diff --git a/src/Polyfills/Polyfill.cs b/src/Polyfills/Polyfill.cs
new file mode 100644
index 0000000000..d00af935d5
--- /dev/null
+++ b/src/Polyfills/Polyfill.cs
@@ -0,0 +1,30 @@
+// <auto-generated />
+
+#pragma warning disable
+
+// Binary-compatibility shim for the (now removed) Polyfill NuGet package.
+//
+// Older Microsoft.Testing extension assemblies (built before Polyfill switched to
+// [Embedded] types) were compiled against `Polyfills.Polyfill.Deconstruct` and, through
+// InternalsVisibleTo, bound that call to the copy declared in the core
+// Microsoft.Testing.Platform assembly. When Polyfill was dropped, that method disappeared,
+// so mixing a newer core with an older extension throws:
+//   System.MissingMethodException: Method not found: 'Void Polyfills.Polyfill.Deconstruct(...)'.
+// Keeping this method on the [Embedded] Polyfill type (see ProcessExtensions.cs) restores the
+// runtime binding without re-exposing the polyfill to source consumers.
+// See https://github.com/microsoft/testfx/issues/9710.
+
+using System;
+using System.Collections.Generic;
+
+namespace Polyfills;
+
+internal static partial class Polyfill
+{
+    [Obsolete("Binary-compatibility shim for pre-[Embedded] Polyfill extension assemblies (see https://github.com/microsoft/testfx/issues/9710). It has no source callers and MUST be removed in the next major version of Microsoft.Testing.Platform.")]
+    public static void Deconstruct<TKey, TValue>(this KeyValuePair<TKey, TValue> target, out TKey key, out TValue value)
+    {
+        key = target.Key;
+        value = target.Value;
+    }
+}