fix(config): don't rewrite explicit gemini-2.5-flash model selection

Issue_29213.md

@@ -0,0 +1,105 @@
+# Unexpected model resolution: gemini-2.5-flash is mapped to gemini-3.5-flash
+
+### What happened?
+
+## `--model gemini-2.5-flash` is resolved to `gemini-3.5-flash` when using Vertex AI
+
+## Summary
+
+When invoking Gemini CLI with `--model gemini-2.5-flash`, the CLI appears to
+resolve the request to `gemini-3.5-flash` instead of the specified model.
+
+As a result, the request fails because my Vertex AI project does not have access
+to `gemini-3.5-flash`.
+
+## Environment
+
+- Gemini CLI version: **0.58.0**
+- Also reproduced on: **0.54.0**
+- Node.js: **v24.13.0**
+- Authentication: **Vertex AI (ADC)**
+
+## Reproduction
+
+Run:
+
+```bash
+echo "hello" | gemini \
+  --model gemini-2.5-flash \
+  --prompt ""
+```
+
+## Actual Behavior
+
+The request fails with the following error:
+
+```text
+Error when talking to Gemini API
+
+ModelNotFoundError: Publisher model
+`projects/.../locations/us-central1/publishers/google/models/gemini-3.5-flash`
+was not found or your project does not have access to it.
+```
+
+Although `gemini-2.5-flash` was specified, the CLI appears to send the request
+to `gemini-3.5-flash`.
+
+## Expected Behavior
+
+The CLI should send the request to `gemini-2.5-flash` when it is explicitly
+specified with:
+
+```bash
+--model gemini-2.5-flash
+```
+
+It should not resolve or rewrite the model name to `gemini-3.5-flash`.
+
+## Additional Information
+
+Using the same environment and authentication, the following command works as
+expected:
+
+```bash
+echo "hello" | gemini \
+  --model gemini-2.5-pro \
+  --prompt ""
+```
+
+### What did you expect to happen?
+
+It should not resolve or rewrite the model name to `gemini-3.5-flash`.
+
+### Client information
+
+<details>
+<summary>Client Information</summary>
+
+Run on Linux
+
+Run `gemini` to enter the interactive CLI, then run the `/about` command.
+
+```console
+> /about
+│ About Gemini CLI                                                                                                                                                                                                                                                                  │
+│                                                                                                                                                                                                                                                                                   │
+│ CLI Version                                                                                     0.54.0                                                                                                                                                                            │
+│ Git Commit                                                                                      a74b483d1                                                                                                                                                                         │
+│ Model                                                                                           gemini-2.5-pro                                                                                                                                                                    │
+│ Sandbox                                                                                         no sandbox                                                                                                                                                                        │
+│ OS                                                                                              linux                                                                                                                                                                             │
+│ Auth Method                                                                                     vertex-ai                                                                                                                                                                         │
+│ GCP Project                                                                                     ***                                                                                                                                                    │
+│ IDE Client                                                                                      VS Code                                                                                                                                                                           │
+│
+```
+
+</details>
+
+### Login information
+
+_No response_
+
+### Anything else we need to know?
+
+_No response_

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: [

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 +788,25 @@ 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 --model gemini-2.5-flash (issue #29213)', () => {
+    afterEach(() => {
+      // Restore the module-level defaults mutated by setFlashModels().
+      setFlashModels('gemini-3-flash-preview', 'gemini-2.5-flash');
+    });
+
+    it('should not rewrite an explicitly requested gemini-2.5-flash to gemini-3.5-flash once GA rollout has flipped the default flash pointer (legacy)', () => {
+      // Simulates hasGemini35FlashGAAccess() mutating the default/preview
+      // flash pointers once GA is available for the user's backend (e.g.
+      // Vertex AI), as happens before routing an explicit --model flag.
+      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 the flash alias to gemini-3.5-flash when useGemini3_5Flash is true, but leave explicitly requested concrete flash models untouched (dynamic)', () => {
     const mockDynamicConfig = {
       getExperimentalDynamicModelConfiguration: () => true,
       modelConfigService,
@@ -803,16 +822,19 @@ describe('resolveModel Gemini 3.5 Flash GA', () => {
         true,
       ),
     ).toBe('gemini-3.5-flash');
+    // An explicitly requested, pinned model ID must not be silently
+    // rewritten to a different model, even when the 3.5 flash GA alias
+    // upgrade is active. See https://github.com/google-gemini/gemini-cli/issues/29213
     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,

packages/core/src/config/models.ts

@@ -260,13 +260,14 @@ export function resolveModel(
 }
 
 function isFlashModel(model: string): boolean {
+  // No `endsWith('flash')` catch-all: that would also match explicitly
+  // pinned model IDs like 'gemini-2.5-flash' and silently upgrade them.
   return (
     model === DEFAULT_GEMINI_FLASH_MODEL ||
     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'
   );
 }