Plugin hooks

Plugin Hooks

The most powerful and most trap-laden extension type. Exit code 1 does not block — only exit code 2 blocks. Matcher syntax has three modes. updatedInput replaces the entire tool input object, not just one field. These three facts cause most hook bugs. Learn them first.

Authoritative source: https://code.claude.com/docs/en/hooks

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When to use this skill

• Writing any hook configuration (hooks/hooks.json, settings.json)
• Debugging "my hook isn't blocking" or "my hook isn't firing"
• Choosing between hook event types for a specific capability
• Understanding exit code behavior (exit 1 vs exit 2 distinction)
• Designing safe auto-approve flows for PermissionRequest
• Setting up file-watching or cwd-change hooks

When NOT to use this skill

• What hook script logic to write (domain-specific) → use the domain skill
• MCP server authoring → use plugin-architecture or mcp-server
• Full plugin structure → use plugin-composition

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Core principles

1. Exit code 1 does not block. Only exit code 2 blocks a tool call, prompt, or stop. Exit 1 is a non-blocking error — the action proceeds. This is the #1 source of hook bugs.
2. Matcher syntax has three modes. * or omitted = match all. Letters/digits/underscore/pipe only = exact string or |-separated list. Any other character = JavaScript regex. mcp__memory is an exact match (matches no real tool); use mcp__memory__.* for regex.
3. updatedInput replaces the entire tool input. In PreToolUse, returning updatedInput in hookSpecificOutput replaces ALL fields. You must echo back unchanged fields or they are lost.
4. Async hooks cannot block. If async: true, the hook runs after the action has already proceeded. Never use async for security/guardrail hooks.

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Core events (most common in practice)

The full documented event set changes over time. references/hook-event-reference.md tracks the current catalog; this section covers the events practitioners reach for most often.

PreToolUse

Fires before any tool call. Can block, modify input, or allow. The most powerful event.

{
  "matcher": "Bash",
  "hooks": [{"type": "command", "command": "${CLAUDE_PLUGIN_ROOT}/scripts/check.sh"}]
}

Input includes: tool_name, tool_input (full schema), tool_use_id.

Decision output:

{"hookSpecificOutput": {"hookEventName": "PreToolUse", "permissionDecision": "deny", "permissionDecisionReason": "Blocked"}}

Precedence when multiple hooks run: deny > defer > ask > allow.

PostToolUse

Fires after a tool succeeds. Cannot block. Common use: auto-format, logging, notifications.

{"matcher": "Edit|Write", "hooks": [{"type": "command", "async": true, "command": "prettier --write \"$file_path\""}]}

PermissionRequest

Fires when a permission dialog would appear. Can auto-approve or deny without showing the dialog.

{"hookSpecificOutput": {"hookEventName": "PermissionRequest", "decision": {"behavior": "allow"}}}

UserPromptSubmit

Fires before Claude processes a user prompt. Can block or add context. No matcher support — fires on every prompt.

SessionStart

Fires at session start, resume, clear, or compact. Can inject context Claude will see.

{"matcher": "compact", "hooks": [{"type": "command", "command": "${CLAUDE_PLUGIN_ROOT}/scripts/inject-context.sh"}]}

Exit 0 stdout is added to Claude's context for SessionStart.

Stop

Fires when Claude finishes responding. Can prevent Claude from stopping to continue working.

⚠️ Infinite loop trap: if your Stop hook always signals "keep going", Claude loops forever. Check stop_hook_active in input:

input=$(cat)
if echo "$input" | python3 -c "import json,sys; d=json.load(sys.stdin); sys.exit(0 if d.get('stop_hook_active') else 1)" 2>/dev/null; then
    exit 0  # already running a stop hook, don't loop
fi

Notification

Fires for: permission_prompt, idle_prompt, auth_success, elicitation_dialog. Common use: desktop notifications.

FileChanged

Fires when a watched file changes on disk. The matcher both builds the watch list (literal filenames split by |) and filters which handlers run. Example watching .envrc:

{"matcher": ".envrc", "hooks": [{"type": "command", "command": "direnv export bash >> \"$CLAUDE_ENV_FILE\""}]}

SubagentStop

Fires when a subagent finishes. Useful for aggregating subagent results.

WorktreeCreate

Fires when a worktree is created. Any non-zero exit fails worktree creation. The hook must print the absolute path of the created worktree to stdout:

echo "/absolute/path/to/worktree"

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Hook handler types (quick reference)

Type	Use when	Key fields
command	Shell script, full OS access	command, async, shell, timeout
http	Remote service, shared state	url, headers, allowedEnvVars, timeout
mcp_tool	Delegate the hook decision to a configured MCP tool	server, tool, arguments, timeout
prompt	LLM evaluation	prompt (use $ARGUMENTS), model, timeout (30s default)
agent	Multi-step tool-using verification	prompt (use $ARGUMENTS), model, timeout (60s default)

See references/hook-handler-types.md for full schemas and examples.

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Matcher syntax (quick reference)

Value	Interpreted as
*, "", or omitted	Match all — fires on every occurrence
Letters/digits/_/| only	Exact string or |-separated list: Bash, Edit|Write
Any other character	JavaScript regex: ^Notebook, mcp__memory__.*

Common mistake: mcp__memory (underscores only) is evaluated as an exact string. It never matches any real tool because real tools are mcp__memory__create_entities etc. Use mcp__memory__.* (has .*) for regex.

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Exit code contract

Exit 0  → success; parse stdout for JSON output
Exit 2  → blocking error; ignore stdout; use stderr as the error message
Exit 1  → NON-BLOCKING error; the action proceeds anyway; stderr shown in transcript
Timeout → treated as non-blocking error

Not every event can be blocked. See references/hook-event-reference.md for the per-event blocking table.

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Plugin hooks configuration

Hooks ship with a plugin in hooks/hooks.json at the plugin root. Example:

{
  "description": "Format TypeScript files after edits",
  "hooks": {
    "PostToolUse": [
      {
        "matcher": "Edit|Write",
        "hooks": [{"type": "command", "command": "${CLAUDE_PLUGIN_ROOT}/scripts/format.sh"}]
      }
    ]
  }
}

Use ${CLAUDE_PLUGIN_ROOT} for paths to scripts bundled with the plugin. Use ${CLAUDE_PLUGIN_DATA} for state that should survive plugin updates.

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References

File	Contents
references/hook-event-reference.md	Current hook event catalog with schemas, blocking behavior, matcher semantics
references/hook-handler-types.md	Command, http, mcp_tool, prompt, agent — full schemas, examples, security
references/hook-anti-patterns.md	Exit code 1 trap, infinite Stop loops, shell profile pollution, partial updatedInput, and 10+ more
references/hook-testing-patterns.md	/hooks menu, debug log, test_hook.sh usage, stdin replay

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Plugin-Dev Authoring Toolkit by Viktor Bezdek — licensed under MIT.