diff -r 2d99f3e24da4 -r 956ecf2c07a0 src/Pure/Build/resources.scala
--- a/src/Pure/Build/resources.scala	Sun Jun 15 22:14:38 2025 +0200
+++ b/src/Pure/Build/resources.scala	Sun Jun 15 22:46:45 2025 +0200
@@ -82,7 +82,7 @@
 
   /* source files of Isabelle/ML bootstrap */
 
-  def source_file(raw_name: String): Option[String] = {
+  def source_file(ml_settings: ML_Settings, raw_name: String): Option[String] = {
     if (Path.is_wellformed(raw_name)) {
       if (Path.is_valid(raw_name)) {
         def check(p: Path): Option[Path] = if (p.is_file) Some(p) else None
@@ -91,9 +91,10 @@
         val path1 =
           if (path.is_absolute || path.is_current) check(path)
           else {
-            check(Path.explode("~~/src/Pure") + path) orElse
-              (if (Isabelle_System.getenv("ML_SOURCES") == "") None
-               else check(Path.explode("$ML_SOURCES") + path))
+            check(Path.explode("~~/src/Pure") + path) orElse {
+              val ml_sources = ml_settings.ml_sources
+              if (ml_sources.is_dir) check(ml_sources + path) else None
+            }
           }
         Some(File.platform_path(path1 getOrElse path))
       }