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Hooks

Hooks allow you to execute custom commands at specific points during agent lifecycle and tool execution. This enables security validation, logging, formatting, context gathering, and other custom behaviors.

Defining Hooks

Hooks are defined in the agent configuration file. See the agent format documentation for the complete syntax and examples.

Hook Event

Hooks receive hook event in JSON format via STDIN:

{
  "hook_event_name": "agentSpawn",
  "cwd": "/current/working/directory"
}

For tool-related hooks, additional fields are included:

  • tool_name: Name of the tool being executed
  • tool_input: Tool-specific parameters (see individual tool documentation)
  • tool_response: Tool execution results (PostToolUse only)

Hook Output

  • Exit code 0: Hook succeeded. STDOUT is captured but not shown to user.
  • Exit code 2: (PreToolUse only) Block tool execution. STDERR is returned to the LLM.
  • Other exit codes: Hook failed. STDERR is shown as warning to user.

Tool Matching

Use the matcher field to specify which tools the hook applies to:

Examples

  • "fs_write" - Exact match for built-in tools
  • "fs_*" - Wildcard pattern for built-in tools
  • "@git" - All tools from git MCP server
  • "@git/status" - Specific tool from git MCP server
  • "*" - All tools (built-in and MCP)
  • "@builtin" - All built-in tools only
  • No matcher - Applies to all tools

For complete tool reference format, see agent format documentation.

Hook Types

AgentSpawn

Runs when agent is activated. No tool context provided.

Hook Event

{
  "hook_event_name": "agentSpawn",
  "cwd": "/current/working/directory"
}

Exit Code Behavior:

  • 0: Hook succeeded, STDOUT is added to agent's context
  • Other: Show STDERR warning to user

UserPromptSubmit

Runs when user submits a prompt. Output is added to conversation context.

Hook Event

{
  "hook_event_name": "userPromptSubmit",
  "cwd": "/current/working/directory",
  "prompt": "user's input prompt"
}

Exit Code Behavior:

  • 0: Hook succeeded, STDOUT is added to agent's context
  • Other: Show STDERR warning to user

PreToolUse

Runs before tool execution. Can validate and block tool usage.

Hook Event

{
  "hook_event_name": "preToolUse",
  "cwd": "/current/working/directory",
  "tool_name": "fs_read",
  "tool_input": {
    "operations": [
      {
        "mode": "Line",
        "path": "/current/working/directory/docs/hooks.md"
      }
    ]
  }
}

Exit Code Behavior:

  • 0: Allow tool execution.
  • 2: Block tool execution, return STDERR to LLM.
  • Other: Show STDERR warning to user, allow tool execution.

PostToolUse

Runs after tool execution with access to tool results.

Hook Event

{
  "hook_event_name": "postToolUse",
  "cwd": "/current/working/directory",
  "tool_name": "fs_read",
  "tool_input": {
    "operations": [
      {
        "mode": "Line",
        "path": "/current/working/directory/docs/hooks.md"
      }
    ]
  },
  "tool_response": {
    "success": true,
    "result": ["# Hooks\n\nHooks allow you to execute..."]
  }
}

Exit Code Behavior:

  • 0: Hook succeeded.
  • Other: Show STDERR warning to user. Tool already ran.

Stop

Runs when the assistant finishes responding to the user (at the end of each turn). This is useful for running post-processing tasks like code compilation, testing, formatting, or cleanup after the assistant's response.

Hook Event

{
  "hook_event_name": "stop",
  "cwd": "/current/working/directory"
}

Exit Code Behavior:

  • 0: Hook succeeded.
  • Other: Show STDERR warning to user.

Note: Stop hooks do not use matchers since they don't relate to specific tools.

MCP Example

For MCP tools, the tool name includes the full namespaced format including the MCP Server name:

Hook Event

{
  "hook_event_name": "preToolUse",
  "cwd": "/current/working/directory",
  "tool_name": "@postgres/query",
  "tool_input": {
    "sql": "SELECT * FROM orders LIMIT 10;"
  }
}

Timeout

Default timeout is 30 seconds (30,000ms). Configure with timeout_ms field.

Caching

Successful hook results are cached based on cache_ttl_seconds:

  • 0: No caching (default)
  • > 0: Cache successful results for specified seconds
  • AgentSpawn hooks are never cached