API handlers should be lightweight and focused on request/response handling, not business logic. Extract specific parameters from requests and delegate complex operations to dedicated services rather than handling everything in the controller. Key principles: - Parse and extract only the specific data needed from requests - Avoid passing entire request objects to service methods - Delegate business logic to application services - Keep handlers focused on HTTP concerns (parsing, validation, response formatting) Example of what to avoid: ```typescript async reassignBooking(bookingUid: string, requestUser: UserWithProfile, request: Request) { // Handler doing too much work with entire request object } ``` Better approach: ```typescript async reassignBooking(bookingUid: string, requestUser: UserWithProfile, teamMemberEmail?: string) { // Extract specific parameter in handler, pass only what's needed const teamMemberEmail = req.query.teamMemberEmail; return this.bookingService.reassignBooking(bookingUid, requestUser, teamMemberEmail); } ``` This pattern improves testability, reduces coupling, and makes the API surface clearer by explicitly showing what data each operation requires.