Question: responseMimeType is always sent in request when using google-gen-ai with GenerationConfig, causing INVALID_ARGUMENT

[shiyii1999]

Question: responseMimeType is always serialized (empty) in generationConfig causing INVALID_ARGUMENT error with Google Gemini API

Description

SOS 👋,

I'm encountering an issue when using spring-ai-google-genai while calling the Google Gemini API with a generationConfig.

Specifically, when a generationConfig is present, the field responseMimeType is always included in the serialized JSON request, even when it is explicitly cleared or set to an empty/optional value.

Example Request Payload

"generationConfig": {
  "responseMimeType": "",
  "responseModalities": [
    "TEXT",
    "IMAGE"
  ],
  "imageConfig": {
    "aspectRatio": "9:16",
    "imageSize": "2K"
  }
}

However, in the Spring AI client:
Even if I call something like clearResponseMimeType()
Or set it to an empty / optional value
➡️ The field still appears in the final JSON request as responseMimeType: "" whenever generationConfig exists.
This makes it impossible to send a valid request when: generationConfig is required but responseMimeType should be omitted entirely/
Expected Behavior
If responseMimeType is: null, empty or explicitly cleared
➡️ It should NOT be serialized into the request JSON at all.
In other words, the generated JSON should look like:

"generationConfig": {
  "responseModalities": [
    "TEXT",
    "IMAGE"
  ],
  "imageConfig": {
    "aspectRatio": "9:16",
    "imageSize": "2K"
  }
}