feat(openai): add negative_prompt field to image generation request (#12031)

Expose a negative_prompt string parameter on the image endpoints,
matching Stable Diffusion WebUI / vLLM-Omni conventions. When both the
negative_prompt parameter and a '|'-suffixed negative prompt in the
main prompt are present, they are joined with a comma so callers can
keep a global negative prompt in negative_prompt and add per-image
negative tags after '|'.


Assisted-by: Pi: DeepSeek V4 Pro

Signed-off-by: Fedor Zuev <Fedor.Zuev@gmail.com>

core/http/endpoints/openai/image.go

@@ -160,9 +160,12 @@ func ImageEndpoint(cl *config.ModelConfigLoader, ml *model.ModelLoader, appConfi
 			for range n {
 				prompts := strings.Split(i, "|")
 				positive_prompt := prompts[0]
-				negative_prompt := ""
+				negative_prompt := strings.TrimSpace(input.NegativePrompt)
 				if len(prompts) > 1 {
-					negative_prompt = prompts[1]
+					if negative_prompt != "" {
+						negative_prompt += ", "
+					}
+					negative_prompt += strings.TrimSpace(prompts[1])
 				}
 
 				step := config.Step

core/schema/openai.go

@@ -210,6 +210,10 @@ type OpenAIRequest struct {
 	Size string `json:"size,omitempty"`
 	// Prompt is read only by completion/image API calls
 	Prompt any `json:"prompt,omitempty" yaml:"prompt"`
+	// NegativePrompt for image generation (matches Stable Diffusion WebUI /
+	// vLLM-Omni conventions). Combined, comma-separated, with any "|"-suffixed
+	// negative tags in Prompt.
+	NegativePrompt string `json:"negative_prompt,omitempty" yaml:"negative_prompt"`
 
 	// Edit endpoint
 	Instruction string `json:"instruction,omitempty" yaml:"instruction"`

docs/content/features/image-generation.md

@@ -24,9 +24,9 @@ curl http://localhost:8080/v1/images/generations -H "Content-Type: application/j
 }'
 ```
 
-Available additional parameters: `mode`, `step`.
+Available additional parameters: `mode`, `step`, `negative_prompt`.
 
-Note: To set a negative prompt, you can split the prompt with `|`, for instance: `a cute baby sea otter|malformed`.
+To set a negative prompt you can either pass a separate `negative_prompt` field or split the prompt with `|`, e.g. `a cute baby sea otter|malformed`. When both are present they are joined with a comma (`negative_prompt` first, then the `|`-suffixed tags), so a global negative prompt can live in `negative_prompt` while per-image negatives are appended after `|`.
 
 ```bash
 curl http://localhost:8080/v1/images/generations -H "Content-Type: application/json" -d '{