fix(core): handle non-string model flags in resolution (#26069)

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

packages/core/src/config/models.test.ts

@@ -273,6 +273,13 @@ describe('isCustomModel', () => {
     expect(isCustomModel(GEMINI_MODEL_ALIAS_AUTO)).toBe(false);
     expect(isCustomModel(GEMINI_MODEL_ALIAS_PRO)).toBe(false);
   });
+
+  it('should not throw if the model is an array (e.g. from yargs)', () => {
+    // @ts-expect-error - testing invalid runtime input
+    expect(() => isCustomModel(['gemini-2.0-flash', 'gpt-4'])).not.toThrow();
+    // @ts-expect-error - testing invalid runtime input
+    expect(isCustomModel(['gemini-2.0-flash', 'gpt-4'])).toBe(true); // last one is custom
+  });
 });
 
 describe('supportsModernFeatures', () => {
@@ -431,6 +438,15 @@ describe('resolveModel', () => {
       const model = resolveModel(customModel);
       expect(model).toBe(customModel);
     });
+
+    it('should handle non-string inputs gracefully', () => {
+      // @ts-expect-error - testing invalid runtime input
+      expect(resolveModel(['a', 'b'])).toBe('b');
+      // @ts-expect-error - testing invalid runtime input
+      expect(resolveModel(true)).toBe('true');
+      // @ts-expect-error - testing invalid runtime input
+      expect(resolveModel(null)).toBe('');
+    });
   });
 
   describe('hasAccessToPreview logic', () => {

packages/core/src/config/models.ts

@@ -109,8 +109,15 @@ export function resolveModel(
   hasAccessToPreview: boolean = true,
   config?: ModelCapabilityContext,
 ): string {
+  // Defensive check against non-string inputs at runtime
+  const normalizedModel = Array.isArray(requestedModel)
+    ? String(requestedModel.at(-1) ?? '').trim() || ''
+    : typeof requestedModel !== 'string'
+      ? String(requestedModel ?? '').trim() || ''
+      : requestedModel.trim() || '';
+
   if (config?.getExperimentalDynamicModelConfiguration?.() === true) {
-    const resolved = config.modelConfigService.resolveModelId(requestedModel, {
+    const resolved = config.modelConfigService.resolveModelId(normalizedModel, {
       useGemini3_1,
       useGemini3_1FlashLite,
       useCustomTools: useCustomToolModel,
@@ -132,7 +139,7 @@ export function resolveModel(
   }
 
   let resolved: string;
-  switch (requestedModel) {
+  switch (normalizedModel) {
     case PREVIEW_GEMINI_MODEL:
     case PREVIEW_GEMINI_MODEL_AUTO:
     case GEMINI_MODEL_ALIAS_AUTO:
@@ -161,7 +168,7 @@ export function resolveModel(
       break;
     }
     default: {
-      resolved = requestedModel;
+      resolved = normalizedModel;
       break;
     }
   }