dotnet-minimal-apis

Minimal APIs are Microsoft's recommended approach for new ASP.NET Core HTTP API projects. They provide a lightweight, lambda-based programming model with first-class OpenAPI support, endpoint filters for cross-cutting concerns, and route groups for organization at scale.

Out of scope: API versioning strategies -- see [skill:dotnet-api-versioning]. Input validation frameworks and patterns -- see [skill:dotnet-input-validation]. Architectural patterns (vertical slices, CQRS, clean architecture) -- see [skill:dotnet-architecture-patterns]. Authentication and authorization implementation -- see [skill:dotnet-api-security]. OpenAPI document generation and customization -- see [skill:dotnet-openapi].

Cross-references: [skill:dotnet-architecture-patterns] for organizing large APIs, [skill:dotnet-input-validation] for request validation, [skill:dotnet-api-versioning] for versioning strategies, [skill:dotnet-openapi] for OpenAPI customization.

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Route Groups

Route groups organize related endpoints under a shared prefix, applying common configuration (filters, metadata, authorization) once. They replace repetitive chaining of MapGet/MapPost with shared prefixes.

var app = builder.Build();

// Group endpoints under /api/products with shared configuration
var products = app.MapGroup("/api/products")
    .WithTags("Products")
    .RequireAuthorization();

products.MapGet("/", async (AppDbContext db) =>
    TypedResults.Ok(await db.Products.ToListAsync()));

products.MapGet("/{id:int}", async (int id, AppDbContext db) =>
    await db.Products.FindAsync(id) is Product product
        ? TypedResults.Ok(product)
        : TypedResults.NotFound());

products.MapPost("/", async (CreateProductDto dto, AppDbContext db) =>
{
    var product = new Product { Name = dto.Name, Price = dto.Price };
    db.Products.Add(product);
    await db.SaveChangesAsync();
    return TypedResults.Created($"/api/products/{product.Id}", product);
});

products.MapDelete("/{id:int}", async (int id, AppDbContext db) =>
{
    if (await db.Products.FindAsync(id) is not Product product)
        return TypedResults.NotFound();

    db.Products.Remove(product);
    await db.SaveChangesAsync();
    return TypedResults.NoContent();
});

Nested Groups

Groups can be nested to compose prefixes and filters:

var api = app.MapGroup("/api")
    .AddEndpointFilter<RequestLoggingFilter>();

var v1 = api.MapGroup("/v1");
var products = v1.MapGroup("/products").WithTags("Products");
var orders = v1.MapGroup("/orders").WithTags("Orders");

// Registers as: GET /api/v1/products
products.MapGet("/", GetProducts);
// Registers as: POST /api/v1/orders
orders.MapPost("/", CreateOrder);

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Endpoint Filters

Endpoint filters provide a pipeline for cross-cutting concerns (logging, validation, authorization enrichment) similar to MVC action filters but specific to Minimal APIs.

IEndpointFilter Interface

public sealed class ValidationFilter<T>(IValidator<T> validator) : IEndpointFilter
    where T : class
{
    public async ValueTask<object?> InvokeAsync(
        EndpointFilterInvocationContext context,
        EndpointFilterDelegate next)
    {
        // Extract the argument of type T from the endpoint parameters
        var argument = context.Arguments
            .OfType<T>()
            .FirstOrDefault();

        if (argument is null)
            return TypedResults.BadRequest("Request body is required");

        var result = await validator.ValidateAsync(argument);
        if (!result.IsValid)
        {
            return TypedResults.ValidationProblem(
                result.ToDictionary());
        }

        return await next(context);
    }
}

Applying Filters

// Apply to a single endpoint
products.MapPost("/", CreateProduct)
    .AddEndpointFilter<ValidationFilter<CreateProductDto>>();

// Apply to an entire route group
var products = app.MapGroup("/api/products")
    .AddEndpointFilter<RequestLoggingFilter>();

// Inline filter using a lambda
products.MapGet("/{id:int}", GetProductById)
    .AddEndpointFilter(async (context, next) =>
    {
        var id = context.GetArgument<int>(0);
        if (id <= 0)
            return TypedResults.BadRequest("ID must be positive");

        return await next(context);
    });

Filter Execution Order

Filters execute in registration order (first registered = outermost). The endpoint handler runs after all filters pass:

Request -> Filter1 -> Filter2 -> Filter3 -> Handler
Response <- Filter1 <- Filter2 <- Filter3 <-

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TypedResults

Always use TypedResults (static factory) instead of Results (interface factory) for Minimal API return values. TypedResults returns concrete types that the OpenAPI metadata generator can inspect at build time, producing accurate response schemas automatically.

