Book: TypeScript in Production — Tooling, Build, and Library Authoring Across Runtimes Also by me: The TypeScript Library — the 5-book collection My project: Hermes IDE | GitHub — an IDE for developers who ship with Claude Code and other AI coding tools Me: xgabriel.com | GitHub You've seen it. A library has two test scripts in the same package.json . One says bun test . The other says vitest run . The CI matrix runs both. The local dev loop runs whichever one the maintainer typed last. The README does not mention either. The reason there are two is that the library exports a function that does some HTTP work, and at some point a maintainer got tired of waiting for Vitest to spin up to re-check a one-line fix in a small unit suite. They tried bun test , it felt noticeably quicker, they kept it. Then a contributor opened a PR that mocked an ESM dependency the way Vitest's docs describe. It worked under Vitest.…