Build-time OpenAPI generation addresses runtime-only generation by enabling pre-commit checks, PR contract reviews, and consistent documentation prior to deployment.

A Code-First approach to .NET ASP.NET Core enables auto-generated OpenAPI documents from metadata, including setting up Scalar UI at /scalar for interactive API exploration.

OpenAPI metadata can be enriched at route and handler levels with extensions like WithTags and WithSummary, and request-bound parameters should be documented with XML comments to populate the OpenAPI spec.

OpenAPI is essential for clear, interactive documentation and powers tools such as Scalar UI, Spectral, and Microsoft Kiota to aid testing, linting, and client generation.

To render Scalar UI and the OpenAPI JSON, include Scalar.AspNetCore and MapScalarApiReference() after MapOpenApi() in Program.cs; the UI appears at /scalar and the raw JSON at /openapi/v1.json.

Build-time generation supports pre-commit linting and PR contract reviews, with strategies to extend metadata via attributes and possibly use Spectral for validation and Problem Details for standardized errors.

The takeaway is that combining the OpenApi package, metadata extensions, XML comments, Scalar UI, and build-time generation makes the API contract transparent and verifiable across development.

A Code-First approach is demonstrated by enabling Scalar UI at /scalar to facilitate interactive API exploration from metadata.

Program.cs can detect build-time execution to conditionally skip runtime-configured services, ensuring reliable OpenAPI generation in CI/CD contexts.

For minimal APIs, install Microsoft.AspNetCore.OpenApi and configure AddOpenApi() and MapOpenApi() so the OpenAPI JSON is served at /openapi/v1.json.

XML documentation and OpenAPI metadata can be further enriched through C# attributes and custom transformers as needed.

XML documentation should target only model-bound parameters, and non-bound dependencies like DI services should not have param entries to avoid stray OpenAPI entries.