From 8a72268b509bb2fb5715d26bb0c79260e8d09935 Mon Sep 17 00:00:00 2001
From: Hariom Jangra <hariomjangra999@gmail.com>
Date: Sun, 6 Sep 2026 03:08:26 +0530
Subject: [PATCH] fix(config): prevent rewriting explicitly pinned flash models

Do not match broad endsWith('flash') in isFlashModel() and remove the auto-upgrade rule for gemini-2.5-flash in defaultModelConfigs, ensuring explicitly pinned models are not rewritten to gemini-3.5-flash.

Fixes #29213
---
 .../core/src/config/defaultModelConfigs.ts    |  6 -----
 packages/core/src/config/models.test.ts       | 22 +++++++++++++++----
 packages/core/src/config/models.ts            |  3 +--
 3 files changed, 19 insertions(+), 12 deletions(-)

diff --git a/packages/core/src/config/defaultModelConfigs.ts b/packages/core/src/config/defaultModelConfigs.ts
index 056b91b351d..0fba322e8b8 100644
--- a/packages/core/src/config/defaultModelConfigs.ts
+++ b/packages/core/src/config/defaultModelConfigs.ts
@@ -492,12 +492,6 @@ export const DEFAULT_MODEL_CONFIGS: ModelConfigServiceConfig = {
         },
       ],
     },
-    'gemini-2.5-flash': {
-      default: 'gemini-2.5-flash',
-      contexts: [
-        { condition: { useGemini3_5Flash: true }, target: 'gemini-3.5-flash' },
-      ],
-    },
     'gemini-3-pro-preview': {
       default: 'gemini-3-pro-preview',
       contexts: [
diff --git a/packages/core/src/config/models.test.ts b/packages/core/src/config/models.test.ts
index 63185f77614..1bf9d2a2c86 100644
--- a/packages/core/src/config/models.test.ts
+++ b/packages/core/src/config/models.test.ts
@@ -4,7 +4,7 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
-import { describe, it, expect } from 'vitest';
+import { describe, it, expect, afterEach } from 'vitest';
 import {
   resolveModel,
   resolveClassifierModel,
@@ -19,6 +19,7 @@ import {
   DEFAULT_GEMINI_FLASH_MODEL,
   DEFAULT_GEMINI_3_5_FLASH_MODEL,
   DEFAULT_GEMINI_FLASH_LITE_MODEL,
+  setFlashModels,
   supportsMultimodalFunctionResponse,
   GEMINI_MODEL_ALIAS_PRO,
   GEMINI_MODEL_ALIAS_FLASH,
@@ -787,7 +788,20 @@ describe('resolveModel Gemini 3.5 Flash GA', () => {
     ).toBe(PREVIEW_GEMINI_FLASH_MODEL);
   });
 
-  it('should resolve all but preview flash models to gemini-3.5-flash when useGemini3_5Flash is true (dynamic)', () => {
+  describe('explicit gemini-2.5-flash', () => {
+    afterEach(() => {
+      setFlashModels('gemini-3-flash-preview', 'gemini-2.5-flash');
+    });
+
+    it('should not rewrite explicitly requested gemini-2.5-flash to gemini-3.5-flash', () => {
+      setFlashModels('gemini-3.5-flash', 'gemini-3.5-flash');
+      expect(
+        resolveModel('gemini-2.5-flash', false, false, true, undefined, true),
+      ).toBe('gemini-2.5-flash');
+    });
+  });
+
+  it('should resolve flash alias to gemini-3.5-flash but leave concrete models untouched (dynamic)', () => {
     const mockDynamicConfig = {
       getExperimentalDynamicModelConfiguration: () => true,
       modelConfigService,
@@ -805,14 +819,14 @@ describe('resolveModel Gemini 3.5 Flash GA', () => {
     ).toBe('gemini-3.5-flash');
     expect(
       resolveModel(
-        DEFAULT_GEMINI_FLASH_MODEL,
+        'gemini-2.5-flash',
         false,
         false,
         true,
         mockDynamicConfig,
         true,
       ),
-    ).toBe('gemini-3.5-flash');
+    ).toBe('gemini-2.5-flash');
     expect(
       resolveModel(
         PREVIEW_GEMINI_FLASH_MODEL,
diff --git a/packages/core/src/config/models.ts b/packages/core/src/config/models.ts
index dd6506b837c..5f74a811d6b 100644
--- a/packages/core/src/config/models.ts
+++ b/packages/core/src/config/models.ts
@@ -265,8 +265,7 @@ function isFlashModel(model: string): boolean {
     model === PREVIEW_GEMINI_FLASH_MODEL ||
     model === DEFAULT_GEMINI_3_5_FLASH_MODEL ||
     model === SECONDARY_GEMINI_3_5_FLASH_MODEL ||
-    model === 'flash' ||
-    model.endsWith('flash')
+    model === 'flash'
   );
 }