diff --git a/modules/ui/src/main/scala/de/nowchess/ui/terminal/TerminalUI.scala b/modules/ui/src/main/scala/de/nowchess/ui/terminal/TerminalUI.scala
index e15979c..90bb91d 100644
--- a/modules/ui/src/main/scala/de/nowchess/ui/terminal/TerminalUI.scala
+++ b/modules/ui/src/main/scala/de/nowchess/ui/terminal/TerminalUI.scala
@@ -48,7 +48,7 @@ class TerminalUI(engine: GameEngine) extends Observer:
 
       case _: PromotionRequiredEvent =>
         println("Promote to: q=Queen, r=Rook, b=Bishop, n=Knight")
-        awaitingPromotion = true
+        synchronized { awaitingPromotion = true }
 
   /** Start the terminal UI game loop. */
   def start(): Unit =
@@ -63,24 +63,26 @@ class TerminalUI(engine: GameEngine) extends Observer:
     // Game loop
     while running do
       val input = Option(StdIn.readLine()).getOrElse("quit").trim
-      if awaitingPromotion then
-        input.toLowerCase match
-          case "q" => awaitingPromotion = false; engine.completePromotion(PromotionPiece.Queen)
-          case "r" => awaitingPromotion = false; engine.completePromotion(PromotionPiece.Rook)
-          case "b" => awaitingPromotion = false; engine.completePromotion(PromotionPiece.Bishop)
-          case "n" => awaitingPromotion = false; engine.completePromotion(PromotionPiece.Knight)
-          case _ =>
-            println("Invalid choice. Enter q, r, b, or n.")
-            println("Promote to: q=Queen, r=Rook, b=Bishop, n=Knight")
-      else
-        input.toLowerCase match
-          case "quit" | "q" =>
-            running = false
-            println("Game over. Goodbye!")
-          case "" =>
-            printPrompt(engine.turn)
-          case _ =>
-            engine.processUserInput(input)
+      synchronized {
+        if awaitingPromotion then
+          input.toLowerCase match
+            case "q" => awaitingPromotion = false; engine.completePromotion(PromotionPiece.Queen)
+            case "r" => awaitingPromotion = false; engine.completePromotion(PromotionPiece.Rook)
+            case "b" => awaitingPromotion = false; engine.completePromotion(PromotionPiece.Bishop)
+            case "n" => awaitingPromotion = false; engine.completePromotion(PromotionPiece.Knight)
+            case _ =>
+              println("Invalid choice. Enter q, r, b, or n.")
+              println("Promote to: q=Queen, r=Rook, b=Bishop, n=Knight")
+        else
+          input.toLowerCase match
+            case "quit" | "q" =>
+              running = false
+              println("Game over. Goodbye!")
+            case "" =>
+              printPrompt(engine.turn)
+            case _ =>
+              engine.processUserInput(input)
+      }
 
     // Unsubscribe when done
     engine.unsubscribe(this)
diff --git a/modules/ui/src/test/scala/de/nowchess/ui/terminal/TerminalUITest.scala b/modules/ui/src/test/scala/de/nowchess/ui/terminal/TerminalUITest.scala
index 54903c9..514ad0d 100644
--- a/modules/ui/src/test/scala/de/nowchess/ui/terminal/TerminalUITest.scala
+++ b/modules/ui/src/test/scala/de/nowchess/ui/terminal/TerminalUITest.scala
@@ -264,4 +264,64 @@ class TerminalUITest extends AnyFunSuite with Matchers {
     capturedPiece should be(Some(PromotionPiece.Rook))
     out.toString should include("Invalid")
   }
+
+  test("TerminalUI routes Bishop promotion choice to engine.completePromotion") {
+    import de.nowchess.api.move.PromotionPiece
+
+    var capturedPiece: Option[PromotionPiece] = None
+
+    val engine = new GameEngine() {
+      override def processUserInput(rawInput: String): Unit =
+        if rawInput.trim == "e7e8" then
+          notifyObservers(PromotionRequiredEvent(
+            Board(Map.empty), GameHistory.empty, Color.White,
+            Square(File.E, Rank.R7), Square(File.E, Rank.R8)
+          ))
+      override def completePromotion(piece: PromotionPiece): Unit =
+        capturedPiece = Some(piece)
+        notifyObservers(MoveExecutedEvent(Board(Map.empty), GameHistory.empty, Color.Black, "e7", "e8", None))
+    }
+
+    val in = new ByteArrayInputStream("e7e8\nb\nquit\n".getBytes)
+    val out = new ByteArrayOutputStream()
+    val ui = new TerminalUI(engine)
+
+    Console.withIn(in) {
+      Console.withOut(out) {
+        ui.start()
+      }
+    }
+
+    capturedPiece should be(Some(PromotionPiece.Bishop))
+  }
+
+  test("TerminalUI routes Knight promotion choice to engine.completePromotion") {
+    import de.nowchess.api.move.PromotionPiece
+
+    var capturedPiece: Option[PromotionPiece] = None
+
+    val engine = new GameEngine() {
+      override def processUserInput(rawInput: String): Unit =
+        if rawInput.trim == "e7e8" then
+          notifyObservers(PromotionRequiredEvent(
+            Board(Map.empty), GameHistory.empty, Color.White,
+            Square(File.E, Rank.R7), Square(File.E, Rank.R8)
+          ))
+      override def completePromotion(piece: PromotionPiece): Unit =
+        capturedPiece = Some(piece)
+        notifyObservers(MoveExecutedEvent(Board(Map.empty), GameHistory.empty, Color.Black, "e7", "e8", None))
+    }
+
+    val in = new ByteArrayInputStream("e7e8\nn\nquit\n".getBytes)
+    val out = new ByteArrayOutputStream()
+    val ui = new TerminalUI(engine)
+
+    Console.withIn(in) {
+      Console.withOut(out) {
+        ui.start()
+      }
+    }
+
+    capturedPiece should be(Some(PromotionPiece.Knight))
+  }
 }