diff --git a/main/Main.scala b/main/Main.scala
index d81b3e2a3..f6737613e 100644
--- a/main/Main.scala
+++ b/main/Main.scala
@@ -173,7 +173,7 @@ object BuiltinCommands
 	{
 		import extracted._
 		val append = Load.transformSettings(Load.projectScope(currentRef), currentRef.build, rootProject, settings)
-		session.appendSettings( append map (a => (a, arg)))
+		session.appendSettings( append map (a => (a, arg.split('\n').toList)))
 	}
 	def inspect = Command(InspectCommand, inspectBrief, inspectDetailed)(inspectParser) { case (s, (option, sk)) =>
 		s.log.info(inspectOutput(s, option, sk))
diff --git a/main/SessionSettings.scala b/main/SessionSettings.scala
old mode 100644
new mode 100755
index 659d1ead2..2a487c0d2
--- a/main/SessionSettings.scala
+++ b/main/SessionSettings.scala
@@ -30,7 +30,7 @@ final case class SessionSettings(currentBuild: URI, currentProject: Map[URI, Str
 }
 object SessionSettings
 {
-	type SessionSetting = (Setting[_], String)
+	type SessionSetting = (Setting[_], List[String])
 	type SessionMap = Map[ProjectRef, Seq[SessionSetting]]
 
 	def reapply(session: SessionSettings, s: State): State =
@@ -80,19 +80,63 @@ object SessionSettings
 	}
 	def saveSomeSettings(s: State)(include: ProjectRef => Boolean): State =
 		withSettings(s){session =>
-			for( (ref, settings) <- session.append if !settings.isEmpty && include(ref) )
-				writeSettings(ref, settings, Project.structure(s))
-			reapply(session.copy(original = session.mergeSettings, append = Map.empty), s)
+			val newSettings =
+				for( (ref, settings) <- session.append if !settings.isEmpty && include(ref) ) yield {
+					val (news, olds) = writeSettings(ref, settings.toList, session.original, Project.structure(s))
+					(ref -> news, olds)
+				}
+      val (newAppend, newOriginal) = newSettings.unzip
+      val newSession = session.copy(append = newAppend.toMap, original = newOriginal.flatten.toSeq)
+			reapply(newSession.copy(original = newSession.mergeSettings, append = Map.empty), s)
 		}
-	def writeSettings(pref: ProjectRef, settings: Seq[SessionSetting], structure: Load.BuildStructure)
+	def writeSettings(pref: ProjectRef, settings: List[SessionSetting], original: Seq[Setting[_]], structure: Load.BuildStructure): (Seq[SessionSetting], Seq[Setting[_]]) =
 	{
 		val project = Project.getProject(pref, structure).getOrElse(error("Invalid project reference " + pref))
-		val appendTo: File = BuildPaths.configurationSources(project.base).headOption.getOrElse(new File(project.base, "build.sbt"))
-		val baseAppend = settingStrings(settings).flatMap("" :: _ :: Nil)
-		val adjustedLines = if(appendTo.isFile && hasTrailingBlank(IO readLines appendTo) ) baseAppend else "" +: baseAppend
-		IO.writeLines(appendTo, adjustedLines, append = true)
+		val writeTo: File = BuildPaths.configurationSources(project.base).headOption.getOrElse(new File(project.base, "build.sbt"))
+		writeTo.createNewFile()
+
+		val path = writeTo.getAbsolutePath
+		val (inFile, other, _) = ((List[Setting[_]](), List[Setting[_]](), Set.empty[ScopedKey[_]]) /: original.reverse) {
+			case ((in, oth, keys), s) =>
+				s.pos match {
+					case RangePosition(`path`, _) if !keys.contains(s.key) => (s::in, oth, keys + s.key)
+					case _ => (in, s::oth, keys)
+				}
+		}
+
+		val (_, oldShifted, replace, lineMap) = ((0, List[Setting[_]](), List[SessionSetting](), Map.empty[Int, (Int, List[String])]) /: inFile) {
+			case ((offs, olds, repl, lineMap), s) =>
+				val RangePosition(_, r@LineRange(start, end)) = s.pos
+				settings find (_._1.key == s.key) match {
+					case Some(ss@(ns, newLines)) if !ns.init.dependencies.contains(ns.key) =>
+						val shifted = ns withPos RangePosition(path, LineRange(start - offs, start - offs + 1))
+						(offs + end - start - newLines.size, shifted::olds, ss::repl, lineMap + (start -> (end, newLines)))
+					case _ =>
+						val shifted = s withPos RangePosition(path, r shift -offs)
+						(offs, shifted::olds, repl, lineMap)
+				}
+		}
+		val newSettings = settings diff replace
+		val (tmpLines, _) = ((List[String](), 1) /: IO.readLines(writeTo).zipWithIndex) {
+			case ((accLines,  n), (line, m)) if n == m + 1 =>
+				lineMap.get(n) match {
+					case Some(Pair(end, lines)) =>  (lines reverse_::: accLines, end)
+					case None => (line::accLines, n + 1)
+				}
+			case (res,_) => res
+		}
+		val exist = tmpLines.reverse
+		val adjusted = if(!newSettings.isEmpty && needsTrailingBlank(exist)) exist :+ "" else exist
+		val lines = adjusted ++ newSettings.map(_._2).flatten.flatMap(_ :: "" :: Nil)
+		IO.writeLines(writeTo, lines)
+		val (newWithPos, _) = ((List[SessionSetting](), adjusted.size + 1) /: newSettings) {
+			case ((acc, line), (s, newLines)) => 
+				val endLine = line + newLines.size
+				((s withPos RangePosition(path, LineRange(line, endLine)), newLines)::acc, endLine + 1)
+ 		}
+		(newWithPos.reverse, other ++ oldShifted)
 	}
-	def hasTrailingBlank(lines: Seq[String]) = lines.takeRight(1).exists(_.trim.isEmpty)
+	def needsTrailingBlank(lines: Seq[String]) = !lines.isEmpty && !lines.takeRight(1).exists(_.trim.isEmpty)
 	def printAllSettings(s: State): State =
 		withSettings(s){ session =>
 			for( (ref, settings) <- session.append if !settings.isEmpty) {
@@ -107,11 +151,9 @@ object SessionSettings
 			s
 		}
 	def printSettings(settings: Seq[SessionSetting]): Unit =
-		for((stringRep, index) <- settingStrings(settings).zipWithIndex)
+		for(((_,stringRep), index) <- settings.zipWithIndex)
 			println("  " + (index+1) + ". " + stringRep)
 
