ServiceComposer handles incoming requests asynchronously. Each handler is executed in a separate Task. Dependencies injected into handlers via DI are created by the root ServiceProvider. They are shared across more than one handler, depending on the lifecycle.
Given that handlers are executed in parallel when resources are shared by more than one handler, they must be thread-safe.
One option is to inject the root IServiceProvider instance in handlers, and create child scopes at execution time.