Correct tests.

src/libraries/Common/tests/System/Net/Prerequisites/NetCoreServer/Handlers/EchoWebSocketHandler.cs

@@ -147,20 +147,14 @@ await socket.CloseAsync(
                     else if (receivedMessage == ".receiveMessageAfterClose")
                     {
                         byte[] buffer = new byte[1024];
-                        string message1 = $"{receivedMessage} 1 {DateTime.Now.ToString("HH:mm:ss")}";
-                        buffer = System.Text.Encoding.UTF8.GetBytes(message1);
-                        var stamp = DateTime.Now.ToString("HH:mm:ss");
-                        Console.WriteLine($"[{stamp}] Server => Sending message: {message1}");
+                        string message = $"{receivedMessage} {DateTime.Now.ToString("HH:mm:ss")}";
+                        buffer = System.Text.Encoding.UTF8.GetBytes(message);
                         await socket.SendAsync(
-                            new ArraySegment<byte>(buffer, 0, message1.Length),
+                            new ArraySegment<byte>(buffer, 0, message.Length),
                             WebSocketMessageType.Text,
                             true,
                             CancellationToken.None);
-                        stamp = DateTime.Now.ToString("HH:mm:ss");
-                        Console.WriteLine($"[{stamp}] Server => Closing the socket");
                         await socket.CloseAsync(WebSocketCloseStatus.NormalClosure, receivedMessage, CancellationToken.None);
-                        stamp = DateTime.Now.ToString("HH:mm:ss");
-                        Console.WriteLine($"[{stamp}] Server => Socket got closed");
                     }
                     else if (socket.State == WebSocketState.Open)
                     {

src/libraries/System.Net.WebSockets.Client/tests/CloseTest.cs

@@ -370,6 +370,31 @@ await cws.SendAsync(
         [ActiveIssue("https://github.com/dotnet/runtime/issues/28957", typeof(PlatformDetection), nameof(PlatformDetection.IsNotBrowser))]
         [ConditionalTheory(nameof(WebSocketsSupported)), MemberData(nameof(EchoServers))]
         public async Task CloseOutputAsync_ServerInitiated_CanReceiveAfterClose(Uri server)
+        {
+            await ReceiveMessageAfterClose(server, false);
+        }
+
+        [ActiveIssue("https://github.com/dotnet/runtime/issues/28957", typeof(PlatformDetection), nameof(PlatformDetection.IsNotBrowser))]
+        [ConditionalTheory(nameof(WebSocketsSupported)), MemberData(nameof(EchoServers))]
+        public async Task CloseOutputAsync_ServerInitiated_CannotReceiveAfterCloseAndStateSync(Uri server)
+        {
+            try
+            {
+                await ReceiveMessageAfterClose(server, true);
+            }
+            catch (WebSocketException e)
+            {
+                Assert.Equal(WebSocketError.InvalidState, e.WebSocketErrorCode);
+                return;
+            }
+            catch (Exception e)
+            {
+                Assert.True(false, $"Unexpected exception: {e}");
+            }
+            Assert.True(false, "Expected WebSocketException not thrown.");
+        }
+
+        private async Task ReceiveMessageAfterClose(Uri server, bool syncState)
         {
             using (ClientWebSocket cws = await GetConnectedWebSocket(server, TimeOutMilliseconds, _output))
             {
@@ -380,23 +405,20 @@ await cws.SendAsync(
                     true,
                     cts.Token);
 
-                int delay = 2000;
-                var stamp = DateTime.Now.ToString("HH:mm:ss");
-                Console.WriteLine($"[{stamp}] Client -> '.receiveMessageAfterClose' was sent, waiting {delay}ms. cws.State={cws.State}");
-                await Task.Delay(delay); // even if we decrease to 100, it still fails with
-                // System.Net.WebSockets.WebSocketException : The WebSocket is in an invalid state ('Closed') for this operation. Valid states are: 'Open, CloseSent'
-                // in the manual demo we can wait even 6 sec and the message arrives just fine
+                await Task.Delay(2000);
+
+                if (syncState)
+                {
+                    var state = cws.State;
+                    Assert.Equal(WebSocketState.Closed, state);
+                    // should not be able to receive after this sync
+                }
 
-                stamp = DateTime.Now.ToString("HH:mm:ss");
-                Console.WriteLine($"[{stamp}] Client -> Attempting to receive data... cws.State={cws.State}");
                 var recvBuffer = new ArraySegment<byte>(new byte[1024]);
                 WebSocketReceiveResult recvResult = await cws.ReceiveAsync(recvBuffer, cts.Token);
                 var message = Encoding.UTF8.GetString(recvBuffer.ToArray(), 0, recvResult.Count);
 
-                stamp = DateTime.Now.ToString("HH:mm:ss");
-                Console.WriteLine($"[{stamp}] Client -> Received message={message}");
-                Assert.Contains(".shutdownAfterTwoMessages 1", message);
-                Console.WriteLine($"cws.State={cws.State}");
+                Assert.Contains(".receiveMessageAfterClose", message);
             }
         }