Address feedback and fix test failure

Author: stephentoub

src/ModelContextProtocol/Protocol/Transport/StdioClientSessionTransport.cs

@@ -86,13 +86,21 @@ protected override async ValueTask CleanupAsync(Exception? error = null, Cancell
         }
         catch { }
 
-        string errorMessage = "MCP server process exited unexpectedly.";
+        string errorMessage = "MCP server process exited unexpectedly";
+
+        string? exitCode = null;
+        try
+        {
+            exitCode = $" (exit code: {(uint)_process.ExitCode})";
+        }
+        catch { }
+
         lock (_stderrRollingLog)
         {
             if (_stderrRollingLog.Count > 0)
             {
                 errorMessage =
-                    $"{errorMessage}{Environment.NewLine}" +
+                    $"{errorMessage}{exitCode}{Environment.NewLine}" +
                     $"Server's stderr tail:{Environment.NewLine}" +
                     $"{string.Join(Environment.NewLine, _stderrRollingLog)}";
             }

src/ModelContextProtocol/Protocol/Transport/StreamClientSessionTransport.cs

@@ -59,11 +59,6 @@ public StreamClientSessionTransport(
     /// <inheritdoc/>
     public override async Task SendMessageAsync(JsonRpcMessage message, CancellationToken cancellationToken = default)
     {
-        if (!IsConnected)
-        {
-            throw new InvalidOperationException("Transport is not connected.");
-        }
-
         string id = "(no id)";
         if (message is JsonRpcMessageWithId messageWithId)
         {
@@ -82,7 +77,7 @@ public override async Task SendMessageAsync(JsonRpcMessage message, Cancellation
         catch (Exception ex)
         {
             LogTransportSendFailed(Name, id, ex);
-            throw new IOException("Failed to send message", ex);
+            throw new IOException("Failed to send message.", ex);
         }
     }
 

src/ModelContextProtocol/Protocol/Transport/StreamServerTransport.cs

@@ -60,7 +60,7 @@ public override async Task SendMessageAsync(JsonRpcMessage message, Cancellation
     {
         if (!IsConnected)
         {
-            throw new InvalidOperationException("Transport is not connected.");
+            return;
         }
 
         using var _ = await _sendLock.LockAsync(cancellationToken).ConfigureAwait(false);

src/ModelContextProtocol/Protocol/Transport/TransportBase.cs

@@ -115,7 +115,7 @@ protected void SetConnected()
                     return;
 
                 case StateDisconnected:
-                    throw new InvalidOperationException("Transport is already disconnected and can't be reconnected.");
+                    throw new IOException("Transport is already disconnected and can't be reconnected.");
 
                 default:
                     Debug.Fail($"Unexpected state: {state}");

src/ModelContextProtocol/Shared/McpEndpoint.cs

@@ -125,7 +125,18 @@ public virtual async ValueTask DisposeUnsynchronizedAsync()
     }
 
     protected McpSession GetSessionOrThrow()
-        => _session ?? throw new InvalidOperationException($"This should be unreachable from public API! Call {nameof(InitializeSession)} before sending messages.");
+    {
+#if NET
+        ObjectDisposedException.ThrowIf(_disposed, this);
+#else
+        if (_disposed)
+        {
+            throw new ObjectDisposedException(GetType().Name);
+        }
+#endif
+
+        return _session ?? throw new InvalidOperationException($"This should be unreachable from public API! Call {nameof(InitializeSession)} before sending messages.");
+    }
 
     [LoggerMessage(Level = LogLevel.Information, Message = "{EndpointName} shutting down.")]
     private partial void LogEndpointShuttingDown(string endpointName);

tests/ModelContextProtocol.Tests/Transport/StdioClientTransportTests.cs

@@ -1,5 +1,6 @@
 using ModelContextProtocol.Client;
 using ModelContextProtocol.Protocol.Transport;
+using System.Runtime.InteropServices;
 
 namespace ModelContextProtocol.Tests.Transport;
 
@@ -8,13 +9,13 @@ public class StdioClientTransportTests
     [Fact]
     public async Task CreateAsync_ValidProcessInvalidServer_Throws()
     {
-        StdioClientTransport transport = new(new() { Command = "echo", Arguments = ["this is a test", "1>&2"] });
+        string id = Guid.NewGuid().ToString("N");
+
+        StdioClientTransport transport = RuntimeInformation.IsOSPlatform(OSPlatform.Windows) ?
+            new(new() { Command = "cmd", Arguments = ["/C", $"echo \"{id}\" >&2"] }) :
+            new(new() { Command = "ls", Arguments = [id] });
 
         IOException e = await Assert.ThrowsAsync<IOException>(() => McpClientFactory.CreateAsync(transport, cancellationToken: TestContext.Current.CancellationToken));
-        string exStr = e.ToString();
-        if (!exStr.Contains("this is a test"))
-        {
-            throw new Exception($"Expected error message not found in exception: {exStr}");
-        }
+        Assert.Contains(id, e.ToString());
     }
 }