diff --git a/main/src/main/scala/sbt/ClassLoaderLayeringStrategy.scala b/main/src/main/scala/sbt/ClassLoaderLayeringStrategy.scala
index 89eae4f05..7245c2e8b 100644
--- a/main/src/main/scala/sbt/ClassLoaderLayeringStrategy.scala
+++ b/main/src/main/scala/sbt/ClassLoaderLayeringStrategy.scala
@@ -112,7 +112,9 @@ object ClassLoaderLayeringStrategy {
    * Add the TestDependencies layer on top of the RuntimeDependencies layer on top of the
    * ScalaInstance layer. This differs from TestDependencies in that it will not reload the
    * runtime classpath. The drawback to using this is that if the test dependencies evict
-   * classes provided in the runtime layer, then tests can fail.
+   * classes provided in the runtime layer, then tests can fail. In order for sharing the runtime
+   * layer to work, it is necessary to set [[Keys.bgCopyClasspath]] to false. Otherwise the
+   * runtime and test classpaths are completely different.
    */
   case object ShareRuntimeDependenciesLayerWithTestDependencies
       extends ScalaInstance
diff --git a/main/src/main/scala/sbt/Defaults.scala b/main/src/main/scala/sbt/Defaults.scala
index 107ff54b3..b504dd5c0 100755
--- a/main/src/main/scala/sbt/Defaults.scala
+++ b/main/src/main/scala/sbt/Defaults.scala
@@ -1781,7 +1781,15 @@ object Defaults extends BuildCommon {
           // by the test task layers may be cached without evicting the runtime classloader layers. The
           // cache size should be a multiple of two to support snapshot layers.
           ClassLoaderCache(4)
-        )
+        ),
+        bgCopyClasspath in bgRun := {
+          val old = (bgCopyClasspath in bgRun).value
+          old && (Test / classLoaderLayeringStrategy).value != ClassLoaderLayeringStrategy.ShareRuntimeDependenciesLayerWithTestDependencies
+        },
+        bgCopyClasspath in bgRunMain := {
+          val old = (bgCopyClasspath in bgRunMain).value
+          old && (Test / classLoaderLayeringStrategy).value != ClassLoaderLayeringStrategy.ShareRuntimeDependenciesLayerWithTestDependencies
+        },
       )
 
   lazy val testSettings: Seq[Setting[_]] = configSettings ++ testTasks ++