diff --git a/testing/src/main/scala/sbt/TestFramework.scala b/testing/src/main/scala/sbt/TestFramework.scala
index 6595fd472..0bbab2058 100644
--- a/testing/src/main/scala/sbt/TestFramework.scala
+++ b/testing/src/main/scala/sbt/TestFramework.scala
@@ -191,7 +191,12 @@ object TestFramework {
       val notInterfaceFilter = (name: String) => !interfaceFilter(name)
       val dual = new DualLoader(scalaInstance.loader, notInterfaceFilter, x => true, getClass.getClassLoader, interfaceFilter, x => false)
       val main = ClasspathUtilities.makeLoader(classpath, dual, scalaInstance, tempDir)
-      ClasspathUtilities.filterByClasspath(interfaceJar +: classpath, main)
+      // TODO - There's actually an issue with the classpath facility such that unmanagedScalaInstances are not added
+      // to the classpath correctly.  We have a temporary workaround here.
+      val cp: Seq[File] =
+        if (scalaInstance.isManagedVersion) interfaceJar +: classpath
+        else scalaInstance.allJars ++ (interfaceJar +: classpath)
+      ClasspathUtilities.filterByClasspath(cp, main)
     }
   def createTestFunction(loader: ClassLoader, taskDef: TaskDef, runner: TestRunner, testTask: TestTask): TestFunction =
     new TestFunction(taskDef, runner, (r: TestRunner) => withContextLoader(loader) { r.run(taskDef, testTask) }) { def tags = testTask.tags }
diff --git a/util/classpath/src/main/scala/sbt/ScalaInstance.scala b/util/classpath/src/main/scala/sbt/ScalaInstance.scala
index c0a207887..b2bf5bc3d 100644
--- a/util/classpath/src/main/scala/sbt/ScalaInstance.scala
+++ b/util/classpath/src/main/scala/sbt/ScalaInstance.scala
@@ -19,6 +19,12 @@ final class ScalaInstance(val version: String, val loader: ClassLoader, val libr
     @deprecated("Only `allJars` and `jars` can be reliably provided for modularized Scala.", "0.13.0") val compilerJar: File,
     @deprecated("Only `allJars` and `jars` can be reliably provided for modularized Scala.", "0.13.0") val extraJars: Seq[File],
     val explicitActual: Option[String]) extends xsbti.compile.ScalaInstance {
+  /**
+   * This tells us if the scalaInstance is from a managed (i.e. ivy-resolved) scala *or*
+   * if it's a free-floating ScalaInstance, in which case we need to do tricks to the classpaths we find
+   * because it won't be on them.
+   */
+  final def isManagedVersion = explicitActual.isDefined
   // These are to implement xsbti.ScalaInstance
   @deprecated("Only `allJars` and `jars` can be reliably provided for modularized Scala.", "0.13.0")
   def otherJars: Array[File] = extraJars.toArray