Contract-first forces the team to agree on data shapes before wild implementation. In NestJS, clean Swagger/OpenAPI becomes the source of truth for API consumers.
Document status codes, error shapes, and pagination from the start. These small details are what most often trigger integration bugs.
Generate frontend types from the spec when possible. Breaking changes must be visible in review, not in production at night.
Don't let documentation rot. Make updating OpenAPI part of the definition of done for every new endpoint.