Claude Code Hooks

Claude Code Hooks

Hooks are shell commands, HTTP endpoints, or MCP tools that execute automatically at lifecycle points: before/after tool calls, on session start/end, when config changes, and on other events. The harness controls execution; Claude cannot suppress them.

Event Types

Hooks fire at specific cadences:

Once per session:

• SessionStart — beginning, resume, or after compaction
• SessionEnd — on session close

Once per turn:

• UserPromptSubmit — before Claude reads prompt (can block)
• UserPromptExpansion — before user command expands to prompt (can block)
• Stop — after Claude responds
• StopFailure — if response failed

Per tool call:

• PreToolUse — before tool executes (can block)
• PermissionRequest — when permission dialog appears
• PermissionDenied — when tool denied; can return {retry: true}
• PostToolUse — after tool succeeds
• PostToolUseFailure — if tool failed
• PostToolBatch — after parallel batch resolves

Async/file watching:

• FileChanged — watched file modified on disk (matcher specifies files)
• CwdChanged — working directory changed
• ConfigChange — settings/skills file modified
• InstructionsLoaded — CLAUDE.md or rules/*.md loaded
• Notification — when Claude Code sends notification (matcher: notification type)
• MessageDisplay — while assistant message displays
• SubagentStart — subagent spawned (matcher: agent type)
• SubagentStop — subagent finished
• TaskCreated, TaskCompleted — task lifecycle
• Setup — on --init-only or CI setup mode
• PreCompact, PostCompact — context compaction boundary
• Elicitation, ElicitationResult — MCP server user input
• WorktreeCreate, WorktreeRemove — git worktree lifecycle
• TeammateIdle — agent team teammate idle

Configuration Locations

1. ~/.claude/settings.json — user-wide (all projects)
2. .claude/settings.json — project-wide (shareable, commitable)
3. .claude/settings.local.json — project-specific (gitignored)
4. Plugin hooks/hooks.json — loaded at plugin install
5. Skill/agent YAML frontmatter — active while component loaded

Hook Structure

{
  "hooks": {
    "EventName": [
      {
        "matcher": "ToolName|OtherTool",
        "hooks": [
          {
            "type": "command|http|mcp_tool|prompt|agent",
            "command": "script.sh",
            "timeout": 30,
            "statusMessage": "Running validation...",
            "async": false,
            "if": "Bash(git *)"
          }
        ]
      }
    ]
  }
}

Matcher Syntax

• Empty or "*" — matches all
• Alphanumeric/_/| — exact match or literal alternatives: Bash, Edit|Write, mcp__github__create_issue
• Other chars (including regex syntax) — regex: ^Notebook, .envrc|.env, mcp__.*__write.*
• SessionStart matchers — special: startup|resume|clear|compact
• Notification matchers — permission_prompt, idle_prompt, auth_success, elicitation_dialog, elicitation_complete, elicitation_response

Command Hook Type

Exec form (preferred; safe for paths with special chars):

{
  "type": "command",
  "command": "node",
  "args": ["${CLAUDE_PLUGIN_ROOT}/script.js", "--fix"]
}

Spawns directly without shell; ${} placeholders safe.

Shell form (full features like pipes, globs):

{
  "type": "command",
  "command": "grep 'error' log.txt | wc -l"
}

Runs in shell; supports &&, pipes, * globs. Avoid untrusted paths.

Common fields:

• timeout — seconds before kill (default: varies by event)
• statusMessage — user-visible spinner text
• async — run in background without blocking
• asyncRewake — background task, wake Claude on exit 2
• if — permission rule: Bash(rm **), Edit(*.ts), Bash(git *)
• once — run once per session, then remove (skills only)
• shell — bash (default) or powershell

Path Placeholders

• ${CLAUDE_PROJECT_DIR} — project root
• ${CLAUDE_PLUGIN_ROOT} — plugin installation directory
• ${CLAUDE_PLUGIN_DATA} — plugin persistent data dir

Always use exec form with args when using placeholders.

