agentmods.dev

vitest

A guide to using Vitest 4, a JavaScript and TypeScript test framework, for writing and maintaining automated tests. It covers test setup, asynchronous tests, mocks, snapshots, environments, assertions, and performance.

In plain words
What is it for?
Use it when writing or debugging Vitest tests, configuring test projects, reviewing changes, investigating flaky or slow tests, or improving test performance in continuous integration.
Why use it?
It helps avoid common testing mistakes, such as incorrect handling of asynchronous code, shared test state, or slow test runs. It also provides guidance for reviewing tests and moving from Jest to Vitest.

Skill for Claude CodeCodex

3 13d ago A scan 46 tokens original MIT
View the source file PyModel/react-frontend-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.

agentmods
npx agentmods add skills/pymodel/react-frontend-skills/vitest
Any agent
npx skills add PyModel/react-frontend-skills --skill vitest
Clone the repo
git clone --depth 1 https://github.com/PyModel/react-frontend-skills

Made for: Claude Code, Codex.

Per session 46 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.
When invoked 1,228 The whole file, excluding the scripts and references it only reads on demand.
Security scan A 0 findings. Scan, not verified.
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.

ModelPer sessionOnce invoked
Fable 5 $0.00046 $0.01228
Opus 5 $0.00023 $0.00614
Sonnet 5 $0.00009 $0.00246
Haiku 4.5 $0.00005 $0.00123

Measured 3d ago against content hash 353024b8238f, method: parsed. Prices are Anthropic first-party input rates as of 2026-08-30, 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 3d 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.

skills/vitest/SKILL.md · 121 lines

How it starts

The opening of the file, as written. The whole thing — 121 lines — stays where its author put it; the contents beside it link to each section on GitHub.

Vitest Best Practices

Vitest 4-compatible testing guide with 44 rules across 8 categories. Vitest 4 requires Node.js 20+ and Vite 6+ when used with Vite.

When to Apply

Reference these guidelines when:

  • Writing new Vitest tests
  • Debugging flaky or slow tests
  • Setting up test configuration
  • Reviewing test code in PRs
  • Migrating from Jest to Vitest
  • Optimizing CI/CD test performance

Jest-specific APIs and configuration outside documented migration differences are out of scope; use Jest's own documentation for tests that still run under Jest.

Rule Categories by Priority

Priority Category Impact Prefix
1 Async Patterns CRITICAL async-
2 Test Setup & Isolation CRITICAL setup-
3 Mocking Patterns HIGH mock-
4 Performance HIGH perf-
5 Snapshot Testing MEDIUM snap-
6 Environment MEDIUM env-
7 Assertions LOW-MEDIUM assert-
8 Test Organization LOW org-

Quick Reference

1. Async Patterns (CRITICAL)

  • async-await-assertions - Await async assertions to prevent false positives
  • async-return-promises - Return promises from test functions
  • async-fake-timers - Use fake timers for time-dependent code
  • async-waitfor-polling - Use vi.waitFor for async conditions
  • async-concurrent-expect - Use test context expect in concurrent tests
  • async-act-wrapper - Await user events to avoid act warnings
  • async-error-handling - Test async error handling properly

2. Test Setup & Isolation (CRITICAL)

  • setup-beforeeach-cleanup - Clean up state in afterEach hooks
  • setup-restore-mocks - Restore mocks after each test
  • setup-avoid-shared-state - Avoid shared mutable state between tests
  • setup-beforeall-expensive - Use beforeAll for expensive one-time setup
  • setup-reset-modules - Reset modules when testing module state
  • setup-test-factories - Use test factories for complex test data

3. Mocking Patterns (HIGH)

Read the full file on GitHub · 121 lines

Files

What ships with it

45 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.

  1. 3d ago First seen · 121 lines · 46 tokens per session scan A 353024b8238f

Subscribe to this mod's changes

vitest is a skill published in the GitHub repository PyModel/react-frontend-skills (3 stars, last pushed 13d ago), licensed MIT. It adds 46 tokens to every session and 1,228 once invoked, about $0.0002 per session on Opus 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.

TestingUnit testingagent-skillsai-agentsai-coding-assistantclaude-codecodexcursor
Related

Other skills, from other repositories

designing-tests

Designs and implements testing strategies for any codebase. Use when adding tests, improving coverage, setting up testing infrastructure, debugging test failures, or when asked about unit tests, integration tests, or E2E testing.

CloudAI-X/claude-workflow-v2 · 48 tokens

backend/testing-guide

后端测试编写指南，包括单元测试、集成测试和E2E测试的编写方法和最佳实践.

echoVic/boss-skill · 30 tokens

testing

Use when writing or reviewing Flutter/Dart tests (unit, widget, golden), fixing flaky tests, adding coverage, or choosing between unit and widget tests.

evanca/flutter-ai-rules · 33 tokens

unit-test-generator

Automatically generate unit tests based on source code, supporting multiple languages and testing frameworks.

JackyST0/awesome-agent-skills · 20 tokens

claude-code-qa

The complete QA skill for Claude Code — turn Claude into an expert QA engineer that picks the right test type, writes reliable Playwright, Cypress, and pytest tests, eliminates flaky tests, enforces coverage, and wires up CI. Claude Code QA testing done right.

PramodDutta/qaskills · 59 tokens

vc-web-testing

Web testing with Playwright, Vitest, k6. E2E/unit/integration/load/security/visual/a11y testing. Use for test automation, flakiness, Core Web Vitals, mobile gestures, cross-browser.

withkynam/vibecode-pro-max-kit · 51 tokens