diff --git a/packages/core/src/config/models.test.ts b/packages/core/src/config/models.test.ts
index 42e3719739..e4e7774abb 100644
--- a/packages/core/src/config/models.test.ts
+++ b/packages/core/src/config/models.test.ts
@@ -272,9 +272,9 @@ describe('isActiveModel', () => {
     expect(isActiveModel(DEFAULT_GEMINI_FLASH_MODEL)).toBe(true);
   });
 
-  it('should return true for unknown models and aliases (to support test models)', () => {
-    expect(isActiveModel('invalid-model')).toBe(true);
-    expect(isActiveModel(GEMINI_MODEL_ALIAS_AUTO)).toBe(true);
+  it('should return true for unknown models and aliases', () => {
+    expect(isActiveModel('invalid-model')).toBe(false);
+    expect(isActiveModel(GEMINI_MODEL_ALIAS_AUTO)).toBe(false);
   });
 
   it('should return false for PREVIEW_GEMINI_MODEL when useGemini3_1 is true', () => {
diff --git a/packages/core/src/config/models.ts b/packages/core/src/config/models.ts
index 85b1e578ee..153919829f 100644
--- a/packages/core/src/config/models.ts
+++ b/packages/core/src/config/models.ts
@@ -208,7 +208,7 @@ export function isActiveModel(
   useCustomToolModel: boolean = false,
 ): boolean {
   if (!VALID_GEMINI_MODELS.has(model)) {
-    return true;
+    return false;
   }
   if (useGemini3_1) {
     if (model === PREVIEW_GEMINI_MODEL) {