2 files changed, 254 insertions, 0 deletions
diff --git a/packages/cli/src/utils/modelCheck.test.ts b/packages/cli/src/utils/modelCheck.test.ts
new file mode 100644
index 00000000..3b1cded8
--- /dev/null
+++ b/packages/cli/src/utils/modelCheck.test.ts
@@ -0,0 +1,179 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest';
+import {
+  getEffectiveModel,
+  type EffectiveModelCheckResult,
+} from './modelCheck.js';
+import {
+  DEFAULT_GEMINI_MODEL,
+  DEFAULT_GEMINI_FLASH_MODEL,
+} from '../config/config.js';
+
+// Mock global fetch
+global.fetch = vi.fn();
+
+// Mock AbortController
+const mockAbort = vi.fn();
+global.AbortController = vi.fn(() => ({
+  signal: { aborted: false }, // Start with not aborted
+  abort: mockAbort,
+  // eslint-disable-next-line @typescript-eslint/no-explicit-any
+})) as any;
+
+describe('getEffectiveModel', () => {
+  const apiKey = 'test-api-key';
+
+  beforeEach(() => {
+    vi.useFakeTimers();
+    vi.clearAllMocks();
+    // Reset signal for each test if AbortController mock is more complex
+    global.AbortController = vi.fn(() => ({
+      signal: { aborted: false },
+      abort: mockAbort,
+      // eslint-disable-next-line @typescript-eslint/no-explicit-any
+    })) as any;
+  });
+
+  afterEach(() => {
+    vi.restoreAllMocks();
+    vi.useRealTimers();
+  });
+
+  describe('when currentConfiguredModel is not DEFAULT_GEMINI_MODEL', () => {
+    it('should return the currentConfiguredModel and switched: false without fetching', async () => {
+      const customModel = 'custom-model-name';
+      const result = await getEffectiveModel(apiKey, customModel);
+      expect(result).toEqual({
+        effectiveModel: customModel,
+        switched: false,
+      });
+      expect(fetch).not.toHaveBeenCalled();
+    });
+  });
+
+  describe('when currentConfiguredModel is DEFAULT_GEMINI_MODEL', () => {
+    it('should switch to DEFAULT_GEMINI_FLASH_MODEL if fetch returns 429', async () => {
+      (fetch as vi.Mock).mockResolvedValueOnce({
+        ok: false,
+        status: 429,
+      });
+      const result: EffectiveModelCheckResult = await getEffectiveModel(
+        apiKey,
+        DEFAULT_GEMINI_MODEL,
+      );
+      expect(result).toEqual({
+        effectiveModel: DEFAULT_GEMINI_FLASH_MODEL,
+        switched: true,
+        originalModelIfSwitched: DEFAULT_GEMINI_MODEL,
+      });
+      expect(fetch).toHaveBeenCalledTimes(1);
+      expect(fetch).toHaveBeenCalledWith(
+        `https://generativelanguage.googleapis.com/v1beta/models/${DEFAULT_GEMINI_MODEL}:generateContent?key=${apiKey}`,
+        expect.any(Object),
+      );
+    });
+
+    it('should return DEFAULT_GEMINI_MODEL if fetch returns 200', async () => {
+      (fetch as vi.Mock).mockResolvedValueOnce({
+        ok: true,
+        status: 200,
+      });
+      const result = await getEffectiveModel(apiKey, DEFAULT_GEMINI_MODEL);
+      expect(result).toEqual({
+        effectiveModel: DEFAULT_GEMINI_MODEL,
+        switched: false,
+      });
+      expect(fetch).toHaveBeenCalledTimes(1);
+    });
+
+    it('should return DEFAULT_GEMINI_MODEL if fetch returns a non-429 error status (e.g., 500)', async () => {
+      (fetch as vi.Mock).mockResolvedValueOnce({
+        ok: false,
+        status: 500,
+      });
+      const result = await getEffectiveModel(apiKey, DEFAULT_GEMINI_MODEL);
+      expect(result).toEqual({
+        effectiveModel: DEFAULT_GEMINI_MODEL,
+        switched: false,
+      });
+      expect(fetch).toHaveBeenCalledTimes(1);
+    });
+
+    it('should return DEFAULT_GEMINI_MODEL if fetch throws a network error', async () => {
+      (fetch as vi.Mock).mockRejectedValueOnce(new Error('Network error'));
+      const result = await getEffectiveModel(apiKey, DEFAULT_GEMINI_MODEL);
+      expect(result).toEqual({
+        effectiveModel: DEFAULT_GEMINI_MODEL,
+        switched: false,
+      });
+      expect(fetch).toHaveBeenCalledTimes(1);
+    });
+
+    it('should return DEFAULT_GEMINI_MODEL if fetch times out', async () => {
+      // Simulate AbortController's signal changing and fetch throwing AbortError
+      const abortControllerInstance = {
+        signal: { aborted: false }, // mutable signal
+        abort: vi.fn(() => {
+          abortControllerInstance.signal.aborted = true; // Use abortControllerInstance
+        }),
+      };
+      (global.AbortController as vi.Mock).mockImplementationOnce(
+        () => abortControllerInstance,
+      );
+
+      (fetch as vi.Mock).mockImplementationOnce(
+        async ({ signal }: { signal: AbortSignal }) => {
+          // Simulate the timeout advancing and abort being called
