diff --git a/sbt/src/sbt-test/project/session-save/build.check.1 b/sbt/src/sbt-test/project/session-save/build.check.1
new file mode 100755
index 000000000..85beb3faa
--- /dev/null
+++ b/sbt/src/sbt-test/project/session-save/build.check.1
@@ -0,0 +1,7 @@
+k1 := {error("k1")}
+
+k2 := {
+}
+
+k2 <<= k1 map identity
+
diff --git a/sbt/src/sbt-test/project/session-save/build.check.2 b/sbt/src/sbt-test/project/session-save/build.check.2
new file mode 100755
index 000000000..d47b51846
--- /dev/null
+++ b/sbt/src/sbt-test/project/session-save/build.check.2
@@ -0,0 +1,7 @@
+k1 := {}
+
+k2 := {
+}
+
+k2 <<= k1 map identity
+
diff --git a/sbt/src/sbt-test/project/session-save/test b/sbt/src/sbt-test/project/session-save/test
index 9011de364..c552f3a31 100755
--- a/sbt/src/sbt-test/project/session-save/test
+++ b/sbt/src/sbt-test/project/session-save/test
@@ -7,16 +7,19 @@
 > session save
 > reload
 -> k2
+$ must-mirror build.sbt build.check.1
 
 > set k1 := {}
 > session save
 > reload
 > k1
 > k2
+$ must-mirror build.sbt build.check.2
 
 > set k1 <<= k1 map {_ => error("k1")}
 > set k2 := {}
 > session save
 > reload
 -> k1
-> k2
\ No newline at end of file
+> k2
+$ must-mirror build.sbt build.check.3
\ No newline at end of file
diff --git a/scripted/base/FileCommands.scala b/scripted/base/FileCommands.scala
index 6c9ca1c68..d425911ad 100644
--- a/scripted/base/FileCommands.scala
+++ b/scripted/base/FileCommands.scala
@@ -24,6 +24,7 @@ class FileCommands(baseDirectory: File) extends BasicStatementHandler
 			"exec" nonEmpty(execute _ ),
 			"copy" copy (to => rebase(baseDirectory, to)),
 			"copy-file" twoArg("Two paths", copyFile _),
+			"must-mirror" twoArg("Two paths", diffFiles _),
 			"copy-flat" copy flat
 		)
 
@@ -42,6 +43,12 @@ class FileCommands(baseDirectory: File) extends BasicStatementHandler
 		IO.copyFile(fromString(from), fromString(to))
 	def makeDirectories(paths: List[String]) =
 		IO.createDirectories(fromStrings(paths))
+	def diffFiles(file1: String, file2: String) = {
+		val lines1 = IO.readLines(fromString(file1))
+		val lines2 = IO.readLines(fromString(file2))
+		if (lines1 != lines2)
+			scriptError("File contents are different:\n" + lines1.mkString("\n") + "\nAnd:\n" + lines2.mkString("\n"))
+	}
 
 	def newer(a: String, b: String) =
 	{
@@ -115,4 +122,4 @@ class FileCommands(baseDirectory: File) extends BasicStatementHandler
 		def wrongArguments(requiredArgs: String, args: List[String]): Some[String] =
 			scriptError("Wrong number of arguments to " + commandName + " command.  " + requiredArgs + " required, found: '" + spaced(args) + "'.")
 	}
-}
\ No newline at end of file
+}