Hooks Automation

Overview

Hooks are user-defined shell commands, LLM prompts, or subagents that execute automatically at specific points in Claude Code's lifecycle. They provide deterministic control over Claude Code's behavior, ensuring certain actions always happen rather than relying on the LLM to choose to run them. Use hooks to enforce project rules, automate repetitive tasks, and integrate Claude Code with your existing tools.

There are three types of hook handlers. Command hooks (type: "command") run a shell command that receives JSON on stdin and communicates results through exit codes and stdout. Prompt hooks (type: "prompt") send a prompt to a Claude model for single-turn yes/no evaluation. Agent hooks (type: "agent") spawn a subagent that can use tools like Read, Grep, and Glob to verify conditions before returning a decision.

Hooks are defined in JSON settings files or in skill/agent YAML frontmatter. The configuration has three levels: choose a hook event (the lifecycle point), add a matcher group (a regex filter for when it fires), and define one or more hook handlers (the command, prompt, or agent that runs).

When to Use Hooks

• •Enforce project rules deterministically (block dangerous commands, protect files)
• •Automate post-edit workflows (formatting, linting, test runs)
• •Send notifications when Claude needs input
• •Inject context at session start or after compaction
• •Validate tool inputs before execution
• •Auto-approve or deny permission requests programmatically
• •Run quality gates before task completion or agent stopping

When NOT to Use Hooks

• •For static context injection, use CLAUDE.md instead
• •For one-time instructions, use the prompt directly
• •For complex multi-step workflows that need judgment, consider skills or subagents
• •For decisions that don't map to a specific lifecycle event

Quick Start Example

The simplest hook blocks a dangerous command. Add this to .claude/settings.json:

{
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "Bash",
        "hooks": [
          {
            "type": "command",
            "command": ".claude/hooks/block-rm.sh"
          }
        ]
      }
    ]
  }
}

The script reads JSON input from stdin, checks the command, and returns a decision:

#!/bin/bash
# .claude/hooks/block-rm.sh
COMMAND=$(jq -r '.tool_input.command')

if echo "$COMMAND" | grep -q 'rm -rf'; then
  jq -n '{
    hookSpecificOutput: {
      hookEventName: "PreToolUse",
      permissionDecision: "deny",
      permissionDecisionReason: "Destructive command blocked by hook"
    }
  }'
else
  exit 0  # allow the command
fi

How it resolves: the PreToolUse event fires, the matcher "Bash" matches the tool name, the hook handler runs, and Claude Code reads the JSON decision to block the tool call.

Quick Reference Router

Hook Lifecycle

Hooks fire at specific points during a Claude Code session. The lifecycle flows as:

SessionStart
    |
    v
UserPromptSubmit  (user submits a prompt)
    |
    v
  [Agentic Loop]
    |
    +---> PreToolUse        (before tool call)
    |         |
    |     PermissionRequest  (if permission dialog shown)
    |         |
    |     PostToolUse        (after tool succeeds)
    |     PostToolUseFailure (after tool fails)
    |         |
    |     SubagentStart      (when subagent spawns)
    |     SubagentStop       (when subagent finishes)
    |         |
    |     Notification       (when notification fires)
    |         |
    +---> PreCompact         (before compaction)
    |
    v
  Stop                       (Claude finishes responding)
  TeammateIdle               (teammate about to go idle)
  TaskCompleted              (task marked as completed)
    |
    v
SessionEnd

When an event fires and a matcher matches, Claude Code passes JSON context about the event to your hook handler. For command hooks, this arrives on stdin. Your handler can then inspect the input, take action, and optionally return a decision.

All 14 Hook Events

Hook Types Summary

Command Hooks (type: "command")

Run a shell command. Receives JSON on stdin, returns decisions via exit codes and stdout. Supports async: true for background execution.

Key fields: command (required), async (optional), timeout (default 600s)

Prompt Hooks (type: "prompt")

Send a prompt to a Claude model (Haiku by default) for single-turn evaluation. The model returns { "ok": true } or { "ok": false, "reason": "..." }.

