diff --git a/main/src/main/scala/sbt/ClassLoaderLayeringStrategy.scala b/main/src/main/scala/sbt/ClassLoaderLayeringStrategy.scala
index 483900728..cfb360fc4 100644
--- a/main/src/main/scala/sbt/ClassLoaderLayeringStrategy.scala
+++ b/main/src/main/scala/sbt/ClassLoaderLayeringStrategy.scala
@@ -96,5 +96,5 @@ object ClassLoaderLayeringStrategy {
   /**
    * Add a layer on top of the ScalaLibrary layer for all of the jar dependencies.
    */
-  object AllLibraryJars extends AllLibraryJars
+  case object AllLibraryJars extends AllLibraryJars
 }
diff --git a/main/src/main/scala/sbt/internal/ClassLoaders.scala b/main/src/main/scala/sbt/internal/ClassLoaders.scala
index 37ace0f72..773ec4813 100644
--- a/main/src/main/scala/sbt/internal/ClassLoaders.scala
+++ b/main/src/main/scala/sbt/internal/ClassLoaders.scala
@@ -105,7 +105,7 @@ private[sbt] object ClassLoaders {
   }
   /*
    * Create a layered classloader. There are up to four layers:
-   * 1) the scala instance class loader
+   * 1) the scala instance class loader (may actually be two layers if scala-reflect is used)
    * 2) the resource layer
    * 3) the dependency jars
    * 4) the rest of the classpath
@@ -132,15 +132,21 @@ private[sbt] object ClassLoaders {
           case _: AllLibraryJars => true
           case _                 => false
         }
-        val allDependenciesSet = allDependencies.toSet
         val scalaLibraryLayer = layer(si.libraryJars, interfaceLoader, cache, resources, tmp)
         val cpFiles = fullCP.map(_._1)
 
+        val scalaReflectJar = allDependencies.find(_.getName == "scala-reflect.jar")
+        val scalaReflectLayer = scalaReflectJar
+          .map { file =>
+            layer(file :: Nil, scalaLibraryLayer, cache, resources, tmp)
+          }
+          .getOrElse(scalaLibraryLayer)
+
         // layer 2 (resources)
         val resourceLayer =
           if (layerDependencies)
-            getResourceLayer(cpFiles, resourceCP, scalaLibraryLayer, cache, resources)
-          else scalaLibraryLayer
+            getResourceLayer(cpFiles, resourceCP, scalaReflectLayer, cache, resources)
+          else scalaReflectLayer
 
         // layer 3 (optional if in the test config and the runtime layer is not shared)
         val dependencyLayer =
@@ -148,7 +154,10 @@ private[sbt] object ClassLoaders {
           else resourceLayer
 
         // layer 4
-        val dynamicClasspath = cpFiles.filterNot(allDependenciesSet ++ si.libraryJars)
+        val filteredSet =
+          if (layerDependencies) allDependencies.toSet ++ si.libraryJars ++ scalaReflectJar
+          else Set(si.libraryJars ++ scalaReflectJar: _*)
+        val dynamicClasspath = cpFiles.filterNot(filteredSet)
         new LayeredClassLoader(dynamicClasspath, dependencyLayer, resources, tmp)
     }
     ClasspathUtilities.filterByClasspath(cpFiles, raw)
diff --git a/sbt/src/sbt-test/classloader-cache/package-private/test b/sbt/src/sbt-test/classloader-cache/package-private/test
index 64ef9d785..dd298596e 100644
--- a/sbt/src/sbt-test/classloader-cache/package-private/test
+++ b/sbt/src/sbt-test/classloader-cache/package-private/test
@@ -1,3 +1,5 @@
+> set Compile / classLoaderLayeringStrategy := ClassLoaderLayeringStrategy.AllLibraryJars
+
 -> run
 
 > set Compile / classLoaderLayeringStrategy := ClassLoaderLayeringStrategy.Flat
diff --git a/sbt/src/sbt-test/classloader-cache/scalatest/test b/sbt/src/sbt-test/classloader-cache/scalatest/test
index 9b34bf4a6..055516751 100644
--- a/sbt/src/sbt-test/classloader-cache/scalatest/test
+++ b/sbt/src/sbt-test/classloader-cache/scalatest/test
@@ -1,3 +1,5 @@
+> set Test / classLoaderLayeringStrategy := ClassLoaderLayeringStrategy.AllLibraryJars
+
 > test
 
 > set Test / classLoaderLayeringStrategy := ClassLoaderLayeringStrategy.Flat
@@ -8,8 +10,14 @@
 
 > test
 
-> set Test / classLoaderLayeringStrategy := ClassLoaderLayeringStrategy.AllLibraryJars
-
 $ copy-file changes/bad.scala src/test/scala/sbt/ScalatestTest.scala
 
 -> test
+
+> set Test / classLoaderLayeringStrategy := ClassLoaderLayeringStrategy.Flat
+
+-> test
+
+> set Test / classLoaderLayeringStrategy := ClassLoaderLayeringStrategy.AllLibraryJars
+
+-> test