openai: add v1/responses support (#13351)

Only supporting the stateless part of the API.

Doc updates to come once this is shipped.

Closes: #9659

middleware/openai.go

@@ -433,3 +433,111 @@ func ChatMiddleware() gin.HandlerFunc {
 		c.Next()
 	}
 }
+
+type ResponsesWriter struct {
+	BaseWriter
+	converter  *openai.ResponsesStreamConverter
+	model      string
+	stream     bool
+	responseID string
+	itemID     string
+}
+
+func (w *ResponsesWriter) writeEvent(eventType string, data any) error {
+	d, err := json.Marshal(data)
+	if err != nil {
+		return err
+	}
+	_, err = w.ResponseWriter.Write([]byte(fmt.Sprintf("event: %s\ndata: %s\n\n", eventType, d)))
+	if err != nil {
+		return err
+	}
+	if f, ok := w.ResponseWriter.(http.Flusher); ok {
+		f.Flush()
+	}
+	return nil
+}
+
+func (w *ResponsesWriter) writeResponse(data []byte) (int, error) {
+	var chatResponse api.ChatResponse
+	if err := json.Unmarshal(data, &chatResponse); err != nil {
+		return 0, err
+	}
+
+	if w.stream {
+		w.ResponseWriter.Header().Set("Content-Type", "text/event-stream")
+
+		events := w.converter.Process(chatResponse)
+		for _, event := range events {
+			if err := w.writeEvent(event.Event, event.Data); err != nil {
+				return 0, err
+			}
+		}
+		return len(data), nil
+	}
+
+	// Non-streaming response
+	w.ResponseWriter.Header().Set("Content-Type", "application/json")
+	response := openai.ToResponse(w.model, w.responseID, w.itemID, chatResponse)
+	return len(data), json.NewEncoder(w.ResponseWriter).Encode(response)
+}
+
+func (w *ResponsesWriter) Write(data []byte) (int, error) {
+	code := w.ResponseWriter.Status()
+	if code != http.StatusOK {
+		return w.writeError(data)
+	}
+	return w.writeResponse(data)
+}
+
+func ResponsesMiddleware() gin.HandlerFunc {
+	return func(c *gin.Context) {
+		var req openai.ResponsesRequest
+		if err := c.ShouldBindJSON(&req); err != nil {
+			c.AbortWithStatusJSON(http.StatusBadRequest, openai.NewError(http.StatusBadRequest, err.Error()))
+			return
+		}
+
+		chatReq, err := openai.FromResponsesRequest(req)
+		if err != nil {
+			c.AbortWithStatusJSON(http.StatusBadRequest, openai.NewError(http.StatusBadRequest, err.Error()))
+			return
+		}
+
+		// Check if client requested streaming (defaults to false)
+		streamRequested := req.Stream != nil && *req.Stream
+
+		// Pass streaming preference to the underlying chat request
+		chatReq.Stream = &streamRequested
+
+		var b bytes.Buffer
+		if err := json.NewEncoder(&b).Encode(chatReq); err != nil {
+			c.AbortWithStatusJSON(http.StatusInternalServerError, openai.NewError(http.StatusInternalServerError, err.Error()))
+			return
+		}
+
+		c.Request.Body = io.NopCloser(&b)
+
+		responseID := fmt.Sprintf("resp_%d", rand.Intn(999999))
+		itemID := fmt.Sprintf("msg_%d", rand.Intn(999999))
+
+		w := &ResponsesWriter{
+			BaseWriter: BaseWriter{ResponseWriter: c.Writer},
+			converter:  openai.NewResponsesStreamConverter(responseID, itemID, req.Model),
+			model:      req.Model,
+			stream:     streamRequested,
+			responseID: responseID,
+			itemID:     itemID,
+		}
+
+		// Set headers based on streaming mode
+		if streamRequested {
+			c.Writer.Header().Set("Content-Type", "text/event-stream")
+			c.Writer.Header().Set("Cache-Control", "no-cache")
+			c.Writer.Header().Set("Connection", "keep-alive")
+		}
+
+		c.Writer = w
+		c.Next()
+	}
+}