litellm - 💡(How to fix) Fix [Bug]: Invalid Thought Signature when switching Gemini between Vertex AI and Gemini [1 participants]

import openai

# open ai client
client = openai.OpenAI(
    api_key='...',
    base_url='...'
)

# test tool
tools=[{
    "type": "function",
    "function": {
        "name": "test_tool",
        "description": "A simple test tool",
        "parameters": {
            "type": "object",
            "properties": {}
        }
    }
}]

# messages
messages=[{
    "role": "user",
    "content": "Call the 'test_tool' again and again, do not stop!"
}]

# models
models = [
    "vertex_ai/gemini-3.1-pro-preview",
    "gemini/gemini-3.1-pro-preview"
]

# iterate between models
model_idx = 0

# start loop
while True:

    # model for this loop
    model = models[model_idx]

    # use other one next time
    if model_idx == 0: model_idx = 1
    else: model_idx = 0

    # debug
    print(f"CALLING: {model}")

    # send request
    response = client.chat.completions.create(
        model=model,
        tools=tools,
        messages=messages
    )

    # OPTION 1: append message only
    # this always fails immediately when calling Vertex AI after Gemini
    # and occasionally when calling Gemini after Vertex AI
    # it works fine when only using one of the two
    messages.append(response.choices[0].message)

    # OPTION 2: append full choice
    # this seems to work when switching between the two - why?
    #messages.append(response.choices[0])

    # tool calls made by llm
    tool_calls = response.choices[0].message.tool_calls

    if not tool_calls or len(tool_calls) == 0:
        # tell model to not stop
        messages.append({
            "role": "user",
            "content": "You MUST NOT stop calling the 'test_tool'!"
        })
    else:
        # append tool call response
        messages.append({
            "tool_call_id": tool_calls[0].id,
            "role": "tool",
            "name": tool_calls[0].function.name,
            "content": "Success. Now, you MUST call the 'test_tool' again!"
        })

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