[BUG] - Google Gemini OpenAI compatible endpoints thought signatures

[ericproulx]

Basic checks

• I searched existing issues - this hasn't been reported
• I can reproduce this consistently
• This is a RubyLLM bug, not my application code

What's broken?

Google Gemini models support extended reasoning through "thinking" modes, which generate thought signatures that need to be preserved when working with tool calls. When using Gemini models through an OpenAI-compatible endpoint, these thought signatures are transmitted in the extra_content.google.thought_signature field, following the OpenAI Realtime API's extension pattern.

How to reproduce

#!/usr/bin/env ruby
# frozen_string_literal: true

require 'bundler/setup'
require 'ruby_llm'
require 'faraday'
require 'json'

# Configure RubyLLM with your OpenAI API key
RubyLLM.configure do |config|
  config.openai_api_key = ENV.fetch('OPENAI_API_KEY', nil)
  config.openai_api_base = "https://generativelanguage.googleapis.com/v1beta/openai"
end

# Define a Weather tool that fetches real weather data
class Weather < RubyLLM::Tool
  description 'Gets current weather for a location'

  params do
    string :latitude, description: 'Latitude (e.g., 52.5200)'
    string :longitude, description: 'Longitude (e.g., 13.4050)'
  end

  def execute(latitude:, longitude:)
    url = "https://api.open-meteo.com/v1/forecast?latitude=#{latitude}&longitude=#{longitude}&current=temperature_2m,wind_speed_10m"

    response = Faraday.get(url)
    data = JSON.parse(response.body)

    current = data['current']
    "Temperature: #{current['temperature_2m']}°C, Wind Speed: #{current['wind_speed_10m']} km/h"
  rescue StandardError => e
    { error: e.message }
  end
end

# Create a chat with the OpenAI provider and attach the Weather tool
chat = RubyLLM.chat(model: 'google:gemini-3-pro-preview', provider: :openai, assume_model_exists: true)
              .with_tool(Weather)

# Ask about the weather
puts "Asking: What's the weather in Berlin? (52.5200, 13.4050)"
puts '-' * 50

response = chat.ask("What's the weather in Berlin? (52.5200, 13.4050)")
puts response.content

Expected behavior

OpenAI provider should add the though_signature if present.

What actually happened

Unable to submit request because function call `weather` in the 2. content block is missing a `thought_signature`. Learn more: https://docs.cloud.google.com/vertex-ai/generative-ai/docs/thought-signatures (RubyLLM::BadRequestError)

Environment

All versions.

[ericproulx]

PR #588