# Playwright, Vitest, pytest

> End-to-end and unit tests that agents can run and fix on their own.

- Verdict: My default
- Vendor: Open source
- Layer: [Build and test](https://cloudlesslabs.com/stack/layers/dev.md). Job: Testing
- Last reviewed: 18 October 2026, by Nicolas Dao (Cloudless Labs, Sydney)
- Web page: https://cloudlesslabs.com/stack/testing/. The whole stack in one file: https://cloudlesslabs.com/stack/brief.md

Playwright for the browser, Vitest for JavaScript, pytest for Python. Codex drives Playwright through the complicated scenarios.

## Use it when

- Every project. Agents need tests to know when they're done.

## Pricing

Free.

## Instead, consider

- [Stryker mutation testing](https://cloudlesslabs.com/stack/stryker/index.md): Checking the tests themselves.

## Where it fits

- [Building with AI agents](https://cloudlesslabs.com/stack/use-cases/ai-dev.md): pick 3. Agents need tests to know when they're done.

## Related

- [Build and test](https://cloudlesslabs.com/stack/layers/dev.md): every tool in this layer, with its verdict
- [The whole stack in one file](https://cloudlesslabs.com/stack/brief.md): every use case and tool, for questions that span many
- [Map of every Markdown file on cloudlesslabs.com, with what each one answers](https://ask.cloudlesslabs.com)