From ff725dea41dd9ed6126b34ce5f394aa07a6e470e Mon Sep 17 00:00:00 2001
From: joshualitt <joshualitt@google.com>
Date: Wed, 19 Nov 2025 21:48:08 -0800
Subject: [PATCH] feat(core): Fix bug with incorrect model overriding. (#13477)

---
 .../src/routing/modelRouterService.test.ts    |  8 +++----
 .../core/src/routing/modelRouterService.ts    | 21 -------------------
 2 files changed, 4 insertions(+), 25 deletions(-)

diff --git a/packages/core/src/routing/modelRouterService.test.ts b/packages/core/src/routing/modelRouterService.test.ts
index da1b659d7c..f282103104 100644
--- a/packages/core/src/routing/modelRouterService.test.ts
+++ b/packages/core/src/routing/modelRouterService.test.ts
@@ -152,7 +152,7 @@ describe('ModelRouterService', () => {
       );
     });
 
-    it('should upgrade to Preview Model when preview features are enabled and model is 2.5 Pro', async () => {
+    it('should NOT upgrade to Preview Model when preview features are enabled and model is 2.5 Pro', async () => {
       vi.spyOn(mockCompositeStrategy, 'route').mockResolvedValue({
         model: DEFAULT_GEMINI_MODEL,
         metadata: { source: 'test', latencyMs: 0, reasoning: 'test' },
@@ -162,7 +162,7 @@ describe('ModelRouterService', () => {
 
       const decision = await service.route(mockContext);
 
-      expect(decision.model).toBe(PREVIEW_GEMINI_MODEL);
+      expect(decision.model).toBe(DEFAULT_GEMINI_MODEL);
     });
 
     it('should NOT upgrade to Preview Model when preview features are disabled', async () => {
@@ -215,7 +215,7 @@ describe('ModelRouterService', () => {
       expect(decision.model).toBe(DEFAULT_GEMINI_MODEL);
     });
 
-    it('should upgrade to Preview Model even if fallback mode is active (probing behavior)', async () => {
+    it('should NOT upgrade to Preview Model even if fallback mode is active (probing behavior)', async () => {
       vi.spyOn(mockCompositeStrategy, 'route').mockResolvedValue({
         model: DEFAULT_GEMINI_MODEL,
         metadata: { source: 'default', latencyMs: 0, reasoning: 'Default' },
@@ -225,7 +225,7 @@ describe('ModelRouterService', () => {
 
       const decision = await service.route(mockContext);
 
-      expect(decision.model).toBe(PREVIEW_GEMINI_MODEL);
+      expect(decision.model).toBe(DEFAULT_GEMINI_MODEL);
     });
   });
 });
diff --git a/packages/core/src/routing/modelRouterService.ts b/packages/core/src/routing/modelRouterService.ts
index 901c565cde..3898ff4100 100644
--- a/packages/core/src/routing/modelRouterService.ts
+++ b/packages/core/src/routing/modelRouterService.ts
@@ -5,10 +5,6 @@
  */
 
 import type { Config } from '../config/config.js';
-import {
-  PREVIEW_GEMINI_MODEL,
-  DEFAULT_GEMINI_MODEL,
-} from '../config/models.js';
 import type {
   RoutingContext,
   RoutingDecision,
@@ -66,23 +62,6 @@ export class ModelRouterService {
         this.config.getBaseLlmClient(),
       );
 
-      // Unified Preview Model Logic:
-      // If the decision is to use 'gemini-2.5-pro' and preview features are enabled,
-      // we attempt to upgrade to 'gemini-3.0-pro' (Preview Model).
-      if (
-        decision.model === DEFAULT_GEMINI_MODEL &&
-        this.config.getPreviewFeatures() &&
-        decision.metadata.source !== 'override'
-      ) {
-        // We ALWAYS attempt to upgrade to Preview Model here.
-        // If we are in fallback mode, the 'previewModelBypassMode' flag (handled in handler.ts/geminiChat.ts)
-        // will ensure we downgrade to 2.5 Pro for the actual API call if needed.
-        // This allows us to "probe" Preview Model periodically (i.e., every new request tries Preview Model first).
-        decision.model = PREVIEW_GEMINI_MODEL;
-        decision.metadata.source += ' (Preview Model)';
-        decision.metadata.reasoning += ' (Upgraded to Preview Model)';
-      }
-
       const event = new ModelRoutingEvent(
         decision.model,
         decision.metadata.source,