Recently our team ran into a production incident caused by a subtle interaction in NestJS’s dependency injection system.

The root cause was a request-scoped provider being injected into a service that was also used by a background worker. At first glance the code looked perfectly valid, but the runtime behavior was very different from what we expected.

This article explains:

• what happened
• why it happens in NestJS
• how dependency scope propagation works
• how to design services to avoid this class of issue