agents-hooks

name	agents-hooks
description	Event-driven hooks for AI coding agents. Use when automating Claude Code or Codex CLI with stdin JSON and decision-control responses.

Claude Code Hooks — Meta Reference

This skill provides the definitive reference for creating Claude Code hooks. Use this when building automation that triggers on Claude Code events.

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When to Use This Skill

• Building event-driven automation for Claude Code
• Creating PreToolUse guards to block dangerous commands
• Implementing PostToolUse formatters, linters, or auditors
• Adding Stop hooks for testing or notifications
• Setting up SessionStart/SessionEnd for environment management
• Integrating Claude Code with CI/CD pipelines (headless mode)

---

Quick Reference

Event	Trigger	Use Case
SessionStart	Session begins/resumes	Initialize environment
UserPromptSubmit	User submits prompt	Preprocess/validate input
PreToolUse	Before tool execution	Validate, block dangerous commands
PermissionRequest	Permission dialog shown	Auto-allow/deny permissions
PostToolUse	After tool succeeds	Format, audit, notify
PostToolUseFailure	After tool fails	Capture failures, add guidance
SubagentStart	Subagent spawns	Inspect subagent metadata
Stop	When Claude finishes	Run tests, summarize
SubagentStop	Subagent finishes	Verify subagent completion
Notification	On notifications	Alert integrations
PreCompact	Before context compaction	Preserve critical context
Setup	--init/--maintenance	Initialize repo/env
SessionEnd	Session ends	Cleanup, save state

Hook Structure

.claude/hooks/
├── pre-tool-validate.sh
├── post-tool-format.sh
├── post-tool-audit.sh
├── stop-run-tests.sh
└── session-start-init.sh

---

Configuration

settings.json

{
  "hooks": {
    "PostToolUse": [
      {
        "matcher": "Edit|Write",
        "hooks": [
          {
            "type": "command",
            "command": "$CLAUDE_PROJECT_DIR/.claude/hooks/post-tool-format.sh"
          }
        ]
      }
    ],
    "PreToolUse": [
      {
        "matcher": "Bash",
        "hooks": [
          {
            "type": "command",
            "command": "$CLAUDE_PROJECT_DIR/.claude/hooks/pre-tool-validate.sh"
          }
        ]
      }
    ],
    "Stop": [
      {
        "hooks": [
          {
            "type": "command",
            "command": "$CLAUDE_PROJECT_DIR/.claude/hooks/stop-run-tests.sh"
          }
        ]
      }
    ]
  }
}

---

Execution Model (Jan 2026)

• Hooks receive a JSON payload via stdin (treat it as untrusted input) and run with your user permissions (outside the Bash tool sandbox).
• Default timeout is 60s per hook command; all matching hooks run in parallel; identical commands are deduplicated.

Hook Input (stdin)

{
  "hook_event_name": "PreToolUse",
  "tool_name": "Bash",
  "tool_input": {
    "command": "ls -la"
  }
}

Environment Variables (shell)

Variable	Description
CLAUDE_PROJECT_DIR	Absolute project root where Claude Code started
CLAUDE_PLUGIN_ROOT	Plugin root (plugin hooks only)
CLAUDE_CODE_REMOTE	"true" in remote/web environments; empty/local otherwise
CLAUDE_ENV_FILE	File path to persist export ... lines (available in SessionStart; check docs for Setup support)

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Exit Codes

Code	Meaning	Notes
0	Success	JSON written to stdout is parsed for structured control
2	Blocking error	stderr becomes the message; JSON in stdout is ignored
Other	Non-blocking error	Execution continues; stderr is visible in verbose mode

Stdout injection note: for UserPromptSubmit, SessionStart, and Setup, non-JSON stdout (exit 0) is injected into Claude’s context; most other events show stdout only in verbose mode.

---

Decision Control + Input Modification (v2.0.10+)

PreToolUse hooks can allow/deny/ask and optionally modify the tool input via updatedInput.

Hook Output Schema

{
  "hookSpecificOutput": {
    "hookEventName": "PreToolUse",
    "permissionDecision": "allow",
    "permissionDecisionReason": "Reason shown to user (and to Claude on deny)",
    "updatedInput": { "command": "echo 'modified'" },
    "additionalContext": "Extra context added before tool runs"
  }
}

Note: older decision/reason fields are deprecated; prefer the hookSpecificOutput.* fields.

See hook-templates.md for full examples: redirect sensitive file edits to /dev/null and strip .env files from git add commands.

---

Prompt-Based Hooks

For complex decisions, use LLM-evaluated hooks (type: "prompt") instead of bash scripts. They are most useful for Stop and SubagentStop decisions.

