diff --git a/buildfile/src/main/scala/sbt/internal/Eval.scala b/buildfile/src/main/scala/sbt/internal/Eval.scala
index aa2fb957b..06ae18ae8 100644
--- a/buildfile/src/main/scala/sbt/internal/Eval.scala
+++ b/buildfile/src/main/scala/sbt/internal/Eval.scala
@@ -43,7 +43,7 @@ class Eval(
       case None      => VirtualDirectory("output")
   private val classpathString = (backingDir.toList ++ classpath)
     .map(_.toString)
-    .mkString(":")
+    .mkString(java.io.File.pathSeparator)
   private lazy val driver: EvalDriver = new EvalDriver
   private lazy val reporter: EvalReporter = mkReporter match
     case Some(f) => f()