Windows Polyglot Pattern

Claude Code on Windows auto-prepends bash to .sh commands. Override by using extensionless filenames with a wrapper:

{
  "command": "\"${CLAUDE_PLUGIN_ROOT}/hooks/run-hook.cmd\" session-start"
}

Wrapper (run-hook.cmd): batch block for cmd.exe, shell code for bash:

: << 'CMDBLOCK'
@echo off
if exist "C:\Program Files\Git\bin\bash.exe" (
    "C:\Program Files\Git\bin\bash.exe" "%~dp0%~1" %2 %3
    exit /b %ERRORLEVEL%
)
where bash >nul 2>nul && bash "%~dp0%~1" %2 %3 && exit /b %ERRORLEVEL%
exit /b 0
CMDBLOCK
SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
exec bash "${SCRIPT_DIR}/$1" "${@:2}"

The : << 'CMDBLOCK' trick: cmd.exe runs batch; bash sees : (no-op) and runs Unix shell code.

Exit Code Protocol

Common Input (JSON stdin)

All hooks receive:

• session_id — unique identifier
• cwd — current working directory
• permission_mode — default, plan, acceptEdits, auto, dontAsk, bypassPermissions
• hook_event_name — event name
• effort.level — low, medium, high, xhigh, max
• transcript_path — path to session transcript (JSONL)
• agent_id, agent_type — when inside subagent
• Event-specific fields (e.g., tool_name, tool_input for PreToolUse)

JSON Output (Exit 0 Only)

Exit 0 with valid JSON stdout to control harness. Output format depends on your platform:

Claude Code (CLAUDE_PLUGIN_ROOT set):

{
  "hookSpecificOutput": {
    "hookEventName": "PreToolUse",
    "additionalContext": "Context for Claude",
    "permissionDecision": "deny",
    "permissionDecisionReason": "Blocked by policy"
  }
}

Cursor (CURSOR_PLUGIN_ROOT set):

{
  "additional_context": "Context for Claude"
}

Copilot CLI (SDK standard):

{
  "additionalContext": "Context for Claude"
}

Universal fields:

• continue: false — stop entire turn
• stopReason — message when stopping
• suppressOutput: true — hide from transcript
• systemMessage — warning shown to user
• terminalSequence — OSC codes (desktop notification, window title)

HTTP Hook Type

{
  "type": "http",
  "url": "http://localhost:8080/hook",
  "headers": {"Authorization": "Bearer ${TOKEN}"},
  "allowedEnvVars": ["TOKEN"]
}

POST with stdin JSON. Whitelist env vars to pass. Return 2xx with JSON body for decisions.

MCP Tool Hook Type

{
  "type": "mcp_tool",
  "server": "my_mcp_server",
  "tool": "tool_name",
  "input": {"file_path": "${tool_input.file_path}"}
}

Call tool on connected MCP server. Fields can interpolate from stdin.

Prompt/Agent Hook Types

{
  "type": "prompt",
  "prompt": "Should this be allowed? $ARGUMENTS",
  "model": "fast-model"
}

Sends yes/no question to Claude. Rarely needed; prefer command hooks for determinism.

Real Example: Block rm -rf

{
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "Bash",
        "if": "Bash(rm -rf *)",
        "hooks": [{
          "type": "command",
          "command": "node",
          "args": ["${CLAUDE_PROJECT_DIR}/.claude/hooks/block-rm.js"]
        }]
      }
    ]
  }
}

Exit code 2 to deny; stderr message shown.

Disable All Hooks

{
  "disableAllHooks": true
}

Top-level key to skip all hooks (useful for debugging).

When to Use Hooks

• Deterministic automation ("each time X, always do Y")
• Security gates (block dangerous commands, validate secrets before commits)
• Context injection (SessionStart loads docs, specs, templates)
• Auto-validation (PostToolUse runs tests, blocks if fails)
• Observability (log commands, audit config changes)

Don't use hooks for:

• One-off tasks Claude should handle directly
• Business logic that belongs in code
• Anything requiring complex reasoning (use prompt/agent types sparingly)

Debug Hooks

• /hooks command: browse configured hooks, view matchers/handlers
• Exit 2 blocks silently; non-0 shows error in transcript
• Test JSON with jq before deploying
• Check script permissions on Unix: chmod +x script.sh
• Use timeout to kill runaway scripts
• Hook stderr shown in transcript (unless suppressOutput: true)

Security

Hooks execute with your user permissions. Always:

• Store hook scripts in .claude/ or plugin dirs (not world-writable)
• Never call untrusted executables
• Whitelist env vars in HTTP hooks
• Review hook code before enabling