How far into application code should FastAPI `Depends` reach?

Keeps FastAPI dependency resolution at transport and composition boundaries while application services use explicit dependencies.

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Depends should normally stay in path operations, dependency functions, and the composition layer that adapts HTTP to an application use case. Those boundary functions can resolve the authenticated actor, repository, or service and then call ordinary Python code with explicit arguments. I do not put Depends defaults inside domain entities or application-service methods. Keeping the boundary narrow makes the same use case callable from tests, background jobs, scripts, or another transport without asking FastAPI to resolve it.

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FastAPI can assemble a request, but application code should not need the framework to express its collaborators.
Depends should normally stay in path operations, dependency functions, and the composition layer that adapts HTTP to an application use case. Those boundary functions can resolve the authenticated actor, repository, or service and then call ordinary Python code with explicit arguments. I do not put Depends defaults inside domain entities or application-service methods. Keeping the boundary narrow makes the same use case callable from tests, background jobs, scripts, or another transport without asking FastAPI to resolve it.
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A small dependency can translate framework context into an application value, such as turning a bearer credential into an or opening a unit of work. The use case then declares that value or collaborator in its constructor or method signature. This still provides convenient request wiring without spreading HTTP exceptions, request objects, and hidden resolution through the codebase. The trade-off is a little explicit composition, which is usually cheaper than framework-coupled tests and surprising runtime lookup.

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Details

Adding Depends defaults to domain or service-layer methods.
Raising transport-specific exceptions from reusable business logic.
Using dependency overrides as the only way to test a use case.
Creating a second hidden object graph alongside explicit construction.

How far into application code should FastAPI `Depends` reach?

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