NestJS "A circular dependency has been detected" in CI

Two providers or modules depend on each other, so when Nest constructs the first, the second is still undefined. Nest warns about the cycle and injection resolves to undefined.

What this error means

Startup logs "A circular dependency has been detected inside X. Please, make sure that each side of a bidirectional relationship is decorated with forwardRef()." Injected dependencies then arrive as undefined.

NestJS

[Nest] WARN A circular dependency has been detected inside "CatsService". Please,
make sure that each side of a bidirectional relationship is decorated with "forwardRef()".
Nest cannot create the module instance.

Common causes

Two providers inject each other

CatsService needs UsersService and UsersService needs CatsService; neither can be constructed first without the other existing.

Two modules import each other

Module A imports Module B and Module B imports Module A, creating a module-level cycle Nest cannot order.

How to fix it

Wrap each side in forwardRef

Identify the two sides of the cycle from the warning.
Wrap the injected reference with forwardRef(() => Other) on both sides.
Do the same for imports when the cycle is between modules.

cats.service.ts

@Injectable()
export class CatsService {
  constructor(
    @Inject(forwardRef(() => UsersService))
    private users: UsersService,
  ) {}
}

Break the cycle by extracting shared code

Move the shared logic into a third provider both sides depend on, so the direct cycle disappears.

How to prevent it

Prefer a unidirectional dependency graph; extract shared logic to a common provider.
Reserve forwardRef for genuine bidirectional needs.
Compile the module graph in a CI test so cycles surface immediately.