Document Microsoft.Extensions.ApiDescription.Client package deprecation breaking change

docs/core/compatibility/10.0.md

@@ -15,6 +15,12 @@ If you're migrating an app to .NET 10, the breaking changes listed here might af
 > [!NOTE]
 > This article is a work in progress. It's not a complete list of breaking changes in .NET 10.
 
+## ASP.NET Core
+
+| Title | Type of change      | Introduced version |
+|-------|---------------------|--------------------|
+| [Microsoft.Extensions.ApiDescription.Client package deprecated](aspnet-core/10/apidescription-client-deprecated.md) | Source incompatible | Preview 7 |
+
 ## Containers
 
 | Title                                                                          | Type of change    | Introduced version |

docs/core/compatibility/aspnet-core/10/apidescription-client-deprecated.md

@@ -0,0 +1,64 @@
+---
+title: "Breaking change - Microsoft.Extensions.ApiDescription.Client package deprecated"
+description: "Learn about the breaking change in ASP.NET Core 10 where the Microsoft.Extensions.ApiDescription.Client package has been deprecated."
+ms.date: 08/07/2025
+ai-usage: ai-assisted
+ms.custom: https://github.com/aspnet/Announcements/issues/518
+---
+
+# Microsoft.Extensions.ApiDescription.Client package deprecated
+
+The Microsoft.Extensions.ApiDescription.Client NuGet package has been deprecated. The package supplied MSBuild targets and CLI support that generated OpenAPI-based client code during the build. Projects that reference the package now receive a warning during build.
+
+## Version introduced
+
+.NET 10 Preview 7
+
+## Previous behavior
+
+Projects could add `<PackageReference Include="Microsoft.Extensions.ApiDescription.Client" ... />` and `<OpenApiReference>` items (or run `dotnet openapi`) to generate strongly typed clients at build time.
+
+```xml
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Microsoft.Extensions.ApiDescription.Client" Version="8.0.0" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <OpenApiReference Include="swagger.json" />
+  </ItemGroup>
+</Project>
+```
+
+## New behavior
+
+The package is now deprecated and projects that reference it receive build warnings. The MSBuild targets and CLI commands are no longer supported.
+
+## Type of breaking change
+
+This change can affect [source compatibility](../../categories.md#source-compatibility).
+
+## Reason for change
+
+- The package has seen minimal updates and maintenance since its introduction.
+- Its abstractions were tightly coupled to certain generators and did not scale well to others. Each generator now ships its own CLI/configuration experience, making the MSBuild middle-layer redundant.
+- Removing the package reduces maintenance burden and clarifies the recommended workflow for client generation.
+
+## Recommended action
+
+- Remove any `<PackageReference Include="Microsoft.Extensions.ApiDescription.Client" … />` from your project.
+- Replace `<OpenApiReference>` items or `dotnet openapi` commands with generator-specific tooling:
+  - NSwag – Use `npx nswag` or `dotnet tool run nswag` with an `.nswag` config file.
+  - Kiota – Install with `dotnet tool install -g Microsoft.OpenApi.Kiota` and run `kiota generate`.
+  - OpenAPI generator – Invoke `openapi-generator-cli` via JAR or Docker.
+- Commit the generated client code or run generation in a custom pre-build step that doesn't rely on the removed package.
+
+## Affected APIs
+
+- MSBuild item `OpenApiReference` (all instances).
+- MSBuild property `OpenApiProjectReference`.
+- CLI command `dotnet openapi`.

docs/core/compatibility/toc.yml

@@ -8,6 +8,10 @@ items:
         items:
           - name: Overview
             href: 10.0.md
+          - name: ASP.NET Core
+            items:
+              - name: Microsoft.Extensions.ApiDescription.Client package deprecated
+                href: aspnet-core/10/apidescription-client-deprecated.md
           - name: Containers
             items:
               - name: Default .NET images use Ubuntu