OpenAPI server URL no longer has a trailing slash when PathBase is empty

The server URL that ASP.NET Core writes into the generated OpenAPI document no longer ends with a trailing slash when the request PathBase is empty. Tooling or contract tests that string-compare servers[0].url need to be updated.

Version introduced

.NET 11

Previous behavior

Previously, OpenApiDocumentService.GetOpenApiServers() used UriHelper.BuildAbsolute(), which appends a trailing slash when both pathBase and path are empty. The generated OpenAPI document contained a servers entry with a trailing slash:

{
  "servers": [
    { "url": "https://example.com/" }
  ]
}

New behavior

Starting in ASP.NET Core 11, the trailing slash is stripped when HttpRequest.PathBase.HasValue is false. The generated OpenAPI document no longer includes the trailing slash:

{
  "servers": [
    { "url": "https://example.com" }
  ]
}

If PathBase has an explicit value (for example, /api), the URL is unchanged and continues to include the path base.

Type of breaking change

This change is a behavioral change.

Reason for change

The previous behavior didn't match the OpenAPI 3.1.0 specification examples, which use server URLs without a trailing slash. The change aligns the generated document with the spec and avoids surprises in downstream tooling that's sensitive to the difference. For more information, see dotnet/aspnetcore#64716.

Recommended action

If your build pipeline or contract tests compare the generated OpenAPI document against a fixture, update the fixture to remove the trailing slash. If you use a code generator or SDK that constructs request URIs by concatenating the server URL with operation paths, verify that it produces the same URLs as before. Most generators normalize repeated slashes, but a few don't.

If you intentionally want the trailing slash (for example, to match an existing fixture), use an OpenAPI document transformer to write it back:

builder.Services.AddOpenApi(options =>
{
    options.AddDocumentTransformer((document, context, ct) =>
    {
        foreach (var server in document.Servers ?? [])
        {
            if (!server.Url.EndsWith('/'))
            {
                server.Url += "/";
            }
        }
        return Task.CompletedTask;
    });
});

Affected APIs

• Microsoft.AspNetCore.OpenApi.OpenApiRouteHandlerBuilderExtensions
• Generated servers[0].url in the OpenAPI document produced by AddOpenApi.