Inappropriate prompt for Google / Gemma

[DinoChiesa]

The prompt sent via chatgpt-shell-google--make-gemini-payload includes a system instruction, and results in a JSON payload, something like this:

{
  "system_instruction": {
    "parts": {
      "text": "You use markdown liberally to structure responses. Always show code snippets in markdown blocks with language labels."
    }
  },
  "contents": [
    {
      "role": "user",
      "parts": [
        {
          "text": "prompt from user input here"
        }
      ]
    }
  ],
  "generation_config": {
    "temperature": 1,
    "topP": 1
  }
}

When you POST this to gemini-2.0-flash, you get a successful response.

Gemma does not support this system_instruction field. When you POST this to https://generativelanguage.googleapis.com/v1beta/models/gemma-3-27b-it:streamGenerateContent , the response is:

{
  "error": {
    "code": 400,
    "message": "Developer instruction is not enabled for models/gemma-3-27b-it",
    "status": "INVALID_ARGUMENT",
    "details": [
      {
        "@type": "type.googleapis.com/google.rpc.DebugInfo",
        "detail": "[ORIGINAL ERROR] generic::invalid_argument: Developer instruction is not enabled for models/gemma-3-27b-it [google.rpc.error_details_ext] { message: \"Developer instruction is not enabled for models/gemma-3-27b-it\" }"
      }
    ]
  }
}

If the system instruction text is migrated to a part of the user text, like this:

{
  "contents": [
    {
      "role": "user",
      "parts": [
        {
          "text": "You use markdown liberally to structure responses. Always show code snippets in markdown blocks with language labels."
        },
        {
          "text": "prompt from user input here"
        }
      ]
    }
  ],
  "generation_config": {
    "temperature": 1,
    "topP": 1
  }
}

... Gemma responds successfully.

This isn't a bug in the elisp; if there is a bug, it is a usability bug. Some of the Google models support system_instruction and some do not. One could imagine special-casing the Gemma model so as not to send the system_instruction field.