API Versioning

Core Principles

1. Version from day one — Adding versioning later is painful. Start with a version in the URL even if you only have v1.
2. URL segment versioning is the default — /api/v1/orders is the most discoverable and cache-friendly strategy.
3. Never break existing versions — Add a new version for breaking changes. Deprecate the old version with a timeline.
4. Version the API, not individual endpoints — All endpoints in a version group share the same version number.

Patterns

Setup with Asp.Versioning

// Program.cs
builder.Services.AddApiVersioning(options =>
{
    options.DefaultApiVersion = new ApiVersion(1, 0);
    options.AssumeDefaultVersionWhenUnspecified = true;
    options.ReportApiVersions = true;
    options.ApiVersionReader = new UrlSegmentApiVersionReader();
})
.AddApiExplorer(options =>
{
    options.GroupNameFormat = "'v'VVV";
    options.SubstituteApiVersionInUrl = true;
});

URL Segment Versioning (Recommended)

var v1 = app.NewApiVersionSet()
    .HasApiVersion(new ApiVersion(1, 0))
    .Build();

var v2 = app.NewApiVersionSet()
    .HasApiVersion(new ApiVersion(2, 0))
    .Build();

app.MapGroup("/api/v{version:apiVersion}/orders")
    .WithApiVersionSet(v1)
    .WithTags("Orders")
    .MapOrderEndpointsV1();

app.MapGroup("/api/v{version:apiVersion}/orders")
    .WithApiVersionSet(v2)
    .WithTags("Orders")
    .MapOrderEndpointsV2();

Header Versioning (Alternative)

options.ApiVersionReader = new HeaderApiVersionReader("X-Api-Version");

// Client sends: X-Api-Version: 2.0

Deprecating a Version

var v1 = app.NewApiVersionSet()
    .HasDeprecatedApiVersion(new ApiVersion(1, 0))
    .HasApiVersion(new ApiVersion(2, 0))
    .Build();

// Response headers will include: api-deprecated-versions: 1.0

Version-Specific Endpoint Groups

public static class OrderEndpointsV1
{
    public static RouteGroupBuilder MapOrderEndpointsV1(this RouteGroupBuilder group)
    {
        group.MapGet("/{id:guid}", GetOrderV1);
        group.MapPost("/", CreateOrderV1);
        return group;
    }

    private static async Task<Results<Ok<OrderResponseV1>, NotFound>> GetOrderV1(
        Guid id, ISender sender, CancellationToken ct)
    {
        // V1 response shape
        var result = await sender.Send(new GetOrder.Query(id), ct);
        return result.IsSuccess
            ? TypedResults.Ok(result.Value.ToV1())
            : TypedResults.NotFound();
    }
}

public static class OrderEndpointsV2
{
    public static RouteGroupBuilder MapOrderEndpointsV2(this RouteGroupBuilder group)
    {
        group.MapGet("/{id:guid}", GetOrderV2);
        group.MapPost("/", CreateOrderV2);
        return group;
    }

    private static async Task<Results<Ok<OrderResponseV2>, NotFound>> GetOrderV2(
        Guid id, ISender sender, CancellationToken ct)
    {
        // V2 response shape — includes new fields
        var result = await sender.Send(new GetOrder.Query(id), ct);
        return result.IsSuccess
            ? TypedResults.Ok(result.Value.ToV2())
            : TypedResults.NotFound();
    }
}

Anti-patterns

Don't Version Individual Endpoints

// BAD — inconsistent versioning within a group
app.MapGet("/api/v1/orders", ListOrdersV1);
app.MapGet("/api/v2/orders/{id}", GetOrderV2); // V2 only for this endpoint?

// GOOD — version the entire group
app.MapGroup("/api/v1/orders").MapOrderEndpointsV1();
app.MapGroup("/api/v2/orders").MapOrderEndpointsV2();

Don't Use Query String Versioning as Default

// BAD for REST APIs — version hidden in query string, not cache-friendly
GET /api/orders?api-version=2.0

// GOOD — version in URL, discoverable and cacheable
GET /api/v2/orders

Decision Guide

Scenario	Recommendation
New public API	URL segment versioning from day one
Internal API between services	Header versioning (cleaner URLs)
Breaking response shape change	New version
Adding new optional fields	Same version (backwards compatible)
Deprecating a version	Mark deprecated, set sunset date, document migration path