prompt shouldn't be required with an image

internal/server/server.go

@@ -761,8 +761,13 @@ func (s *Server) CreateSession(w http.ResponseWriter, r *http.Request, taskID st
 		return
 	}
 
-	if strings.TrimSpace(req.Prompt) == "" {
-		writeError(w, http.StatusBadRequest, errors.New("prompt is required"))
+	images, ok := s.images(w, r, req.Images)
+	if !ok {
+		return
+	}
+
+	if strings.TrimSpace(req.Prompt) == "" && len(images) == 0 {
+		writeError(w, http.StatusBadRequest, errors.New("prompt or images are required"))
 		return
 	}
 
@@ -783,11 +788,6 @@ func (s *Server) CreateSession(w http.ResponseWriter, r *http.Request, taskID st
 		model = *req.Model
 	}
 
-	images, ok := s.images(w, r, req.Images)
-	if !ok {
-		return
-	}
-
 	sess, err := s.agents.Create(r.Context(), t, u, req.Prompt, model, images...)
 	if err != nil {
 		writeAgentError(w, err)