vitest

A test earns its place by failing when the behavior breaks, and at no other time. Three biases decide most calls:

• Mock the boundary, never the unit under test. Every mock asserts that the real thing cannot run here. Most such assertions are false, and each false one buys a test that keeps passing after the code is wrong.
• Prefer a fixture to a hook. test.extend initializes on demand, tears down in its own scope, and declares its dependencies in the signature. A beforeEach runs for every test in scope whether that test needs it or not.
• The installed major decides what may be written, not the shape recalled from an older one.

Version and Toolchain

Vitest 4 is the target. Read the installed version out of the lockfile before touching a config file — the configuration surface moved in 3.2, 4.0 and 4.1, and a key from the wrong one is silently ignored or loudly rejected.

Vitest 4.1 requires Node ^20 || ^22 || >=24 and Vite ^6 || ^7 || ^8. Vitest 4.0 shipped requiring Vite 6 and Node 20; 4.1 added Vite 8 and uses the installed vite rather than a bundled copy.

What changed, by version. Every removal below is rejected or ignored under Vitest 4; the 4.1 entries are additions and need 4.1 installed.

• 3.2 — workspace deprecated in favor of projects; fixture scopes (test, file, worker) added; AST-aware v8 coverage remapping added behind coverage.experimentalAstAwareRemapping
• 4.0 — workspace removed; poolOptions.* flattened to top level; maxThreads/maxForks became maxWorkers; singleThread/singleFork became maxWorkers: 1, isolate: false; minWorkers removed; coverage.all and coverage.extensions removed; environmentMatchGlobs and poolMatchGlobs removed; deps.external/deps.inline moved under server.deps; browser providers became separate packages taking a factory; the basic reporter and the pre-3.0 reporter hooks removed; the third-argument options object on test and describe removed
• 4.1 — aroundEach and aroundAll hooks; the test.extend builder pattern with type inference; test.override; test tags with --tags-filter; --detect-async-leaks; vi.defineHelper; vi.setTimerTickMode; experimental.viteModuleRunner

Read [${CLAUDE_SKILL_DIR}/references/configuration.md] when reconciling a config written against an older major — it carries the rename inventory entry by entry, each with the major that made the change.