+          vi.advanceTimersByTime(2000);
+          if (signal.aborted) {
+            throw new DOMException('Aborted', 'AbortError');
+          }
+          // Should not reach here in a timeout scenario
+          return { ok: true, status: 200 };
+        },
+      );
+
+      const resultPromise = getEffectiveModel(apiKey, DEFAULT_GEMINI_MODEL);
+      // Ensure timers are advanced to trigger the timeout within getEffectiveModel
+      await vi.advanceTimersToNextTimerAsync(); // Or advanceTimersByTime(2000) if more precise control is needed
+
+      const result = await resultPromise;
+
+      expect(mockAbort).toHaveBeenCalledTimes(0); // setTimeout calls controller.abort(), not our direct mockAbort
+      expect(abortControllerInstance.abort).toHaveBeenCalledTimes(1);
+      expect(result).toEqual({
+        effectiveModel: DEFAULT_GEMINI_MODEL,
+        switched: false,
+      });
+      expect(fetch).toHaveBeenCalledTimes(1);
+    });
+
+    it('should correctly pass API key and model in the fetch request', async () => {
+      (fetch as vi.Mock).mockResolvedValueOnce({ ok: true, status: 200 });
+      const specificApiKey = 'specific-key-for-this-test';
+      await getEffectiveModel(specificApiKey, DEFAULT_GEMINI_MODEL);
+
+      expect(fetch).toHaveBeenCalledWith(
+        `https://generativelanguage.googleapis.com/v1beta/models/${DEFAULT_GEMINI_MODEL}:generateContent?key=${specificApiKey}`,
+        expect.objectContaining({
+          method: 'POST',
+          headers: { 'Content-Type': 'application/json' },
+          body: JSON.stringify({
+            contents: [{ parts: [{ text: 'test' }] }],
+            generationConfig: {
+              maxOutputTokens: 1,
+              temperature: 0,
+              topK: 1,
+              thinkingConfig: { thinkingBudget: 0, includeThoughts: false },
+            },
+          }),
+        }),
+      );
+    });
+  });
+});
diff --git a/packages/cli/src/utils/modelCheck.ts b/packages/cli/src/utils/modelCheck.ts
new file mode 100644
index 00000000..1634656e
--- /dev/null
+++ b/packages/cli/src/utils/modelCheck.ts
@@ -0,0 +1,75 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import {
+  DEFAULT_GEMINI_MODEL,
+  DEFAULT_GEMINI_FLASH_MODEL,
+} from '../config/config.js';
+
+export interface EffectiveModelCheckResult {
+  effectiveModel: string;
+  switched: boolean;
+  originalModelIfSwitched?: string;
+}
+
+/**
+ * Checks if the default "pro" model is rate-limited and returns a fallback "flash"
+ * model if necessary. This function is designed to be silent.
+ * @param apiKey The API key to use for the check.
+ * @param currentConfiguredModel The model currently configured in settings.
+ * @returns An object indicating the model to use, whether a switch occurred,
+ *          and the original model if a switch happened.
+ */
+export async function getEffectiveModel(
+  apiKey: string,
+  currentConfiguredModel: string,
+): Promise<EffectiveModelCheckResult> {
+  if (currentConfiguredModel !== DEFAULT_GEMINI_MODEL) {
+    // Only check if the user is trying to use the specific pro model we want to fallback from.
+    return { effectiveModel: currentConfiguredModel, switched: false };
+  }
+
+  const modelToTest = DEFAULT_GEMINI_MODEL;
+  const fallbackModel = DEFAULT_GEMINI_FLASH_MODEL;
+  const endpoint = `https://generativelanguage.googleapis.com/v1beta/models/${modelToTest}:generateContent?key=${apiKey}`;
+  const body = JSON.stringify({
+    contents: [{ parts: [{ text: 'test' }] }],
+    generationConfig: {
+      maxOutputTokens: 1,
+      temperature: 0,
+      topK: 1,
+      thinkingConfig: { thinkingBudget: 0, includeThoughts: false },
+    },
+  });
+
+  const controller = new AbortController();
+  const timeoutId = setTimeout(() => controller.abort(), 2000); // 500ms timeout for the request
+
+  try {
+    const response = await fetch(endpoint, {
+      method: 'POST',
+      headers: { 'Content-Type': 'application/json' },
+      body,
+      signal: controller.signal,
+    });
+
+    clearTimeout(timeoutId);
+
+    if (response.status === 429) {
+      return {
+        effectiveModel: fallbackModel,
+        switched: true,
+        originalModelIfSwitched: modelToTest,
+      };
+    }
+    // For any other case (success, other error codes), we stick to the original model.
+    return { effectiveModel: currentConfiguredModel, switched: false };
+  } catch (_error) {
+    clearTimeout(timeoutId);
+    // On timeout or any other fetch error, stick to the original model.
+    return { effectiveModel: currentConfiguredModel, switched: false };
+  }
+}