diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index d4dc50691..eb8c90661 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -28,6 +28,19 @@ This project has adopted the [Microsoft Open Source Code of Conduct](https://ope
 
 > **Package version:** All projects reference `Microsoft.WindowsAppSDK` **2.0.1** (public NuGet). The version is centralized in `Directory.Build.props` — update it there to change the version for every project at once.
 
+> **Clone to a short path (Windows):** Clone into a short root directory such as
+> `C:\src\` (for example `C:\src\microsoft-ui-reactor`) rather than a deeply
+> nested or OneDrive-synced folder. The Windows App SDK XAML markup compiler
+> (`XamlCompiler.exe`) and resource indexer (`MakePri.exe`) are 32-bit tools with
+> no long-path support, so when the repository sits on a long path a project's
+> generated `obj`/`bin` output can exceed the Windows 260-character `MAX_PATH`
+> limit and the build fails with errors such as `MSB3073`, `PRI210`, or
+> `APPX0002`. This is most common on ARM64 dev boxes, where these tools run under
+> x86 emulation. Enabling `git config --global core.longpaths true` lets Git
+> itself check out long paths, but the Windows App SDK build tools still require
+> the short checkout path. See
+> [issue #678](https://github.com/microsoft/microsoft-ui-reactor/issues/678).
+
 ---
 
 ## Building
diff --git a/README.md b/README.md
index 112553f02..a8bf5c739 100644
--- a/README.md
+++ b/README.md
@@ -83,6 +83,15 @@ cd MyApp
 dotnet run -p:Platform=x64
 ```
 
+> ⚠️ **Clone to a short path**: Clone into a short root directory such as
+> `C:\src\` (e.g. `C:\src\microsoft-ui-reactor`), not a deeply nested or
+> OneDrive-synced folder. The Windows App SDK's XAML and resource compilers are
+> 32-bit tools with no long-path support, so a long checkout path can push a
+> project's generated `obj`/`bin` output past the Windows 260-character
+> `MAX_PATH` limit and break the build (`MSB3073` / `PRI210`) — most often on
+> ARM64 dev boxes, where those tools run under emulation. See
+> [#678](https://github.com/microsoft/microsoft-ui-reactor/issues/678).
+
 > ⚠️ **Platform flag required**: Always build with an explicit platform:
 > `dotnet build -p:Platform=x64` (or `ARM64`). Omitting `-p:Platform=...`
 > causes `WindowsAppSDKSelfContained` errors. This applies to `dotnet build`,