Claude Code Hooks — Meta Reference Skill

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.

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) |

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

Identify repetitive, low-risk command prefixes (for example: test runners, read-only diagnostics).
Approve narrow prefixes instead of full commands.
Keep destructive or broad shells (rm, git reset --hard, generic interpreters with arbitrary input) out of auto-approval rules.
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.

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)

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.

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.

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

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Prefer primary sources; report source links and dates for volatile information.
If web access is unavailable, state the limitation and mark guidance as unverified.

Agent Skills: Claude Code Hooks — Meta Reference

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