What Is a Tool Cache?
A tool cache is a directory on a CI runner that holds extracted, ready-to-use versions of languages and tools such as Node, Python, or Go. A setup action looks here first and adds a matching version to the path instead of downloading and unpacking it. It makes toolchain setup near-instant when the requested version is already present.
Why it matters
Downloading and extracting a runtime on every job adds avoidable seconds or minutes. A populated tool cache turns setup into a path edit. On managed runners, baking common versions into the image keeps the tool cache warm so jobs skip the download entirely.
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