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Vitest unit testing AGENTS.md

AGENTS.md for unit and component tests with Vitest 3 and Testing Library in Vite/TypeScript projects.

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Save this as AGENTS.md in your repository root, commit it, and your agent will pick it up automatically. Edit the commands to match your project.

# AGENTS.md: Testing with Vitest

## Commands
- `npx vitest run` (CI) · `npx vitest <pattern>` (watch) · `npx vitest run --coverage`

## Where and how
- Tests sit next to code: `foo.ts` -> `foo.test.ts`. Component tests use `@testing-library/react` (or `/vue`, `/svelte`) with `@testing-library/user-event`.
- Arrange / act / assert, one behavior per test, and names that read as a sentence: `it('shows an error when the email is invalid')`.
- Query the DOM like a user: `getByRole`, `getByLabelText`. Use `findBy*` for async UI; avoid `container.querySelector`.
- Interact with `const user = userEvent.setup(); await user.click(...)`, not `fireEvent`.
- Mock at the boundary: network with MSW (`msw/node`), modules with `vi.mock` only for things you don't own. Reset with `vi.restoreAllMocks()` in `afterEach` (or `restoreMocks: true` in config).
- Time: `vi.useFakeTimers()` + `vi.setSystemTime()`; restore real timers after.
- Use `test.each` for tables of inputs.
- Snapshot tests only for small, stable output; review snapshot diffs instead of blindly updating with `-u`.

## Rules for agents
- When fixing a bug, write a failing test first, then the fix.
- Never delete, skip (`.skip`), or weaken an assertion to make a test pass. If a test looks wrong, say so.
- Don't test implementation details (internal state, private functions).

## Done
`npx vitest run` passes and new code paths have tests.