Description

When using @ai-sdk/google-vertex with Gemini models (e.g. gemini-3.5-flash) via streamText, tool calls work correctly at the LLM reasoning step, but sending the tool result back causes a 400 Bad Request from the Gemini API. Plain text-only responses work fine.

Error message received:

Invalid JSON payload received. Unknown name "id" at 'contents[1].parts[0].function_call': Cannot find field.
Invalid JSON payload received. Unknown name "id" at 'contents[2].parts[0].function_response': Cannot find field.

Steps to reproduce

1. Set up an agent using streamText with @ai-sdk/google-vertex targeting gemini-3.5-flash.
2. Define at least one tool.
3. Send a prompt that causes the model to invoke a tool.
4. Return the tool result to continue the conversation.
5. Observe 400 from aiplatform.googleapis.com on the second turn.

Environment

Error payload

{
  "errorUrl": "https://aiplatform.googleapis.com/v1/publishers/google/models/gemini-3.5-flash:streamGenerateContent?alt=sse",
  "modelName": "gemini-3.5-flash",
  "errorMessage": "Invalid JSON payload received. Unknown name \"id\" at 'contents[1].parts[0].function_call': Cannot find field.\nInvalid JSON payload received. Unknown name \"id\" at 'contents[2].parts[0].function_response': Cannot find field.",
  "errorStatusCode": 400
}

• This only reproduces on multi-turn conversations where a tool call is made and a result is returned — single-turn text-only completions are unaffected.
• Other providers (e.g. OpenAI, Anthropic) are not affected since they accept or require the id field.
• This may be a regression introduced when normalizing tool call message shapes across providers in the v6 message format.

• This only reproduces on multi-turn conversations where a tool call is made and a result is returned — single-turn text-only completions are unaffected.
• Other providers (e.g. OpenAI, Anthropic) are not affected since they accept or require the id field.
• This may be a regression introduced when normalizing tool call message shapes across providers in the v6 message format.