MGS1017: magusfile is not a spell

A magusfile imported magus/spell/magusfile, or named magusfile in its project's "spells" list. Neither has any effect: magus binds that driver to every project it discovers.

A spell is a library of tool-native operations for one toolchain - the go spell exposes go-build/go-test/go-vet, the rust spell exposes cargo-build/cargo-clippy. The magusfile driver adapts no toolchain and contributes no operations. It is what makes the targets in the file you are writing runnable at all, which is not something an author opts into.

Resolution

Delete the import, and drop magusfile from the "spells" list:

 import "magus";
-import "magus/spell/magusfile";
 import "magus/spell/go";

 magus\project({
-    "spells": [magusfile, go],
+    "spells": [go],
 });

A project whose targets all come from its magusfile binds no toolchain at all, so it needs no list:

 magus\project({
-    "spells": [magusfile],
 });

Your targets keep running exactly as before. magus ls will stop reporting spell: magusfile for the project and report the toolchain it actually binds, or none.

Why this is an error rather than a warning

Binding the driver became implicit because listing it was never the author's job. That left the import and the list entry as no-ops which still taught every reader - and every generated example - that magusfile was a spell like go or buf. Failing loudly with the one-line fix ends that, rather than leaving a declaration that looks load-bearing and is not.