File lists are options, not something to merge: mergeConfig concatenates arrays.
// vitest.config.ts
import { defineConfig, mergeConfig } from 'vitest/config';
import { vitestConfig } from '@helpers4/presets/vitest';
export default mergeConfig(
vitestConfig({ coverage: 100, include: ['src/**/*.test.ts'] }),
defineConfig({ test: { globals: false } })
);
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