Vitest default threads pool leaks process.env across files
Migrating a Jest test suite to Vitest when some tests mutate process.env to exercise configuration-dependent code paths.
Jest's default worker isolation gives each test file a fresh process.env. Vitest defaults to pool: 'threads', which SHARES process.env across all test files in the same worker thread. Tests that do process.env.FOO = 'bar' leak into other tests — sometimes producing opposite pass/fail depending on file alphabetical order. Maddening to debug. Fix: set pool: 'forks' in vitest.config.ts to restore Jest's isolation. Forks are ~15% slower on typical suites but safe for env-mutation patterns.
Jest → Vitest migrations with env-mutating tests need pool: 'forks' in vitest config. The thread default is faster but leaks shared globals across tests in ways that bite weeks later.