LiteLLM Proxy: Vertex AI Gemini models fail for tools with empty parameters

[wenkangjing]

Description

When using OpenCode with LiteLLM proxy configured for Vertex AI Gemini models (e.g., gemini-3-pro-image-preview), tool calls fail because function declarations with empty parameters don't include "type": "object" in the parameters schema. Vertex AI strictly requires this field, while OpenAI-compatible APIs typically accept schemas without it.

Error Message

litellm.ServiceUnavailableError: litellm.BadRequestError: Vertex_ai_betaException BadRequestError - 
{
  "error": {
    "code": 400,
    "message": "Unable to submit request because `todoread` functionDeclaration parameters schema should be of type OBJECT. Learn more: https://cloud.google.com/vertex-ai/generative-ai/docs/multimodal/function-calling",
    "status": "INVALID_ARGUMENT"
  }
}

Vertex AI Gemini requires:

{
  name: todoread,
  description: Use this tool to read your todo list,
  parameters: {
    type: object,  // ← REQUIRED
    properties: {},
    required: []
  }
}

Provider Settings

  "provider": {
    "a-ramdon-name": {
      "npm": "@ai-sdk/openai-compatible",
      "name": "a ramdon name",
      "options": {
        "baseURL": "{baseUIL}/v1",
        "apiKey": "a litellm key"
      },
      "models": {
        "gemini-3-pro-image-preview": {
          "name": "gemini-3-pro-image-preview"
        }
      }
    }

Proposed Solution

When serializing tool schemas for Vertex AI models, ensure, always include "type": "object" in parameters schema

Plugins

Oh My OpenCode

OpenCode version

1.1.25

Steps to reproduce

1. Configure LiteLLM proxy with a Vertex AI Gemini model
2. Ask OpenCode to generate a random image
3. Observe the Vertex AI 400 error

Screenshot and/or share link

https://opncd.ai/share/Ud6K8Z1N

Operating System

macOS SequoiaVersion 15.7.3

Terminal

Ghostty