// PREFERRED: TypedResults -- concrete return types, auto-generates OpenAPI metadata
products.MapGet("/{id:int}", async Task<Results<Ok<Product>, NotFound>> (
    int id, AppDbContext db) =>
    await db.Products.FindAsync(id) is Product product
        ? TypedResults.Ok(product)
        : TypedResults.NotFound());

// AVOID: Results -- returns IResult, OpenAPI generator cannot infer response types
products.MapGet("/{id:int}", async (int id, AppDbContext db) =>
    await db.Products.FindAsync(id) is Product product
        ? Results.Ok(product)
        : Results.NotFound());

Union Return Types

Use Results<T1, T2, ...> to declare all possible response types for a single endpoint. This enables accurate OpenAPI documentation with multiple response codes:

products.MapPost("/", async Task<Results<Created<Product>, ValidationProblem, Conflict>> (
    CreateProductDto dto, AppDbContext db) =>
{
    if (await db.Products.AnyAsync(p => p.Sku == dto.Sku))
        return TypedResults.Conflict();

    var product = new Product { Name = dto.Name, Sku = dto.Sku, Price = dto.Price };
    db.Products.Add(product);
    await db.SaveChangesAsync();
    return TypedResults.Created($"/api/products/{product.Id}", product);
});

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OpenAPI 3.1 Integration

.NET 10 adds built-in OpenAPI 3.1 support via Microsoft.AspNetCore.OpenApi. Minimal APIs generate OpenAPI metadata from TypedResults, parameter bindings, and attributes automatically.

builder.Services.AddOpenApi();

var app = builder.Build();

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

Enriching Metadata

products.MapGet("/{id:int}", GetProductById)
    .WithName("GetProductById")
    .WithSummary("Get a product by its ID")
    .WithDescription("Returns the product details for the specified ID, or 404 if not found.")
    .Produces<Product>(StatusCodes.Status200OK)
    .ProducesProblem(StatusCodes.Status404NotFound);

For advanced OpenAPI customization (document transformers, operation transformers, schema customization), see [skill:dotnet-openapi].

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Organization Patterns for Scale

As an API grows beyond a handful of endpoints, organize endpoints into separate static classes or extension methods.

Extension Method Pattern

// ProductEndpoints.cs
public static class ProductEndpoints
{
    public static RouteGroupBuilder MapProductEndpoints(this IEndpointRouteBuilder routes)
    {
        var group = routes.MapGroup("/api/products")
            .WithTags("Products");

        group.MapGet("/", GetAll);
        group.MapGet("/{id:int}", GetById);
        group.MapPost("/", Create);
        group.MapPut("/{id:int}", Update);
        group.MapDelete("/{id:int}", Delete);

        return group;
    }

    private static async Task<Ok<List<Product>>> GetAll(AppDbContext db) =>
        TypedResults.Ok(await db.Products.ToListAsync());

    private static async Task<Results<Ok<Product>, NotFound>> GetById(
        int id, AppDbContext db) =>
        await db.Products.FindAsync(id) is Product p
            ? TypedResults.Ok(p)
            : TypedResults.NotFound();

    private static async Task<Created<Product>> Create(
        CreateProductDto dto, AppDbContext db)
    {
        var product = new Product { Name = dto.Name, Price = dto.Price };
        db.Products.Add(product);
        await db.SaveChangesAsync();
        return TypedResults.Created($"/api/products/{product.Id}", product);
    }

    private static async Task<Results<NoContent, NotFound>> Update(
        int id, UpdateProductDto dto, AppDbContext db)
    {
        var product = await db.Products.FindAsync(id);
        if (product is null) return TypedResults.NotFound();

        product.Name = dto.Name;
        product.Price = dto.Price;
        await db.SaveChangesAsync();
        return TypedResults.NoContent();
    }

    private static async Task<Results<NoContent, NotFound>> Delete(
        int id, AppDbContext db)
    {
        var product = await db.Products.FindAsync(id);
        if (product is null) return TypedResults.NotFound();

        db.Products.Remove(product);
        await db.SaveChangesAsync();
        return TypedResults.NoContent();
    }
}

// Program.cs
app.MapProductEndpoints();
app.MapOrderEndpoints();
app.MapCustomerEndpoints();

Carter Library

For projects that prefer auto-discovery of endpoint modules, the Carter library provides an ICarterModule interface:

