diff --git a/backend/internal/pkg/gemini/models.go b/backend/internal/pkg/gemini/models.go
index 882d2ebd..fac79d18 100644
--- a/backend/internal/pkg/gemini/models.go
+++ b/backend/internal/pkg/gemini/models.go
@@ -2,6 +2,8 @@
 // It is used when upstream model listing is unavailable (e.g. OAuth token missing AI Studio scopes).
 package gemini
 
+import "strings"
+
 type Model struct {
 	Name                       string   `json:"name"`
 	DisplayName                string   `json:"displayName,omitempty"`
@@ -23,10 +25,27 @@ func DefaultModels() []Model {
 		{Name: "models/gemini-3-flash-preview", SupportedGenerationMethods: methods},
 		{Name: "models/gemini-3-pro-preview", SupportedGenerationMethods: methods},
 		{Name: "models/gemini-3.1-pro-preview", SupportedGenerationMethods: methods},
+		{Name: "models/gemini-3.1-pro-preview-customtools", SupportedGenerationMethods: methods},
 		{Name: "models/gemini-3.1-flash-image", SupportedGenerationMethods: methods},
 	}
 }
 
+func HasFallbackModel(model string) bool {
+	trimmed := strings.TrimSpace(model)
+	if trimmed == "" {
+		return false
+	}
+	if !strings.HasPrefix(trimmed, "models/") {
+		trimmed = "models/" + trimmed
+	}
+	for _, model := range DefaultModels() {
+		if model.Name == trimmed {
+			return true
+		}
+	}
+	return false
+}
+
 func FallbackModelsList() ModelsListResponse {
 	return ModelsListResponse{Models: DefaultModels()}
 }
diff --git a/backend/internal/pkg/gemini/models_test.go b/backend/internal/pkg/gemini/models_test.go
index b80047fb..1d20c0e6 100644
--- a/backend/internal/pkg/gemini/models_test.go
+++ b/backend/internal/pkg/gemini/models_test.go
@@ -2,7 +2,7 @@ package gemini
 
 import "testing"
 
-func TestDefaultModels_ContainsImageModels(t *testing.T) {
+func TestDefaultModels_ContainsFallbackCatalogModels(t *testing.T) {
 	t.Parallel()
 
 	models := DefaultModels()
@@ -13,6 +13,7 @@ func TestDefaultModels_ContainsImageModels(t *testing.T) {
 
 	required := []string{
 		"models/gemini-2.5-flash-image",
+		"models/gemini-3.1-pro-preview-customtools",
 		"models/gemini-3.1-flash-image",
 	}
 
@@ -26,3 +27,17 @@ func TestDefaultModels_ContainsImageModels(t *testing.T) {
 		}
 	}
 }
+
+func TestHasFallbackModel_RecognizesCustomtoolsModel(t *testing.T) {
+	t.Parallel()
+
+	if !HasFallbackModel("gemini-3.1-pro-preview-customtools") {
+		t.Fatalf("expected customtools model to exist in fallback catalog")
+	}
+	if !HasFallbackModel("models/gemini-3.1-pro-preview-customtools") {
+		t.Fatalf("expected prefixed customtools model to exist in fallback catalog")
+	}
+	if HasFallbackModel("gemini-unknown") {
+		t.Fatalf("did not expect unknown model to exist in fallback catalog")
+	}
+}