An ASP.NET Core API can be fast and still be difficult to integrate with. Inconsistent errors, unclear versioning, leaky domain models, and poorly defined authorization create more downstream cost than the choice of controller or minimal API syntax.
Good API design starts with contracts and operational behavior. This guide outlines practical boundaries, validation, security, and compatibility decisions for APIs expected to serve multiple products or enterprise customers.
Design stable contracts
Define resources around business capabilities rather than database tables. Use explicit request and response models, predictable status codes, pagination, and a consistent error format. Generate an OpenAPI document, but review it as a consumer would instead of assuming generated documentation is automatically usable.
Separate transport from domain logic
Keep endpoint handlers focused on authentication context, validation, mapping, and response construction. Put business rules in application or domain services that can be tested without HTTP. This avoids duplicated behavior across APIs, background workers, and scheduled jobs.
Build security into each operation
Apply policy-based authorization at the resource and action level, not only at the route group. Validate identifiers against tenant context, limit request sizes, and protect expensive operations. Record security-relevant decisions without logging credentials or sensitive payloads.
Plan for change and failure
Use cancellation tokens, timeouts, idempotency for retried commands, and health checks that distinguish readiness from liveness. Define compatibility rules before publishing version one. A practical next step is reviewing one critical endpoint from contract through telemetry, then applying the discovered standards consistently.