openapi

name

openapi

description

Built-in OpenAPI support for .NET 10 applications. Covers document generation, transformers, TypedResults metadata, security schemes, XML comments, build-time generation, and multiple document support. No Swashbuckle needed. Load this skill when setting up API documentation, customizing OpenAPI output, adding security schemes to docs, or when the user mentions "OpenAPI", "AddOpenApi", "MapOpenApi", "document transformer", "operation transformer", "schema transformer", "OpenAPI 3.1", "API documentation", "Swashbuckle replacement", "Produces", "WithSummary", "WithDescription", "ProblemDetails", "Kiota", or "client generation".

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