Add virtual model support for multi-key expansion

Virtual models generated from multi-key expansion are now marked and
filtered during config persistence. Virtual models display with a badge
in the UI and cannot be set as default.

web/backend/api/models.go

@@ -42,6 +42,7 @@ type modelResponse struct {
 	// Meta
 	Configured bool `json:"configured"`
 	IsDefault  bool `json:"is_default"`
+	IsVirtual  bool `json:"is_virtual"`
 }
 
 // handleListModels returns all model_list entries with masked API keys.
@@ -86,6 +87,7 @@ func (h *Handler) handleListModels(w http.ResponseWriter, r *http.Request) {
 			ExtraBody:      m.ExtraBody,
 			Configured:     configured[i],
 			IsDefault:      m.ModelName == defaultModel,
+			IsVirtual:      m.IsVirtual(),
 		})
 	}
 
@@ -288,11 +290,13 @@ func (h *Handler) handleSetDefaultModel(w http.ResponseWriter, r *http.Request)
 		return
 	}
 
-	// Verify the model_name exists in model_list
+	// Verify the model_name exists in model_list and is not a virtual model
 	found := false
+	isVirtual := false
 	for _, m := range cfg.ModelList {
 		if m.ModelName == req.ModelName {
 			found = true
+			isVirtual = m.IsVirtual()
 			break
 		}
 	}
@@ -300,6 +304,10 @@ func (h *Handler) handleSetDefaultModel(w http.ResponseWriter, r *http.Request)
 		http.Error(w, fmt.Sprintf("Model %q not found in model_list", req.ModelName), http.StatusNotFound)
 		return
 	}
+	if isVirtual {
+		http.Error(w, fmt.Sprintf("Cannot set virtual model %q as default", req.ModelName), http.StatusBadRequest)
+		return
+	}
 
 	cfg.Agents.Defaults.ModelName = req.ModelName
 

web/backend/api/models_test.go

@@ -356,6 +356,46 @@ func TestHandleAddModel_PersistsAPIKey(t *testing.T) {
 	}
 }
 
+// TestHandleSetDefaultModel_RejectsNonexistentModel tests that setting a non-existent
+// model as default returns 404. This covers the case where virtual models (which are
+// filtered by SaveConfig) cannot be set as default.
+func TestHandleSetDefaultModel_RejectsNonexistentModel(t *testing.T) {
+	configPath, cleanup := setupOAuthTestEnv(t)
+	defer cleanup()
+
+	// First save a valid config with a primary model
+	cfg, err := config.LoadConfig(configPath)
+	if err != nil {
+		t.Fatalf("LoadConfig() error = %v", err)
+	}
+	cfg.ModelList = []*config.ModelConfig{
+		{ModelName: "gpt-4", Model: "openai/gpt-4o"},
+	}
+	if err := config.SaveConfig(configPath, cfg); err != nil {
+		t.Fatalf("SaveConfig() error = %v", err)
+	}
+
+	// Try to set a non-existent model (like a virtual model name) as default
+	h := NewHandler(configPath)
+	mux := http.NewServeMux()
+	h.RegisterRoutes(mux)
+
+	rec := httptest.NewRecorder()
+	req := httptest.NewRequest(http.MethodPost, "/api/models/default", bytes.NewBufferString(`{
+		"model_name": "gpt-4__key_1"
+	}`))
+	req.Header.Set("Content-Type", "application/json")
+	mux.ServeHTTP(rec, req)
+
+	// Should return 404 because the virtual model doesn't exist in the persisted config
+	if rec.Code != http.StatusNotFound {
+		t.Fatalf("status = %d, want %d, body=%s", rec.Code, http.StatusNotFound, rec.Body.String())
+	}
+	if !strings.Contains(rec.Body.String(), "not found") {
+		t.Fatalf("error message should mention 'not found', got: %s", rec.Body.String())
+	}
+}
+
 func TestMaskAPIKey(t *testing.T) {
 	tests := []struct {
 		name string