Add example for skipping AllowAnonymous endpoints in OpenAPI docs (#36902)

* Add example for skipping AllowAnonymous endpoints in OpenAPI operation transformer

* Update customize-openapi.md with schema transformer guidance

Added missing code fence closure on example.

* Apply suggestion from @wadepickett

Update the ms.date to show the article has recently changed.

* Update aspnetcore/fundamentals/openapi/customize-openapi.md

Co-authored-by: Wade Pickett <wpickett@microsoft.com>

* Update aspnetcore/fundamentals/openapi/customize-openapi.md

Co-authored-by: Wade Pickett <wpickett@microsoft.com>

* Update aspnetcore/fundamentals/openapi/customize-openapi.md

Co-authored-by: Wade Pickett <wpickett@microsoft.com>

---------

Co-authored-by: Wade Pickett <wpickett@microsoft.com>

Author: Mohammed-Atef2004

aspnetcore/fundamentals/openapi/customize-openapi.md

@@ -5,7 +5,7 @@ description: Learn how to customize OpenAPI documents in an ASP.NET Core app
 ms.author: wpickett
 monikerRange: '>= aspnetcore-9.0'
 ms.custom: mvc
-ms.date: 10/29/2025
+ms.date: 03/20/2026
 uid: fundamentals/openapi/customize-openapi
 ---
 # Customize OpenAPI documents
@@ -106,6 +106,52 @@ Operation transformers can also be added to specific endpoint with the <xref:Mic
 
 [!code-csharp[](~/fundamentals/openapi/samples/10.x/WebMinOpenApi/Program.cs?name=snippet_operationtransformer2)]
 
+### Conditionally applying security requirements
+
+In some scenarios, developers may want to apply security requirements to all endpoints except those explicitly marked with the `AllowAnonymous` attribute.
+
+Use an operation transformer, which has access to endpoint metadata through the associated <xref:Microsoft.AspNetCore.Mvc.ApiExplorer.ApiDescription>.
+
+The following example demonstrates how to skip adding a security requirement for endpoints that have the `AllowAnonymousAttribute` applied:
+
+```csharp
+internal sealed class AuthOperationTransformer : IOpenApiOperationTransformer
+{
+    public Task TransformAsync(
+        OpenApiOperation operation,
+        OpenApiOperationTransformerContext context,
+        CancellationToken cancellationToken)
+    {
+        var hasAllowAnonymous = context.Description.ActionDescriptor.EndpointMetadata
+            .OfType<AllowAnonymousAttribute>()
+            .Any();
+
+        if (hasAllowAnonymous)
+        {
+            return Task.CompletedTask;
+        }
+
+        operation.Security ??= new List<OpenApiSecurityRequirement>();
+
+        operation.Security.Add(new OpenApiSecurityRequirement
+        {
+            [new OpenApiSecurityScheme
+            {
+                Reference = new OpenApiReference
+                {
+                    Id = "Bearer",
+                    Type = ReferenceType.SecurityScheme
+                }
+            }] = Array.Empty<string>()
+        });
+
+        return Task.CompletedTask;
+    }
+}
+```
+
+Use this approach instead of document transformers when conditional logic based on endpoint metadata is required. This transformer adds security *requirements* per operation and assumes the security *scheme* is already registered at the document level. For an example of registering the Bearer security scheme, see the `BearerSecuritySchemeTransformer` in the [Use document transformers](#use-document-transformers) section.
+
 ## Use schema transformers
 
 Schemas are the data models that are used in request and response bodies in an OpenAPI document. Schema transformers are useful when a modification: