my-skill

name	add_hook
description	Adds a deterministic hook that fires automatically on tool events. Use when asked to enforce a rule on every commit, block a dangerous pattern, validate output, or gate any tool call.
when_to_use	Use when the user wants to automate a check or action that runs every time a specific event occurs — e.g. "block pushes to main", "run linter before every commit", "log every Bash command".
argument-hint	<hook-name> <event> <description>
allowed-tools	Read Write Bash

Step 1 — Choose an event

Pick the event that matches when the hook should fire:

Tool execution (most common):

• PreToolUse — before a tool runs; output block JSON to block it
• PostToolUse — after a tool runs; cannot block, can log or validate
• PostToolBatch — after a batch of parallel tool calls completes
• PermissionRequest — when Claude asks for permission; can auto-approve

Session lifecycle:

• SessionStart — when a session begins
• Stop — when Claude stops generating (end of turn)
• StopFailure — when a turn fails

User input:

• UserPromptSubmit — when the user submits a prompt
• UserPromptExpansion — when slash commands expand

Agent/task events:

• SubagentStart / SubagentStop — subagent lifecycle
• TaskCreated / TaskCompleted — task tracking events

File/config events:

• FileChanged, CwdChanged, ConfigChange

Step 2 — Write the hook script

Create .claude/hooks/<name>.sh:

#!/usr/bin/env bash
# Hook receives JSON on stdin from Claude Code.
INPUT=$(cat)

# exit 0 = allow (or no-op for non-blocking events)
# stdout JSON with {"decision":"block","reason":"..."} = block (PreToolUse)
# stdout JSON with {"additionalContext":"..."} = inject context
# Always exit 0 — exit 1 means hook error, not a block

# Example: block pushes to main
CMD=$(echo "$INPUT" | python3 -c "
import json, sys
try:
    data = json.load(sys.stdin)
except Exception:
    sys.exit(0)
print((data.get('tool_input', {}) or {}).get('command', '') or '')
" 2>/dev/null || true)

[ -z "$CMD" ] && exit 0

if echo "$CMD" | grep -qE 'git push.*(main|master)'; then
  python3 -c "import json,sys; print(json.dumps({'decision':'block','reason':sys.argv[1]}))" \
    "Direct push to main is not allowed. Use a branch and PR."
  exit 0
fi
exit 0

Make it executable: chmod +x .claude/hooks/<name>.sh

Step 3 — Wire it in settings.json

Add to .claude/settings.json:

{
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "Bash",
        "hooks": [{"type": "command", "command": "\"$CLAUDE_PROJECT_DIR\"/.claude/hooks/<name>.sh", "timeout": 10}]
      }
    ]
  }
}

Matchers: tool name (Bash, Write, Edit), pipe-separated (Write|Edit), or * for all tools.

Alternative — Inline hook in a skill

Hooks can also live in a skill's frontmatter and fire only while the skill is active:

---
name: my-skill
hooks:
  PreToolUse:
    - matcher: "Bash"
      hooks:
        - type: command
          command: "./.claude/hooks/check.sh"
---

Step 4 — Test

Trigger the wired tool and verify the hook fires. Check $CLAUDE_PROJECT_DIR is set correctly in the command path.