Conventions

Naming

Public API namespace: mostly flat FastEndpoints (file-scoped / namespace-per-file patterns common).
Endpoint types often named Endpoint inside a feature namespace (see harness), not FooEndpoint.
Private fields: camelCase with _ prefix (editorconfig / ReSharper rules).
Parameters: camelCase; private constants: PascalCase.
Packages: FastEndpoints.<Area> NuGet IDs matching project folders.

C# latest language version; nullable enable; implicit usings.
Indent 4 spaces; LF; UTF-8 (.editorconfig).
Prefer existing partial-class splits for large types (Endpoint.*.cs).
XML docs generated for packages (GenerateDocumentationFile); suppress noise via project NoWarn.
Accessibility modifiers: editorconfig prefers not requiring explicit modifiers in some cases; follow neighboring code.
CS8618 suppressed for *Request.cs, *Response.cs, *Model*.cs, *Endpoint.cs patterns.

Request validation: FluentValidation via Validator<TRequest> / endpoint Validator configuration.
Validation failures flow through endpoint pipeline; error response shape controlled by Config.Errors (including ProblemDetails helpers).
Prefer framework hooks (OnBeforeValidate, send helpers) over ad-hoc MVC filters.

REPR: Request DTO + Endpoint + optional Response DTO; configure routes/verbs/auth in Configure().
Pre/post processors: IPreProcessor<TRequest>, IPostProcessor<TRequest,TResponse>.
Mappers: IMapper / request-response mappers; stateless (singleton lifetime expectation).
Commands/events: implement ICommand / IEvent (+ handlers); job queue builds on commands.
HTTP idempotency: services.AddIdempotency() + endpoint Idempotency() (output-cache policy). Distinct from job-queue IdempotencyKeyFor.
Feature flags: implement IFeatureFlag, call FeatureFlag<T>() in Configure().
Optional attributes: DontRegister, DontInject, HideFromDocs, RegisterService, etc. in Attributes package.

Global options: UseFastEndpoints(c => { c.Serializer…; c.Endpoints…; }) → FastEndpoints.Config (Cfg alias).
Endpoint ctor DI supported; property injection possible with attributes/options.
Manual resolve façade: Endpoint/Group/Mapper inherit ServiceResolverClient; Validator/BinderContext/HttpContext extensions forward via ServiceResolverClient.Forward (do not re-copy the eight Resolve methods).
Service registration generator can emit registration from attributes when generator is referenced.
Central package versions only in Directory.Packages.props; do not hardcode versions in csproj except intentional VersionOverride / constrained ranges already present.
FastEndpoints.AspVersioning: on .NET 10, AddVersioning(...) takes an optional trailing Action<VersionedOpenApiOptions> so consumers can configure the versioned OpenAPI documents from Asp.Versioning.OpenApi (that package is net10.0-only, hence the #if NET10_0_OR_GREATER guard and conditional package ref in Src/AspVersioning/).

Integration: AppFixture<TProgram> / collection fixtures from FastEndpoints.Testing.
Prefer typed HTTP helpers (POSTAsync<TEndpoint, TRequest, TResponse>) over raw URLs when endpoint types exist.
Mark flaky/heavy tests with [Trait("ExcludeInCiCd", "Yes")] to match CI filter.

Keep changes minimal and consistent with surrounding package boundaries.
Do not reintroduce NSwag/Swagger paths as default when OpenApi is the active harness path unless task is legacy package work.