Skip to content

Latest commit

 

History

History
178 lines (141 loc) · 7.82 KB

reference.md

File metadata and controls

178 lines (141 loc) · 7.82 KB

Hooks Reference

This document provides the technical specification for Gemini CLI hooks, including the JSON schemas for input and output, exit code behaviors, and the stable model API.

Communication Protocol

Hooks communicate with Gemini CLI via standard streams and exit codes:

  • Input: Gemini CLI sends a JSON object to the hook's stdin.
  • Output: The hook sends a JSON object (or plain text) to stdout.
  • Exit Codes: Used to signal success or blocking errors.

Exit Code Behavior

Exit Code Meaning Behavior
0 Success stdout is parsed as JSON. If parsing fails, it's treated as a systemMessage.
2 Blocking Error Interrupts the current operation. stderr is shown to the agent (for tool events) or the user.
Other Warning Execution continues. stderr is logged as a non-blocking warning.

Input Schema (stdin)

Every hook receives a base JSON object. Extra fields are added depending on the specific event.

Base Fields (All Events)

Field Type Description
session_id string Unique identifier for the current CLI session.
transcript_path string Path to the session's JSON transcript (if available).
cwd string The current working directory.
hook_event_name string The name of the firing event (e.g., BeforeTool).
timestamp string ISO 8601 timestamp of the event.

Event-Specific Fields

Tool Events (BeforeTool, AfterTool)

  • tool_name: (string) The internal name of the tool (e.g., write_file, run_shell_command).
  • tool_input: (object) The arguments passed to the tool.
  • tool_response: (object, AfterTool only) The raw output from the tool execution.
  • mcp_context: (object, optional) Present only for MCP tool invocations. Contains server identity information:
    • server_name: (string) The configured name of the MCP server.
    • tool_name: (string) The original tool name from the MCP server.
    • command: (string, optional) For stdio transport, the command used to start the server.
    • args: (string[], optional) For stdio transport, the command arguments.
    • cwd: (string, optional) For stdio transport, the working directory.
    • url: (string, optional) For SSE/HTTP transport, the server URL.
    • tcp: (string, optional) For WebSocket transport, the TCP address.

Agent Events (BeforeAgent, AfterAgent)

  • prompt: (string) The user's submitted prompt.
  • prompt_response: (string, AfterAgent only) The final response text from the model.
  • stop_hook_active: (boolean, AfterAgent only) Indicates if a stop hook is already handling a continuation.

Model Events (BeforeModel, AfterModel, BeforeToolSelection)

  • llm_request: (LLMRequest) A stable representation of the outgoing request. See Stable Model API.
  • llm_response: (LLMResponse, AfterModel only) A stable representation of the incoming response.

Session & Notification Events

  • source: (startup | resume | clear, SessionStart only) The trigger source.
  • reason: (exit | clear | logout | prompt_input_exit | other, SessionEnd only) The reason for session end.
  • trigger: (manual | auto, PreCompress only) What triggered the compression event.
  • notification_type: (ToolPermission, Notification only) The type of notification being fired.
  • message: (string, Notification only) The notification message.
  • details: (object, Notification only) Payload-specific details for the notification.

Output Schema (stdout)

If the hook exits with 0, the CLI attempts to parse stdout as JSON.

Common Output Fields

Field Type Description
decision string One of: allow, deny, block, ask, approve.
reason string Explanation shown to the agent when a decision is deny or block.
systemMessage string Message displayed in Gemini CLI terminal to provide warning or context to the user
continue boolean If false, immediately terminates the agent loop for this turn.
stopReason string Message shown to the user when continue is false.
suppressOutput boolean If true, the hook execution is hidden from the CLI transcript.
hookSpecificOutput object Container for event-specific data (see below).

hookSpecificOutput Reference

Field Supported Events Description
additionalContext SessionStart, BeforeAgent, AfterTool Appends text directly to the agent's context.
llm_request BeforeModel A Partial<LLMRequest> to override parameters of the outgoing call.
llm_response BeforeModel A full LLMResponse to bypass the model and provide a synthetic result.
llm_response AfterModel A Partial<LLMResponse> to modify the model's response before the agent sees it.
toolConfig BeforeToolSelection Object containing mode (AUTO/ANY/NONE) and allowedFunctionNames.

Stable Model API

Gemini CLI uses a decoupled format for model interactions to ensure hooks remain stable even if the underlying Gemini SDK changes.

LLMRequest Object

Used in BeforeModel and BeforeToolSelection.

💡 Note: In v1, model hooks are primarily text-focused. Non-text parts (like images or function calls) provided in the content array will be simplified to their string representation by the translator.

{
  "model": string,
  "messages": Array<{
    "role": "user" | "model" | "system",
    "content": string | Array<{ "type": string, [key: string]: any }>
  }>,
  "config"?: {
    "temperature"?: number,
    "maxOutputTokens"?: number,
    "topP"?: number,
    "topK"?: number
  },
  "toolConfig"?: {
    "mode"?: "AUTO" | "ANY" | "NONE",
    "allowedFunctionNames"?: string[]
  }
}

LLMResponse Object

Used in AfterModel and as a synthetic response in BeforeModel.

{
  "text"?: string,
  "candidates": Array<{
    "content": {
      "role": "model",
      "parts": string[]
    },
    "finishReason"?: "STOP" | "MAX_TOKENS" | "SAFETY" | "RECITATION" | "OTHER",
    "index"?: number,
    "safetyRatings"?: Array<{
      "category": string,
      "probability": string,
      "blocked"?: boolean
    }>
  }>,
  "usageMetadata"?: {
    "promptTokenCount"?: number,
    "candidatesTokenCount"?: number,
    "totalTokenCount"?: number
  }
}