Document .NET 10 Preview 7 breaking change: Deprecation of WithOpenApi extension method

docs/core/compatibility/10.0.md

@@ -19,6 +19,7 @@ If you're migrating an app to .NET 10, the breaking changes listed here might af
 
 | Title | Type of change    | Introduced version |
 |-------|-------------------|--------------------|
+| [Deprecation of WithOpenApi extension method](aspnet-core/10/withopenapi-deprecated.md) | Source incompatible | Preview 7 |
 | [IActionContextAccessor and ActionContextAccessor are obsolete](aspnet-core/10/iactioncontextaccessor-obsolete.md) | Source incompatible/behavioral change | Preview 7 |
 | [IncludeOpenAPIAnalyzers property and MVC API analyzers are deprecated](aspnet-core/10/openapi-analyzers-deprecated.md) | Source incompatible | Preview 7 |
 | [Microsoft.Extensions.ApiDescription.Client package deprecated](aspnet-core/10/apidescription-client-deprecated.md) | Source incompatible | Preview 7 |

docs/core/compatibility/aspnet-core/10/withopenapi-deprecated.md

@@ -0,0 +1,98 @@
+---
+title: "Breaking change: Deprecation of WithOpenApi extension method"
+description: "Learn about the breaking change in ASP.NET Core 10 where WithOpenApi extension methods have been deprecated and produce a compiler warning."
+ms.date: 08/07/2025
+ai-usage: ai-assisted
+ms.custom: https://github.com/aspnet/Announcements/issues/519
+---
+
+# Deprecation of WithOpenApi extension method
+
+The <xref:Microsoft.AspNetCore.Builder.OpenApiEndpointConventionBuilderExtensions.WithOpenApi*> methods have been deprecated in .NET 10. Invoking these methods now produces the compile-time diagnostic `ASPDEPR002` and a standard `Obsolete` warning that states:
+
+> WithOpenApi is deprecated and will be removed in a future release. For more information, visit <https://aka.ms/aspnet/deprecate/002>.
+
+## Version introduced
+
+.NET 10 Preview 7
+
+## Previous behavior
+
+Previously, you could use the `WithOpenApi` extension method without any warnings:
+
+```csharp
+app.MapGet("/weather", () => ...)
+   .WithOpenApi();   // No warnings.
+```
+
+## New behavior
+
+Starting in .NET 10, using the `WithOpenApi` extension method produces a compiler warning:
+
+```csharp
+app.MapGet("/weather", () => ...)
+   .WithOpenApi();   // Warning ASPDEPR002: WithOpenApi is deprecated...
+```
+
+However, the call still compiles and executes.
+
+## Type of breaking change
+
+This change can affect [source compatibility](../../categories.md#source-compatibility).
+
+## Reason for change
+
+<xref:Microsoft.AspNetCore.Builder.OpenApiEndpointConventionBuilderExtensions.WithOpenApi*> duplicated functionality now provided by the built-in OpenAPI document generation pipeline. Deprecating it simplifies the API surface and prepares for its eventual removal.
+
+## Recommended action
+
+Remove `.WithOpenApi()` calls from your code.
+
+- If you used `Microsoft.AspNetCore.OpenApi` for document generation, use the <xref:Microsoft.AspNetCore.Builder.OpenApiEndpointConventionBuilderExtensions.AddOpenApiOperationTransformer``1(``0,System.Func{Microsoft.OpenApi.OpenApiOperation,Microsoft.AspNetCore.OpenApi.OpenApiOperationTransformerContext,System.Threading.CancellationToken,System.Threading.Tasks.Task})> extension method.
+
+  Before:
+
+  ```csharp
+  using Microsoft.AspNetCore.OpenApi;
+
+  var builder = WebApplication.CreateBuilder();
+  var app = builder.Build();
+
+  app.MapGet("/weather", () => ...)
+     .WithOpenApi(operation =>
+     {
+         // Per-endpoint tweaks
+         operation.Summary     = "Gets the current weather report.";
+         operation.Description = "Returns a short description and emoji.";
+         return operation;
+     });
+
+  app.Run();
+  ```
+
+  After:
+
+  ```csharp
+  using Microsoft.AspNetCore.OpenApi;
+
+  var builder = WebApplication.CreateBuilder();
+  var app = builder.Build();
+
+  app.MapGet("/weather", () => ...)
+     .AddOpenApiOperationTransformer((operation, context, ct) =>
+     {
+         // Per-endpoint tweaks
+         operation.Summary     = "Gets the current weather report.";
+         operation.Description = "Returns a short description and emoji.";
+         return Task.CompletedTask;
+     });
+
+  app.Run();
+  ```
+
+- If you used `Swashbuckle` for document generation, use the `IOperationFilter` API.
+- If you used `NSwag` for document generation, use the `IOperationProcessor` API.
+
+## Affected APIs
+
+- <xref:Microsoft.AspNetCore.Builder.OpenApiEndpointConventionBuilderExtensions.WithOpenApi%2A?displayProperty=fullName>

docs/core/compatibility/toc.yml

@@ -10,6 +10,8 @@ items:
             href: 10.0.md
           - name: ASP.NET Core
             items:
+              - name: Deprecation of WithOpenApi extension method
+                href: aspnet-core/10/withopenapi-deprecated.md
               - name: IActionContextAccessor and ActionContextAccessor are obsolete
                 href: aspnet-core/10/iactioncontextaccessor-obsolete.md
               - name: IncludeOpenAPIAnalyzers property and MVC API analyzers are deprecated