[keithwinstein]

I think you're both right in a sense. Bazel doesn't (in general) prevent filesystem access, e.g. to library headers in /usr/include. If those headers change (maybe because a Debian package got upgraded or whatever), Bazel won't know it has to invalidate the build cache. I think the FAQ is still technically correct because upgrading the Debian package for a random library dependency counts as "chang[ing] the toolchain" in this context. But I don't think you'd call it hermetic by default.

Check out the previous discussion at https://news.ycombinator.com/item?id=23184843 and below:

> Under the hood there's a default auto-configured toolchain that finds whatever is installed locally in the system. Since it has no way of knowing what files an arbitrary "cc" might depend on, you lose hermeticity by using it.