A backend developer agent handles server-side implementation work: writing business logic, designing service boundaries, and maintaining the data layer. In a fleet, it picks up tickets marked ready and produces pull requests without waiting for a human to start the work.
A role-specific prompt matters because backend work has strong conventions: how your team structures service layers, which error types get wrapped vs. surfaced, whether you use repository pattern or active record. A generic coding agent ignores those contracts; a backend agent tuned to your codebase follows them from the first commit.