diff --git a/src/Pure/Admin/sync_repos.scala b/src/Pure/Admin/sync_repos.scala
--- a/src/Pure/Admin/sync_repos.scala
+++ b/src/Pure/Admin/sync_repos.scala
@@ -1,100 +1,113 @@
 /*  Title:      Pure/Admin/sync_repos.scala
     Author:     Makarius
 
 Synchronize Isabelle + AFP repositories.
 */
 
 package isabelle
 
 
 object Sync_Repos {
   def sync_repos(target: String,
     progress: Progress = new Progress,
     verbose: Boolean = false,
     thorough: Boolean = false,
+    preserve_jars: Boolean = false,
     dry_run: Boolean = false,
     clean: Boolean = false,
     rev: String = "",
     afp_root: Option[Path] = None,
     afp_rev: String = ""
   ): Unit = {
     val target_dir = if (target.endsWith(":") || target.endsWith("/")) target else target + "/"
 
     val isabelle_hg = Mercurial.repository(Path.ISABELLE_HOME)
     val afp_hg = afp_root.map(Mercurial.repository(_))
 
+    val more_filter = if (preserve_jars) List("include *.jar", "protect *.jar") else Nil
+
     def sync(hg: Mercurial.Repository, dest: String, r: String, filter: List[String] = Nil): Unit =
       hg.sync(dest, rev = r, progress = progress, verbose = verbose, thorough = thorough,
-        dry_run = dry_run, clean = clean, filter = filter)
+        dry_run = dry_run, clean = clean, filter = filter ::: more_filter)
 
     progress.echo("\n* Isabelle repository:")
     sync(isabelle_hg, target, rev, filter = List("protect /AFP", "protect /etc/ISABELLE_ID"))
 
     if (!dry_run) {
       Isabelle_System.with_tmp_dir("sync") { tmp_dir =>
         val id_path = tmp_dir + Path.explode("ISABELLE_ID")
         File.write(id_path, isabelle_hg.id(rev = rev))
         Isabelle_System.rsync(thorough = thorough,
           args = List(File.standard_path(id_path), target_dir + "etc/"))
       }
     }
 
     for (hg <- afp_hg) {
       progress.echo("\n* AFP repository:")
       sync(hg, target_dir + "AFP", afp_rev)
     }
   }
 
   val isabelle_tool =
     Isabelle_Tool("sync_repos", "synchronize Isabelle + AFP repositories",
       Scala_Project.here, { args =>
         var afp_root: Option[Path] = None
         var clean = false
+        var preserve_jars = false
         var thorough = false
         var afp_rev = ""
         var dry_run = false
         var rev = ""
         var verbose = false
 
         val getopts = Getopts("""
 Usage: isabelle sync_repos [OPTIONS] TARGET
 
   Options are:
     -A ROOT      include AFP with given root directory
+    -J           preserve *.jar files
     -C           clean all unknown/ignored files on target
                  (implies -n for testing; use option -f to confirm)
     -T           thorough check of file content (default: time and size)
     -a REV       explicit AFP revision (default: state of working directory)
     -f           force changes: no dry-run
     -n           no changes: dry-run
     -r REV       explicit revision (default: state of working directory)
     -v           verbose
 
   Synchronize Isabelle + AFP repositories; see also "isabelle hg_sync".
 
-  Example (without -f as "dry-run"):
+  Examples (without -f as "dry-run"):
 
-    isabelle sync_repos -A '$AFP_BASE' -C testmachine:test/isabelle_afp
+   * quick testing
+
+      isabelle sync_repos -A '$AFP_BASE' -J -C testmachine:test/isabelle_afp
+
+   * accurate testing
+
+      isabelle sync_repos -A '$AFP_BASE' -T -C testmachine:test/isabelle_afp
 """,
           "A:" -> (arg => afp_root = Some(Path.explode(arg))),
+          "J" -> (_ => preserve_jars = true),
           "C" -> { _ => clean = true; dry_run = true },
           "T" -> (_ => thorough = true),
           "a:" -> (arg => afp_rev = arg),
           "f" -> (_ => dry_run = false),
           "n" -> (_ => dry_run = true),
           "r:" -> (arg => rev = arg),
           "v" -> (_ => verbose = true))
 
         val more_args = getopts(args)
         val target =
           more_args match {
             case List(target) => target
             case _ => getopts.usage()
           }
 
         val progress = new Console_Progress
         sync_repos(target, progress = progress, verbose = verbose, thorough = thorough,
-          dry_run = dry_run, clean = clean, rev = rev, afp_root = afp_root, afp_rev = afp_rev)
+          preserve_jars = preserve_jars, dry_run = dry_run, clean = clean, rev = rev,
+          afp_root = afp_root, afp_rev = afp_rev)
       }
     )
 }