Add documentation for endpoint-specific OpenAPI operation transformers

Author: Sander ten Brinke

aspnetcore/release-notes/aspnetcore-10.0.md

@@ -4,7 +4,7 @@ author: wadepickett
 description: Learn about the new features in ASP.NET Core in .NET 10.
 ms.author: wpickett
 ms.custom: mvc
-ms.date: 09/15/2025
+ms.date: 11/11/2025
 uid: aspnetcore-10
 ---
 # What's new in ASP.NET Core in .NET 10
@@ -67,6 +67,8 @@ This section describes new features for OpenAPI.
 
 [!INCLUDE[](~/release-notes/aspnetcore-10/includes/OpenApiSchemasInTransformers.md)]
 
+[!INCLUDE[](~/release-notes/aspnetcore-10/includes/endpoint-specific-operation-transformers.md)]
+
 [!INCLUDE[](~/release-notes/aspnetcore-10/includes/upgrade-microsoft-openapi-2.md)]
 
 [!INCLUDE[](~/release-notes/aspnetcore-10/includes/openapi-schema-enhancements.md)]

aspnetcore/release-notes/aspnetcore-10/includes/endpoint-specific-operation-transformers.md

@@ -0,0 +1,39 @@
+### Endpoint-specific OpenAPI operation transformers
+
+OpenAPI [operation transformers](<xref:fundamentals/openapi/customize-openapi#use-operation-transformers>) can now be registered directly on individual endpoints using the `AddOpenApiOperationTransformer` extension method. This allows customization of OpenAPI metadata to be colocated with the endpoint definition, rather than requiring global configuration with path-based conditional logic.
+
+Prior to this feature, modifying OpenAPI data for specific endpoints required registering a global operation transformer and implementing conditional logic to target specific endpoints. For example:
+
+```csharp
+builder.Services.AddOpenApi(options =>
+{
+    options.AddOperationTransformer((operation, context, ct) =>
+    {
+        if (context.Description.RelativePath == "deprecated-endpoint")
+        {
+            operation.Deprecated = true;
+        }
+
+        return Task.CompletedTask;
+    });
+});
+```
+
+With the new `AddOpenApiOperationTransformer` API, the same customization can be applied directly to the endpoint, with access to the full operation context as well:
+
+```csharp
+app.MapGet("/deprecated-endpoint", () => "This endpoint is old and should not be used anymore")
+.AddOpenApiOperationTransformer((operation, context, cancellationToken) =>
+{
+    operation.Deprecated = true;
+
+    return Task.CompletedTask;
+});
+```
+
+This approach improves code readability and maintainability by keeping endpoint-specific customizations close to the endpoint definitions. The `AddOpenApiOperationTransformer` method provides access to the full <xref:Microsoft.AspNetCore.OpenApi.OpenApiOperationTransformerContext>, enabling comprehensive customization of the OpenAPI operation, including:
+
+* Setting operation properties like `Deprecated`, `Summary`, or `Description`
+* Customizing response descriptions
+* Adding or modifying security requirements
+* Modifying parameters or request bodies