The federation library is reported in graphitron’s voice, not thrown in the library’s

An author whose schema carries a federation @link, the directive that imports the Apollo federation spec’s directives into their schema, learns from graphitron whether the library that supplies those definitions is present and whether it supports the spec version their @link names. Today they learn it from the library instead: federation-graphql-java-support throws UnsupportedFederationVersionException for a spec version it does not implement, and that exception reaches the build with the library’s vocabulary rather than a sentence naming the @link, its file and line, and the versions that would work. The outcome is a rejection graphitron writes itself, consistent with how it reports every other schema problem, so an author who bumped an @link URL is told what to do about it.

The facts this rests on are cheap to establish and none of them are recorded today. Whether the library is on the classpath at all is a classpath question the census already answers for every other jar. Which version resolved is available from the artifact. Which spec versions that version implements is a property of the library’s own bundled SDL, and the two are related by a table the library effectively owns: v2.6 and v2.7 ship a duplicate @tag declaration that FederationLinkApplier.buildCollisionMessage already has to explain to the author, which is the shape of remediation this item generalises. Presence and version are separately worth having in the store, where a diagnostics reader can see them without re-resolving a classpath.

Scope note, since the neighbouring question is easy to conflate with this one. This is not about which source owns the injected definition rows or which refresh deletes them. That is settled elsewhere: the @link is the trigger, the library only supplies the text, and the rows follow the @link. This item is about telling someone what is wrong with their build, so the library’s identity is a fact to report and not a partition to attribute rows to.