Configuration

{
  "hooks": {
    "Stop": [
      {
        "hooks": [
          {
            "type": "prompt",
            "prompt": "Evaluate whether Claude should stop. Context JSON: $ARGUMENTS. Return {\"ok\": true} if all tasks are complete, otherwise {\"ok\": false, \"reason\": \"what remains\"}.",
            "timeout": 30
          }
        ]
      }
    ]
  }
}

Response Schema

• Allow: {"ok": true}
• Block: {"ok": false, "reason": "Explanation shown to Claude"}

Combining Command and Prompt Hooks

Use command hooks for fast, deterministic checks. Use prompt hooks for nuanced decisions:

{
  "Stop": [
    {
      "hooks": [
        { "type": "command", "command": ".claude/hooks/quick-check.sh" },
        { "type": "prompt", "prompt": "Verify code quality meets standards" }
      ]
    }
  ]
}

---

Permission Prompt Fatigue Reduction Pattern

Use this when frequent approval dialogs slow down repeated safe workflows.

Strategy

1. Identify repetitive, low-risk command prefixes (for example: test runners, read-only diagnostics).
2. Approve narrow prefixes instead of full commands.
3. Keep destructive or broad shells (rm, git reset --hard, generic interpreters with arbitrary input) out of auto-approval rules.
4. Re-check approved prefixes periodically; remove stale ones.

Practical Guardrails

• Allow only task-scoped prefixes (example: npm run test:e2e), not unrestricted executors.
• Keep separate policy for write-outside-workspace actions.
• Pair allow-rules with deny-rules for dangerous patterns.

Outcome

This reduces repeated permission interruptions while preserving high-safety boundaries.

Runtime Preflight Hooks (Mandatory for Tool Reliability)

Add a lightweight runtime preflight hook when workflows depend on specific local tool versions (for example Node for JS REPL, test runners, linters).

Preflight Responsibilities

• Verify required binaries exist.
• Verify minimum version constraints.
• Emit a clear remediation message when requirements are not met.
• Fail fast before expensive task execution starts.

Recommended Trigger

• SessionStart for general runtime checks.
• Setup for repository bootstrap checks.

Minimal Policy

• Keep checks deterministic and fast (<1s target).
• Do not auto-install dependencies silently in hooks.
• Print exact command the operator should run to remediate.

Hook Templates

Copy-paste templates for the five most common hook scenarios: PreToolUse validation, PostToolUse formatting, PostToolUse security audit, Stop test runner, and SessionStart environment check.

See references/hook-templates.md for all scripts.

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Matchers

Matchers filter which tool triggers the hook:

• Exact match: Write matches only the Write tool
• Regex: Edit|Write or Notebook.*
• Match all: * (also works with "" or omitted matcher)

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Security Best Practices

Hooks run with full user permissions outside the Bash tool sandbox. Key rules: validate all stdin input, quote every variable ("$VAR"), use absolute paths, never eval untrusted data, and set -euo pipefail.

See references/hook-security.md for the full checklist, command injection prevention, path traversal defense, credential protection, and ShellCheck requirements.

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Hook Composition

Multiple Hooks on Same Event

{
  "PostToolUse": [
    {
      "matcher": "Edit|Write",
      "hooks": [
        { "type": "command", "command": ".claude/hooks/format.sh" },
        { "type": "command", "command": ".claude/hooks/audit.sh" },
        { "type": "command", "command": ".claude/hooks/notify.sh" }
      ]
    }
  ]
}

All matching hooks run in parallel. If you need strict ordering (format → lint → test), make one wrapper script that runs them sequentially.

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Debugging Hooks

# Test a PostToolUse hook manually (stdin JSON)
export CLAUDE_PROJECT_DIR="$(pwd)"
echo '{"hook_event_name":"PostToolUse","tool_name":"Edit","tool_input":{"file_path":"'"$(pwd)"'/src/app.ts"}}' \
  | bash .claude/hooks/post-tool-format.sh

# Check exit code
echo $?

---

Navigation

Resources

• references/hook-templates.md — Copy-paste hook scripts
• references/hook-patterns.md — Common patterns
• references/hook-security.md — Security guide
• references/runtime-preflight-hooks.md — Runtime/version preflight patterns for SessionStart and Setup
• data/sources.json — Documentation links
• assets/template-preflight-runtime-hook.sh — Shell hook template for runtime/tool version checks

Related Skills

• ../agents-subagents/SKILL.md — Agent creation
• ../agents-skills/SKILL.md — Skill creation
• ../ops-devops-platform/SKILL.md — CI/CD integration

Fact-Checking

• Use web search/web fetch to verify current external facts, versions, pricing, deadlines, regulations, or platform behavior before final answers.
• Prefer primary sources; report source links and dates for volatile information.
• If web access is unavailable, state the limitation and mark guidance as unverified.