APIs designed for desktop web clients on reliable broadband do not perform well on mobile without deliberate adaptation. Mobile clients operate under constraints that server-side developers often underestimate: variable and often poor network conditions, limited battery budgets where every radio transmission costs energy, smaller screens that need different data shapes than desktop, and the expectation of usable offline states that desktop web apps rarely require. Building an API that serves mobile clients well requires understanding these constraints and making design decisions that account for them.