API versioning strategies for ASP.NET Core. Covers Asp.Versioning library, URL segment, header, and query string strategies, version deprecation, and OpenAPI integration. Load this skill when adding versioning to an API, evolving an API with breaking changes, or when the user mentions "API version", "versioning", "v1/v2", "Asp.Versioning", "deprecation", "breaking change", or "backward compatibility".
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