Vitest

• Docs: if unsure about an API, fetch https://vitest.dev/llms.txt. Do not guess.
• Config file: use vitest.config.{ts,mts,js,mjs} for test / test.projects. Vitest-only-in-vite.config merges badly with multiple projects and Storybook.
• Gate: no vitest.config.* but vite.config has test: (or no Vitest file), ask before adding projects or Storybook whether to introduce vitest.config.ts and move test out of Vite.
• page.viewport(width, height): import from vitest/browser.
• test.projects for mixed node/browser and Storybook. Either extends: true (inherit merged root: Storybook Vitest example with mergeConfig(viteConfig, …)) or extends: './vite.config.ts' per project; put test + plugins on each project (svelte-5:storybook-vitest).
• Browser concurrency: vitest-browser-svelte/pure when you need no auto-cleanup.
• vitest-browser-svelte v3: render is async-only (await render(...)); usage in svelte-5:testing-svelte. v3 gives /pure full export parity with the main entry and exports the RenderResult type.
• Assertions: value, not presence. Presence-only assertions (toBeInTheDocument, not.toBe(''), toHaveBeenCalled) ship regressions. Every interaction test must compute an expected value from the action and assert equality. See references/assertions.md for the pattern, callback-payload rules, and how to tell a harness failure from a behavior failure.
• Locators: prefer accessible queries, avoid data-testid. data-testid is an a11y smell (tkdodo, 2025), adding a testid means the element has no accessible name, role, or text for users of AT either. Fix the a11y hole, not the test. Use page.getByRole(...), getByLabelText, getByText, getByPlaceholderText, getByTitle, getByAltText first. They mirror how screen readers find elements and fail when a11y regresses. Reach for data-testid only as a last resort: charting canvases, truly decorative content, or legacy markup you cannot change, and when you do, leave a comment explaining why no accessible selector worked.
• Upgrade: bump all @vitest/* together.
• browser.provider: import { playwright } from '@vitest/browser-playwright' to provider: playwright() (factory, not a string).
• Use import { page } from 'vitest/browser': not @vitest/browser/context.
• In test.projects, plugins / resolve / optimizeDeps belong inside each project, not only at root: projects do not inherit root plugins by default (extends: true inherits the merged root).
• optimizeDeps.exclude: Svelte 5 runes in .svelte.js (e.g. Melt UI) so vite-plugin-svelte handles them, not Vite's dependency pre-bundler (Rolldown since Vite 8, esbuild before).
• optimizeDeps.include: deps that trigger mid-test optimization (e.g. minisearch) to reduce flakes.
• Storybook: vitest.config.ts + storybookTest + browser playwright(): svelte-5:storybook-vitest, manual setup.