hook-development
Guide for creating event-driven hooks that validate, log, or modify Claude's behavior. Use when the user asks to "create a hook", "add PreToolUse hook", "validate tool use", "hook events", "hooks.json", or mentions hook events (PreToolUse, PostToolUse, Stop).
What this skill does
# Hook Development
Guide for creating event-driven hooks in `hooks/hooks.json`.
## hooks.json Format
```json
{
"description": "What these hooks do",
"hooks": {
"PreToolUse": [
{
"matcher": "Write|Edit",
"hooks": [
{
"type": "command",
"command": "${CLAUDE_PLUGIN_ROOT}/hooks/validate.js",
"timeout": 30
}
]
}
]
}
}
```
## Hook Types
**Command hooks** execute a script (see format above). **Prompt-based hooks** let Claude evaluate conditions — useful for semantic checks hard to script:
```json
{
"type": "prompt",
"prompt": "Check if this file modification follows coding standards. If it violates any standards, explain why and set permissionDecision to 'deny'."
}
```
## Hook Events
| Event | When It Fires |
|-------|---------------|
| `SessionStart` | Session begins or resumes |
| `UserPromptSubmit` | User submits a prompt |
| `PreToolUse` | Before tool execution |
| `PostToolUse` | After tool succeeds |
| `Stop` | Claude finishes responding |
| `SubagentStart` | When spawning a subagent |
| `SubagentStop` | When subagent finishes |
| `PreCompact` | Before context compaction |
| `SessionEnd` | Session terminates |
| `Notification` | Claude sends notifications |
| `Setup` | With --init or --maintenance flags |
## Environment Variables
| Variable | Scope | Description |
|----------|-------|-------------|
| `${CLAUDE_PLUGIN_ROOT}` | Plugin hooks | Absolute path to plugin |
| `"$CLAUDE_PROJECT_DIR"` | Project hooks | Project root (quoted) |
**Critical:** Plugin hooks use `${CLAUDE_PLUGIN_ROOT}` with braces. Project hooks use `"$CLAUDE_PROJECT_DIR"` with quotes.
## Exit Codes
| Code | Meaning |
|------|---------|
| 0 | Success — allow action |
| 2 | Block — deny action, show stderr |
| Other | Non-blocking error |
## Common Matchers
- `Write|Edit` — File modifications
- `Bash` — Shell commands
- `*` or `""` — All tools
- `mcp__.*` — All MCP tools
## Temporarily Active Hooks
Hooks can be enabled/disabled via `.local.md` config:
```javascript
const config = parseLocalMd(pluginRoot);
if (!config.hooks?.validation_enabled) {
process.exit(0); // Skip hook
}
```
See `/claude-plugin-dev:plugin-settings` for configuration patterns.
## JSON Output (PreToolUse)
```json
{
"hookSpecificOutput": {
"hookEventName": "PreToolUse",
"permissionDecision": "allow",
"permissionDecisionReason": "Explanation"
}
}
```
Permission values: `allow`, `deny`, `ask`
## Hook Script Templates
### Bash Hook (Preferred for validation/logging)
```bash
#!/bin/bash
# Hook: PreToolUse - Description
# Exit 0 = allow, Exit 2 = block (stderr shown to Claude)
set -e
INPUT=$(cat)
TOOL_NAME=$(echo "$INPUT" | jq -r '.tool_name // "unknown"')
TOOL_INPUT=$(echo "$INPUT" | jq -c '.tool_input // {}')
# ... validation logic ...
exit 0
```
### Node.js Hook (Cross-Platform Shebang Required)
**Never use `#!/usr/bin/env node`** — it fails on macOS when Node is installed via version managers (nvm, fnm, volta). Use this polyglot preamble instead:
```javascript
#!/bin/sh
":" //; export PATH="/opt/homebrew/bin:/usr/local/bin:$HOME/.volta/bin:$HOME/.fnm/aliases/default/bin:$HOME/.asdf/shims:$HOME/.local/share/mise/shims:$HOME/.local/bin:$PATH"
":" //; command -v node >/dev/null 2>&1 || { [ -s "${NVM_DIR:-$HOME/.nvm}/nvm.sh" ] && . "${NVM_DIR:-$HOME/.nvm}/nvm.sh" 2>/dev/null; }
":" //; exec node "$0" "$@"
'use strict';
let input = '';
process.stdin.setEncoding('utf8');
process.stdin.on('data', chunk => input += chunk);
process.stdin.on('end', () => {
try {
const data = JSON.parse(input || '{}');
// ... logic ...
console.log(JSON.stringify({ continue: true }));
} catch (err) {
process.exit(0); // Always fail open
}
});
```
For detailed patterns, bash examples, and best practices: read `references/patterns.md`.
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