A set of instructions for configuring Vitest, a JavaScript and TypeScript test runner, including projects that run in Node.js or in a browser. It also covers browser testing with Playwright and Svelte.

Use it when editing Vitest configuration, separating Node and browser test projects, upgrading Vitest packages, or debugging browser tests.

It helps avoid configuration conflicts, flaky browser tests, and assertions that check only whether something exists instead of whether it has the right result.

vitest — Skill for Claude Code by fubits1/svelte-skills

Install

Getting it into your agent

One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.

Any agent

npx skills add fubits1/svelte-skills --skill vitest

Clone the repo

git clone --depth 1 https://github.com/fubits1/svelte-skills

Made for: Claude Code.

Or install svelte-5, the plugin that ships this one along with the rest of its 8 skills.

Wrote this? Show the measurements

A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.

README.md

[![agentmods](https://agentmods.dev/badge/skills/fubits1/svelte-skills/vitest/github.svg)](https://agentmods.dev/skills/fubits1/svelte-skills/vitest)

Your own site

<a href="https://agentmods.dev/skills/fubits1/svelte-skills/vitest"><img src="https://agentmods.dev/badge/skills/fubits1/svelte-skills/vitest/github.svg" alt="Measured on agentmods" height="20"></a>

Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.

Your own site · 80×15

<a href="https://agentmods.dev/skills/fubits1/svelte-skills/vitest"><img src="https://agentmods.dev/badge/skills/fubits1/svelte-skills/vitest.svg" alt="Reviewed on agentmods" width="80" height="20"></a>

Per session 49 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.

When invoked 1,821 The whole file, excluding the scripts and references it only reads on demand.

Security scan A 0 findings. A grade says what 26 rules found in the file — not that it is safe.

Origin original No closer match found in the catalogue.

Token cost

What it costs to keep this loaded

Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.

Model	Per session	Once invoked
Fable 5.1	$0.00049	$0.01821
Opus 5.5	$0.00020	$0.00728
Sonnet 5.5	$0.00010	$0.00364
Haiku 4.5	$0.00005	$0.00182

Measured 12d ago against content hash da0040a49e2d, method: parsed. Prices are Anthropic first-party input rates as of 2026-10-07, from the pricing page.

Security

Grade A, and why

vitest scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 12d ago.

A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.

Nothing flagged

None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.

plugins/svelte-5/skills/vitest/SKILL.md · 39 lines

How it starts

The opening of the file, as written. The whole thing — 39 lines — stays where its author put it; the contents beside it link to each section on GitHub.

Vitest

Config

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
Projects inherit the root (Vitest 5): inline test.projects get the root's plugins, resolve, optimizeDeps, and setupFiles; extends: false opts out. NEVER repeat an inherited root plugin inside a project: a second svelte() compiles components twice. Inheritance rules, sharedViteServer, tags: references/config-projects.md
Upgrade: bump all @vitest/* together. Vitest 4 to 5 breaking changes and post-upgrade cleanup: references/vitest-5-upgrade.md
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
rune_outside_svelte: a rune "is only available inside .svelte and .svelte.js/ts files"; in browser mode, until the page has loaded Svelte's client runtime (a component, vitest-browser-svelte or an import from svelte itself, e.g. tick), a $state call shows ReferenceError: $state is not defined instead (the form in a node project is not covered here). Cause: the rune sits in a .js/.ts file without the .svelte. infix (foo.svelte.ts and foo.svelte.test.ts have it); rename it, update its imports, and for a test file check which project's include and exclude the new name matches. On Vite 8, a dependency's file must end in .svelte.js/.svelte.ts; report one that does not to the library. More in svelte-5:testing-svelte troubleshooting
optimizeDeps.exclude does not fix rune_outside_svelte on Vite 8: the file name decides whether vite-plugin-svelte compiles the runes, pre-bundled or not (pre-bundling uses Rolldown since Vite 8, esbuild before)
optimizeDeps.include: deps that trigger mid-test optimization (e.g. minisearch) to reduce flakes; once you list one msw entry point, list every one you import, or narrow frames with frame.protocol (svelte-5:msw). Symptoms, fixes and optimizeDeps.entries: references/flake-hygiene.md
Storybook: vitest.config.ts + storybookTest + browser playwright(): svelte-5:storybook-vitest, manual setup
Reports and coverage: file-reporter paths go in test.outputFile (reporter options beat the CLI; the HTML reporter takes outputDir); coverage thresholds take flat metric keys, a Jest-style global key enforces nothing: references/config-projects.md

Read the full file on GitHub · 39 lines

Files

What ships with it

7 files beside SKILL.md in the same directory: the scripts, references and assets a skill reads on demand. Not counted in the per-session cost; read them before you install if any of them is executable.

Changes

What this file has done since we first saw it

Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.

12d ago Changed · +1 lines da0040a49e2d
1mo ago Changed · -119 lines scan C → A c3938939d1fa
2mo ago First seen · 157 lines · 49 tokens per session scan C 4a693f84f416

Subscribe to this mod's changes

vitest is a skill published in the GitHub repository fubits1/svelte-skills (10 stars, last pushed 12d ago), licensed MIT. It adds 49 tokens to every session and 1,821 once invoked, about $0.0002 per session on Opus 5.5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-31.

E2E testing Testing

vitest next to 6 skills from other repositories
Mod	Type	Stars	Last push	Licence	Scan	Tokens	Works with
vitestfubits1/svelte-skills	Skill	10	12d ago	MIT	A	49	Claude Code
verify-omnigentomnigent-ai/omnigent	Skill	11k	yesterday	Apache-2.0	A	100	Claude Code, Codex
test-masterJeffallan/claude-skills	Skill	12k	7d ago	MIT	A	104	Claude Code
eval-dataset-designbojieli/ai-agent-book	Skill	57k	29d ago	Apache-2.0	A	138	Claude Code, Codex
dogfoodcallstack/agent-device	Skill	5.3k	yesterday	MIT	A	55	Claude Code
cua-driverdavidondrej/skills	Skill	4.3k	3d ago	MIT	A	68	Claude Code, Codex
test-warp-uiwarpdotdev/warp	Skill	66k	yesterday	AGPL-3.0	A	55	Claude Code