-	def settingStrings(s: Seq[SessionSetting]): Seq[String] = s.map(_._2)
-
 	def Help = """session <command>
 
 Manipulates session settings, which are temporary settings that do not persist past the current sbt execution (that is, the current session).
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..1e25d0391
--- /dev/null
+++ b/sbt/src/sbt-test/project/session-save/build.check.1
@@ -0,0 +1,4 @@
+k1 := {error("k1")}
+
+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..42c414288
--- /dev/null
+++ b/sbt/src/sbt-test/project/session-save/build.check.2
@@ -0,0 +1,4 @@
+k1 := {}
+
+k2 <<= k1 map identity
+
diff --git a/sbt/src/sbt-test/project/session-save/build.check.3 b/sbt/src/sbt-test/project/session-save/build.check.3
new file mode 100755
index 000000000..ad18cca9f
--- /dev/null
+++ b/sbt/src/sbt-test/project/session-save/build.check.3
@@ -0,0 +1,6 @@
+k1 := {}
+
+k2 := {}
+
+k1 <<= k1 map {_ => error("k1")}
+
diff --git a/sbt/src/sbt-test/project/session-save/build.sbt b/sbt/src/sbt-test/project/session-save/build.sbt
new file mode 100755
index 000000000..fcf95068c
--- /dev/null
+++ b/sbt/src/sbt-test/project/session-save/build.sbt
@@ -0,0 +1,6 @@
+k1 := {
+}
+
+k2 := {
+}
+
diff --git a/sbt/src/sbt-test/project/session-save/project/Build.scala b/sbt/src/sbt-test/project/session-save/project/Build.scala
new file mode 100755
index 000000000..d90a26779
--- /dev/null
+++ b/sbt/src/sbt-test/project/session-save/project/Build.scala
@@ -0,0 +1,8 @@
+import sbt._
+
+object TestBuild extends Build {
+	val k1 = TaskKey[Unit]("k1")
+	val k2 = TaskKey[Unit]("k2")
+
+	lazy val root = Project("root", file("."))
+}
diff --git a/sbt/src/sbt-test/project/session-save/test b/sbt/src/sbt-test/project/session-save/test
new file mode 100755
index 000000000..c552f3a31
--- /dev/null
+++ b/sbt/src/sbt-test/project/session-save/test
@@ -0,0 +1,25 @@
+> set k1 := {error("k1")}
+> session save
+> reload
+-> k1
+
+> set k2 <<= k1 map identity
+> 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
+$ 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
+}