OpenAPI

Core Principles

1. Built-in, not Swashbuckle — .NET 10 ships Microsoft.AspNetCore.OpenApi as the official, framework-maintained OpenAPI solution. Swashbuckle was removed from templates in .NET 9 and is no longer recommended.
2. TypedResults drive the schema — TypedResults.Ok<T>() automatically generates correct OpenAPI response schemas. Results.Ok() does not. Always use TypedResults.
3. Transformers over workarounds — Document, operation, and schema transformers compose cleanly. Use them for security schemes, global responses, and schema customization.
4. Metadata on every endpoint — Use .WithName(), .WithSummary(), .WithTags() on every endpoint. This metadata feeds directly into the OpenAPI spec and client generators.

Patterns

Basic Setup

var builder = WebApplication.CreateBuilder(args);
builder.Services.AddOpenApi();

var app = builder.Build();

if (app.Environment.IsDevelopment())
{
    app.MapOpenApi();  // Serves at /openapi/v1.json
}

Endpoint Metadata

group.MapPost("/", CreateOrder)
    .WithName("CreateOrder")
    .WithSummary("Create a new order")
    .WithDescription("Creates a new order for the specified customer.")
    .Produces<OrderResponse>(StatusCodes.Status201Created)
    .ProducesValidationProblem()
    .ProducesProblem(StatusCodes.Status500InternalServerError);

With TypedResults, response metadata is inferred automatically:

static async Task<Results<Created<OrderResponse>, ValidationProblem>> CreateOrder(
    CreateOrderRequest request, ISender sender, CancellationToken ct)
{
    var result = await sender.Send(new CreateOrder.Command(request), ct);
    return result.IsSuccess
        ? TypedResults.Created($"/api/orders/{result.Value.Id}", result.Value)
        : TypedResults.ValidationProblem(result.Errors);
}

Bearer Token Security Scheme

builder.Services.AddOpenApi(options =>
{
    options.AddDocumentTransformer<BearerSecuritySchemeTransformer>();
});

internal sealed class BearerSecuritySchemeTransformer(
    IAuthenticationSchemeProvider authSchemeProvider) : IOpenApiDocumentTransformer
{
    public async Task TransformAsync(OpenApiDocument document,
        OpenApiDocumentTransformerContext context, CancellationToken ct)
    {
        var schemes = await authSchemeProvider.GetAllSchemesAsync();
        if (!schemes.Any(s => s.Name == "Bearer"))
            return;

        document.Components ??= new OpenApiComponents();
        document.Components.SecuritySchemes = new Dictionary<string, IOpenApiSecurityScheme>
        {
            ["Bearer"] = new OpenApiSecurityScheme
            {
                Type = SecuritySchemeType.Http,
                Scheme = "bearer",
                BearerFormat = "JWT",
                In = ParameterLocation.Header
            }
        };

        foreach (var operation in document.Paths.Values.SelectMany(p => p.Operations))
        {
            operation.Value.Security ??= [];
            operation.Value.Security.Add(new OpenApiSecurityRequirement
            {
                [new OpenApiSecuritySchemeReference("Bearer", document)] = []
            });
        }
    }
}

Document Info Transformer

builder.Services.AddOpenApi(options =>
{
    options.AddDocumentTransformer((document, context, ct) =>
    {
        document.Info = new()
        {
            Title = "Checkout API",
            Version = "v1",
            Description = "API for processing orders and payments."
        };
        return Task.CompletedTask;
    });
});

Multiple OpenAPI Documents

builder.Services.AddOpenApi("v1");
builder.Services.AddOpenApi("internal", options =>
{
    options.AddDocumentTransformer<BearerSecuritySchemeTransformer>();
});

// Endpoints choose their document via WithGroupName
app.MapGet("/public", () => "Hello").WithGroupName("v1");
app.MapGet("/admin", () => "Secret").WithGroupName("internal");

Endpoints without .WithGroupName() appear in all documents.

