Quick answer: Unity asmdef compile failing with circular reference between two assemblies? Domain reload doesn't show the cycle clearly - inspect via Window > Analysis > Assembly Definition.
Adding a new reference triggers asmdef circular reference. Unclear which two are at fault.
Use the asmdef analyzer
Window > Analysis > Assembly Definition. Visual graph; cycle is red.
Or extract shared types
Common types into a third assembly. Both reference; no cycle.
Audit asmdef graph
Per-asmdef refs. Cycles = restructure candidates.
“Circular references are an architecture smell. The cure is a third assembly.”
If your project has asmdef cycles, the extraction is mandatory. Document the new structure.