fix: the model was speaking but nobody told it to answer in pictures.

responseModalities: ["TEXT", "IMAGE"] — two words that Google made
mandatory around March 12th without updating the deprecation calendar.
the model name was fine. the parameters were fine. the API just stopped
inferring that an image generation request should return images.

the old request: here is a picture, here is a prompt, make me a
picture. the API: I see no explicit instruction to return images.
request denied. "Request contains an invalid argument." the argument
it wanted was permission to do what it was already designed to do.

reverted gemini-3.1-pro-image-preview back to gemini-3-pro-image-preview.
the model was never deprecated. only its willingness to cooperate
without paperwork.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: quinnypig

pkg/gemini/client.go

@@ -12,7 +12,7 @@ import (
 )
 
 const (
-	ModelName       = "gemini-3.1-pro-image-preview"
+	ModelName       = "gemini-3-pro-image-preview"
 	ModelNameFrugal = "gemini-3.1-flash-image-preview" // Nano Banana 2
 	BaseURL         = "https://generativelanguage.googleapis.com/v1beta/models"
 	FilenameSuffix  = "\n\nAfter generating the image, respond with a short (2-4 word) evocative filename for it. Just the words, no extension."
@@ -86,7 +86,8 @@ type GenerateRequest struct {
 
 // GenerationConfig represents generation configuration
 type GenerationConfig struct {
-	ImageConfig *ImageConfig `json:"imageConfig,omitempty"`
+	ResponseModalities []string     `json:"responseModalities,omitempty"`
+	ImageConfig        *ImageConfig `json:"imageConfig,omitempty"`
 }
 
 // ImageConfig represents image-specific configuration
@@ -301,7 +302,8 @@ func (c *Client) GenerateContentWithFullOptions(prompt string, imagesBase64 []st
 	imageConfig.ImageSize = imageSize
 
 	reqBody.GenerationConfig = &GenerationConfig{
-		ImageConfig: imageConfig,
+		ResponseModalities: []string{"TEXT", "IMAGE"},
+		ImageConfig:        imageConfig,
 	}
 
 	jsonData, err := json.Marshal(reqBody)