hook
This skill should be used when building PreToolUse, PostToolUse, SessionStart, or other hook types for Claude Code plugins. It provides patterns, security best practices, and configuration guidance.
What this skill does
# Hook Development Guide
Create Claude Code hooks that intercept events in the lifecycle and can validate, modify, or block operations.
## Hook Types
1. **Command Hooks** (recommended) -- Execute a script for deterministic checks
2. **Prompt Hooks** -- Use LLM reasoning for context-aware decisions
3. **Agent Hooks** (v2.1.0+) -- Leverage agent capabilities for complex workflows
## Hook Events
| Event | Trigger | Common Uses |
|-------|---------|-------------|
| `PreToolUse` | Before any tool executes | Block dangerous commands, validate inputs |
| `PostToolUse` | After tool completes | Format code, run linters, log results |
| `SessionStart` | When session begins | Check environment, load config |
| `SessionEnd` | When session ends | Cleanup, save state |
| `Stop` | When agent stops | Verify task completion |
| `SubagentStop` | When subagent stops | Validate subagent work |
| `UserPromptSubmit` | When user sends message | Process user input |
| `PreCompact` | Before context compression | Preserve critical info |
| `Notification` | System notifications | React to events |
| `PermissionRequest` | Permission dialogs (v2.1.0) | Custom permission handling |
## Configuration
### hooks.json Format
```json
{
"hooks": {
"PreToolUse": [
{
"matcher": "Bash",
"hooks": [
{
"type": "command",
"command": "${CLAUDE_PLUGIN_ROOT}/hooks/check-bash.py",
"timeout": 10
}
]
}
]
}
}
```
**Matchers:** `"Bash"`, `"Edit"`, `"Read"` (case-sensitive tool names), or `"*"` for all.
## Environment Variables
| Variable | Description |
|----------|-------------|
| `CLAUDE_PLUGIN_ROOT` | Plugin directory (always use for portable paths) |
| `CLAUDE_PROJECT_DIR` | Current project root |
| `CLAUDE_ENV_FILE` | Persist variables from SessionStart |
## Writing Command Hooks
Copy and customize a template from `templates/`:
| Template | Purpose |
|----------|---------|
| `templates/pretooluse-bash.py` | PreToolUse hook for Bash commands |
| `templates/pretooluse-read.py` | PreToolUse hook for file reads |
| `templates/posttooluse-edit.py` | PostToolUse hook for formatting |
| `templates/sessionstart.sh` | SessionStart initialization |
```bash
cp ${CLAUDE_PLUGIN_ROOT}/skills/create-hook/templates/pretooluse-bash.py \
plugins/my-hook/hooks/my-hook.py
chmod +x plugins/my-hook/hooks/my-hook.py
```
### Essential Patterns
**Always check safe patterns before blocking (allowlist-first):**
```python
SAFE_PATTERNS = [r"rm\s+-rf\s+/tmp/"]
BLOCKED_PATTERNS = [(r"rm\s+-rf\s+", "rm -rf is destructive")]
for pattern in SAFE_PATTERNS:
if re.search(pattern, command):
c.output.exit_success()
for pattern, reason in BLOCKED_PATTERNS:
if re.search(pattern, command):
c.output.exit_block(reason)
```
**Exit methods:**
- `c.output.exit_success()` -- allow operation
- `c.output.exit_block("reason")` -- block with message
- `c.output.exit_modify({"command": modified})` -- modify input (PreToolUse only)
**Early exit when hook doesn't apply:**
```python
if c.tool_name != "Bash":
c.output.exit_success()
```
## Creating a New Hook Plugin
1. Create directory structure:
```bash
mkdir -p plugins/my-hook/.claude-plugin plugins/my-hook/hooks
```
2. Create `plugins/my-hook/.claude-plugin/plugin.json`:
```json
{"name": "my-hook", "version": "1.0.0", "description": "What this hook does"}
```
3. Create `plugins/my-hook/hooks/hooks.json` with event matchers (see Configuration above)
4. Copy and customize a template from this skill's `templates/` directory
5. Validate: `claude plugin validate .`
## Debugging
Test hooks directly by piping JSON input:
```bash
echo '{"tool_name": "Bash", "tool_input": {"command": "rm -rf /"}}' | \
python plugins/my-hook/hooks/my-hook.py
```
Validate hooks.json: `cat plugins/my-hook/hooks/hooks.json | jq .`
## Reference Examples
Existing hooks in this repository:
- Block destructive commands: `plugins/guards/security/safety-guard/`
- Enforce commit format: `plugins/guards/policy/conventional-commits/`
- Format on save: `plugins/guards/quality/python-format/`
- Protect sensitive files: `plugins/guards/security/protect-env/`
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