// <PackageReference Include="Carter" Version="8.*" />
public sealed class ProductModule : ICarterModule
{
    public void AddRoutes(IEndpointRouteBuilder app)
    {
        var group = app.MapGroup("/api/products").WithTags("Products");

        group.MapGet("/", async (AppDbContext db) =>
            TypedResults.Ok(await db.Products.ToListAsync()));

        group.MapGet("/{id:int}", async (int id, AppDbContext db) =>
            await db.Products.FindAsync(id) is Product p
                ? TypedResults.Ok(p)
                : TypedResults.NotFound());
    }
}

// Program.cs
builder.Services.AddCarter();
var app = builder.Build();
app.MapCarter(); // Auto-discovers and registers all ICarterModule implementations

Vertical Slice Organization

For projects using vertical slice architecture (see [skill:dotnet-architecture-patterns]), each feature owns its endpoints, handlers, and models in a single directory:

Features/
  Products/
    GetProducts.cs       # Endpoint + handler + response DTO
    CreateProduct.cs     # Endpoint + handler + request/response DTOs
    UpdateProduct.cs
    DeleteProduct.cs
    ProductEndpoints.cs  # Route group registration

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JSON Configuration

Minimal APIs use System.Text.Json by default. Configure JSON options globally for all Minimal API endpoints:

// ConfigureHttpJsonOptions applies to Minimal APIs ONLY, not MVC controllers
builder.Services.ConfigureHttpJsonOptions(options =>
{
    options.SerializerOptions.PropertyNamingPolicy = JsonNamingPolicy.CamelCase;
    options.SerializerOptions.DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull;
    options.SerializerOptions.Converters.Add(new JsonStringEnumConverter());
});

Gotcha: ConfigureHttpJsonOptions configures JSON serialization for Minimal APIs only. MVC controllers use a separate pipeline -- configure via builder.Services.AddControllers().AddJsonOptions(...). Mixing them up has no effect.

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Parameter Binding

Minimal APIs bind parameters from route, query, headers, body, and DI automatically based on type and attribute annotations.

// Route parameter (from URL segment)
app.MapGet("/products/{id:int}", (int id) => ...);

// Query string
app.MapGet("/products", ([FromQuery] int page, [FromQuery] int pageSize) => ...);

// Header
app.MapGet("/products", ([FromHeader(Name = "X-Correlation-Id")] string correlationId) => ...);

// Body (JSON deserialized)
app.MapPost("/products", (CreateProductDto dto) => ...);

// DI-injected services (resolved automatically)
app.MapGet("/products", (AppDbContext db, ILogger<Program> logger) => ...);

// AsParameters: bind a complex object from multiple sources
app.MapGet("/products", ([AsParameters] ProductQuery query) => ...);

public record ProductQuery(
    [FromQuery] int Page = 1,
    [FromQuery] int PageSize = 20,
    [FromQuery] string? SortBy = null);

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Agent Gotchas

1. Do not use Results when TypedResults is available -- Results.Ok(value) returns IResult and the OpenAPI generator cannot infer response schemas. Use TypedResults.Ok(value) to enable automatic schema generation.
2. Do not forget ConfigureHttpJsonOptions only applies to Minimal APIs -- MVC controllers need .AddControllers().AddJsonOptions() separately.
3. Do not apply validation logic inline in every endpoint -- use endpoint filters or cross-reference [skill:dotnet-input-validation] for centralized validation patterns.
4. Do not register filters in the wrong order -- first-registered filter is outermost. Put broad filters (logging) first, specific filters (validation) closer to the handler.
5. Do not put all endpoints in Program.cs -- organize into extension method classes or Carter modules once you have more than a handful of endpoints.

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Prerequisites

• .NET 8.0+ (LTS baseline for Minimal APIs with route groups and endpoint filters)
• .NET 10.0 for built-in OpenAPI 3.1, SSE, and built-in validation support
• Microsoft.AspNetCore.OpenApi for OpenAPI document generation
• Carter (optional) for auto-discovery endpoint modules

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Knowledge Sources

Minimal API patterns in this skill are grounded in guidance from:

• David Fowler -- AspNetCoreDiagnosticScenarios (github.com/davidfowl/AspNetCoreDiagnosticScenarios). Authoritative source on ASP.NET Core request pipeline design, middleware best practices, and diagnostic anti-patterns.

These sources inform the patterns and rationale presented above. This skill does not claim to represent or speak for any individual.

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References

• Minimal APIs Overview
• Route Groups
• Endpoint Filters
• OpenAPI in ASP.NET Core
• Carter Library