loaderStamp

What the symbol jar's bytes depend on: the loader's binpatches are baked into it, so its identity is part of the cache key. Bukkit's brand AND build ride along because a hybrid (Youer, Silkard) patches a four-figure number of MC classes on top of its base loader while reporting only the base loader's version — and its brand is a constant across its own builds, so the build half is the load-bearing one.