Key fields: prompt (required, use $ARGUMENTS for hook input), model (optional), timeout (default 30s)

Agent Hooks (type: "agent")

Spawn a subagent with tool access (Read, Grep, Glob) for multi-turn verification. Same response format as prompt hooks. Up to 50 turns.

Key fields: prompt (required), model (optional), timeout (default 60s)

Matcher Patterns Overview

The matcher field is a regex string that filters when hooks fire. Omit or use "" / "*" to match all occurrences. Different event types match on different fields:

• •Tool events (PreToolUse, PostToolUse, PostToolUseFailure, PermissionRequest): match on tool name (e.g., Bash, Edit|Write, mcp__.*)
• •SessionStart: match on how session started (startup, resume, clear, compact)
• •SessionEnd: match on exit reason (clear, logout, prompt_input_exit, other)
• •Notification: match on type (permission_prompt, idle_prompt, auth_success, elicitation_dialog)
• •SubagentStart / SubagentStop: match on agent type (Bash, Explore, Plan, or custom names)
• •PreCompact: match on trigger (manual, auto)
• •UserPromptSubmit, Stop, TeammateIdle, TaskCompleted: no matcher support, always fire

MCP tools follow the pattern mcp__<server>__<tool> and can be matched with regex like mcp__memory__.* or mcp__.*__write.*.

Hook Input and Output Overview

All hooks receive JSON on stdin with common fields (session_id, transcript_path, cwd, permission_mode, hook_event_name) plus event-specific fields. Hooks communicate results through exit codes, stdout JSON, and stderr.

Exit Code Semantics

• •Exit 0: Success. Action proceeds. stdout parsed for JSON output.
• •Exit 2: Blocking error. Action blocked. stderr fed back as error message.
• •Any other code: Non-blocking error. stderr logged, execution continues.

JSON Output

On exit 0, you can print a JSON object to stdout for structured control. Universal fields include continue (stop Claude entirely if false), stopReason, suppressOutput, and systemMessage. Event-specific fields vary; see hook-events-reference.md for details.

Decision Control Patterns

Async Hooks

Set "async": true on command hooks to run in the background without blocking Claude. Async hooks cannot block or return decisions. Results (systemMessage or additionalContext) are delivered on the next conversation turn. Only type: "command" supports async.

Security Considerations

• •Hooks execute with your full user permissions
• •Validate and sanitize inputs; never trust input data blindly
• •Always quote shell variables: use "$VAR" not $VAR
• •Block path traversal: check for .. in file paths
• •Use absolute paths for scripts, using "$CLAUDE_PROJECT_DIR" for the project root
• •Avoid processing .env, .git/, keys, and other sensitive files

Configuration Locations

Debugging Hooks

• •Run claude --debug for full execution details (matched hooks, exit codes, output)
• •Toggle verbose mode with Ctrl+O to see hook progress in the transcript
• •Test hooks manually: echo '{"tool_name":"Bash","tool_input":{"command":"ls"}}' | ./my-hook.sh
• •Use /hooks menu to view, add, and delete hooks interactively

Reference File Index

1. •

   references/hook-events-reference.md - Complete table of all 14 hook events with firing conditions, input/output schemas, matcher patterns, and decision control options for each event.

2. •

   references/hook-configuration.md - hooks.json format, hook locations (user/project/plugin/ skill-scoped), configuration hierarchy, ${CLAUDE_PLUGIN_ROOT} for plugin hooks, matcher patterns, and cross-platform considerations.

3. •

   references/prompt-and-agent-hooks.md - How prompt hooks work (type: "prompt" with model field), agent hooks (type: "agent"), differences from command hooks, when to use each, cost considerations, and complete examples.

4. •

   references/hook-recipes.md - Working recipes: desktop notifications, auto-format with Prettier, protected files, context re-injection after compaction, rm -rf blocking, async test runners, MCP tool logging, session cleanup, and troubleshooting patterns.