diff --git a/frontend/src/__tests__/vitest-config.test.ts b/frontend/src/__tests__/vitest-config.test.ts
new file mode 100644
index 00000000..30bcd8d5
--- /dev/null
+++ b/frontend/src/__tests__/vitest-config.test.ts
@@ -0,0 +1,20 @@
+import { describe, expect, it } from 'vitest';
+import fs from 'fs';
+import path from 'path';
+
+// Regression guard for https://github.com/LabsCrypt/flowfi/issues/1281:
+// having both vitest.config.ts and vitest.config.mts at the same time makes
+// the resolved test config ambiguous (Vite/Vitest pick one based on
+// extension resolution order, which is not obvious to contributors and can
+// silently disable the "other" config's environment, setup file, and
+// coverage thresholds). Only one vitest.config.* should ever exist.
+describe('vitest config', () => {
+  it('has exactly one vitest.config.* file in the frontend package', () => {
+    const frontendRoot = path.resolve(__dirname, '..', '..');
+    const configFiles = fs
+      .readdirSync(frontendRoot)
+      .filter((f) => /^vitest\.config\.(ts|mts|cts|js|mjs|cjs)$/.test(f));
+
+    expect(configFiles).toEqual(['vitest.config.ts']);
+  });
+});
diff --git a/frontend/src/test-setup.ts b/frontend/src/test-setup.ts
deleted file mode 100644
index 3310558c..00000000
--- a/frontend/src/test-setup.ts
+++ /dev/null
@@ -1,30 +0,0 @@
-import '@testing-library/jest-dom';
-import { vi } from 'vitest';
-
-const localStorageMock = (() => {
-  let store: Record<string, string> = {};
-  return {
-    getItem: vi.fn((key: string) => store[key] || null),
-    setItem: vi.fn((key: string, value: string) => {
-      store[key] = value.toString();
-    }),
-    clear: vi.fn(() => {
-      store = {};
-    }),
-    removeItem: vi.fn((key: string) => {
-      delete store[key];
-    }),
-    key: vi.fn((index: number) => Object.keys(store)[index] || null),
-    length: 0,
-  };
-})();
-
-vi.stubGlobal('localStorage', localStorageMock);
-if (typeof window !== 'undefined') {
-  Object.defineProperty(window, 'localStorage', {
-    value: localStorageMock,
-    configurable: true,
-    writable: true,
-  });
-}
-
diff --git a/frontend/vitest.config.mts b/frontend/vitest.config.mts
deleted file mode 100644
index 1e79ea10..00000000
--- a/frontend/vitest.config.mts
+++ /dev/null
@@ -1,18 +0,0 @@
-import { defineConfig } from 'vitest/config';
-import react from '@vitejs/plugin-react';
-import path from 'path';
-
-export default defineConfig({
-  // @ts-expect-error type mismatch between vite versions in monorepo
-  plugins: [react()],
-  resolve: {
-    alias: {
-      '@': path.resolve(__dirname, './src'),
-    },
-  },
-  test: {
-    environment: 'jsdom',
-    globals: true,
-    setupFiles: ['./src/test-setup.ts'],
-  },
-});