XML Documentation Comments (.NET 10)

Enable in the project file — the source generator extracts <summary>, <param>, <response> tags automatically:

<PropertyGroup>
    <GenerateDocumentationFile>true</GenerateDocumentationFile>
</PropertyGroup>

/// <summary>Retrieves a project board by ID.</summary>
/// <param name="id">The project board ID.</param>
/// <response code="200">Returns the project board.</response>
/// <response code="404">Board not found.</response>
static async Task<Results<Ok<Board>, NotFound>> GetBoard(int id, AppDbContext db)
{
    var board = await db.Boards.FindAsync(id);
    return board is not null ? TypedResults.Ok(board) : TypedResults.NotFound();
}

XML comments on lambdas are not captured by the compiler. Use named methods.

Schema Transformer

options.AddSchemaTransformer((schema, context, ct) =>
{
    if (context.JsonTypeInfo.Type == typeof(decimal))
    {
        schema.Format = "decimal";
    }
    return Task.CompletedTask;
});

Per-Endpoint Operation Transformer (.NET 10)

app.MapGet("/old", () => "deprecated")
    .AddOpenApiOperationTransformer((operation, context, ct) =>
    {
        operation.Deprecated = true;
        return Task.CompletedTask;
    });

Build-Time Document Generation

<PackageReference Include="Microsoft.Extensions.ApiDescription.Server" Version="*" />
<PropertyGroup>
    <OpenApiDocumentsDirectory>.</OpenApiDocumentsDirectory>
</PropertyGroup>

The spec file is generated in the output directory during build.

YAML Endpoint (.NET 10)

app.MapOpenApi("/openapi/{documentName}.yaml");

Anti-patterns

Don't Use Swashbuckle for New Projects

// BAD — removed from .NET 9+ templates, maintenance concerns
builder.Services.AddSwaggerGen();
app.UseSwagger();
app.UseSwaggerUI();

// GOOD — built-in OpenAPI
builder.Services.AddOpenApi();
app.MapOpenApi();

Don't Use WithOpenApi() in .NET 10

// BAD — deprecated, produces ASPDEPR002 warning
app.MapGet("/", () => "hello").WithOpenApi(op => { op.Deprecated = true; return op; });

// GOOD — use per-endpoint operation transformer
app.MapGet("/", () => "hello")
    .AddOpenApiOperationTransformer((op, ctx, ct) =>
    {
        op.Deprecated = true;
        return Task.CompletedTask;
    });

Don't Use Untyped Results

// BAD — Results.Ok doesn't contribute to OpenAPI schema
static async Task<IResult> GetOrder(Guid id, AppDbContext db)
{
    var order = await db.Orders.FindAsync(id);
    return order is not null ? Results.Ok(order) : Results.NotFound();
}

// GOOD — TypedResults with union return type
static async Task<Results<Ok<Order>, NotFound>> GetOrder(Guid id, AppDbContext db)
{
    var order = await db.Orders.FindAsync(id);
    return order is not null ? TypedResults.Ok(order) : TypedResults.NotFound();
}

Don't Skip WithName on Endpoints

// BAD — client generators produce poor method names without operationId
group.MapGet("/{id:guid}", GetOrder);

// GOOD — operationId feeds into generated client method names
group.MapGet("/{id:guid}", GetOrder).WithName("GetOrder");

Don't Use OpenApiAny in .NET 10

// BAD — OpenApiAny types removed in Microsoft.OpenApi v2.x
schema.Example = new OpenApiString("2025-01-01");

// GOOD — use JsonNode from System.Text.Json.Nodes
schema.Example = JsonValue.Create("2025-01-01");

Decision Guide

Scenario	Recommendation
New API project	AddOpenApi() + MapOpenApi() (built-in)
API documentation UI	Scalar (MapScalarApiReference())
Security schemes in docs	Document transformer with IOpenApiDocumentTransformer
Response documentation	TypedResults with union return types
XML doc integration	<GenerateDocumentationFile>true</GenerateDocumentationFile>
Multiple API versions	Multiple AddOpenApi("v1") calls + WithGroupName()
Client code generation	Kiota (Microsoft recommended) or NSwag
Build-time spec	Microsoft.Extensions.ApiDescription.Server package
OpenAPI version	3.1 (default in .NET 10), force 3.0 if consumers require it
Per-endpoint customization	.AddOpenApiOperationTransformer() on the endpoint