How do you version an API's OpenAPI/Swagger documentation to show separate docs per version?

Using Swashbuckle or NSwag together with the versioning library's IApiVersionDescriptionProvider, you can generate a separate OpenAPI document per registered API version, configuring Swagger UI to expose a dropdown letting consumers switch between documented versions (e.g., /swagger/v1/swagger.json and /swagger/v2/swagger.json).

builder.Services.AddSwaggerGen();
builder.Services.ConfigureOptions<ConfigureSwaggerOptions>();

app.UseSwaggerUI(options => {
    foreach (var description in provider.ApiVersionDescriptions)
        options.SwaggerEndpoint($"/swagger/{description.GroupName}/swagger.json", description.GroupName.ToUpperInvariant());
});