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Register a provider for a type, then annotate a resolver parameter with that type. 
from fastql import Field, Query, provides


class UserRepository:
    def get(self, id: str) -> User | None: ...


@provides(UserRepository)
def provide_users(context):
    return context.user_repository


@Query
class QueryRoot:
    @Field
    def user(self, id: str, users: UserRepository) -> User | None:
        return users.get(id)
register_dependency(Type, provider) is the imperative form. Providers receive the execution context, may return awaitables, and are resolved once per execution before reuse by matching resolver parameters. Dependency types are removed from the public GraphQL argument list. Prefer concrete, purpose-specific service types so classification remains unambiguous.