Fix stdio encoding to always be UTF8

Author: stephentoub

src/ModelContextProtocol/Logging/Log.cs

@@ -113,8 +113,8 @@ internal static partial class Log
     [LoggerMessage(Level = LogLevel.Error, Message = "Transport not connected for {endpointName}")]
     internal static partial void TransportNotConnected(this ILogger logger, string endpointName);
 
-    [LoggerMessage(Level = LogLevel.Information, Message = "Transport sending message for {endpointName} with ID {messageId}, JSON {json}")]
-    internal static partial void TransportSendingMessage(this ILogger logger, string endpointName, string messageId, string json);
+    [LoggerMessage(Level = LogLevel.Information, Message = "Transport sending message for {endpointName} with ID {messageId}")]
+    internal static partial void TransportSendingMessage(this ILogger logger, string endpointName, string messageId);
 
     [LoggerMessage(Level = LogLevel.Information, Message = "Transport message sent for {endpointName} with ID {messageId}")]
     internal static partial void TransportSentMessage(this ILogger logger, string endpointName, string messageId);

src/ModelContextProtocol/Protocol/Transport/HttpListenerSseServerTransport.cs

@@ -76,7 +76,7 @@ public override async Task SendMessageAsync(IJsonRpcMessage message, Cancellatio
             if (_logger.IsEnabled(LogLevel.Debug))
             {
                 var json = JsonSerializer.Serialize(message, McpJsonUtilities.DefaultOptions.GetTypeInfo<IJsonRpcMessage>());
-                _logger.TransportSendingMessage(EndpointName, id, json);
+                _logger.TransportSendingMessage(EndpointName, id);
             }
 
             await _sseResponseStreamTransport.SendMessageAsync(message, cancellationToken).ConfigureAwait(false);

src/ModelContextProtocol/Protocol/Transport/StdioClientTransport.cs

@@ -1,4 +1,5 @@
 using System.Diagnostics;
+using System.Text;
 using System.Text.Json;
 using ModelContextProtocol.Configuration;
 using ModelContextProtocol.Logging;
@@ -59,7 +60,7 @@ public async Task ConnectAsync(CancellationToken cancellationToken = default)
 
             _shutdownCts = new CancellationTokenSource();
 
-            var startInfo = new ProcessStartInfo
+            ProcessStartInfo startInfo = new()
             {
                 FileName = _options.Command,
                 RedirectStandardInput = true,
@@ -68,6 +69,8 @@ public async Task ConnectAsync(CancellationToken cancellationToken = default)
                 UseShellExecute = false,
                 CreateNoWindow = true,
                 WorkingDirectory = _options.WorkingDirectory ?? Environment.CurrentDirectory,
+                StandardOutputEncoding = Encoding.UTF8,
+                StandardErrorEncoding = Encoding.UTF8,
             };
 
             if (!string.IsNullOrWhiteSpace(_options.Arguments))
@@ -92,11 +95,39 @@ public async Task ConnectAsync(CancellationToken cancellationToken = default)
             // Set up error logging
             _process.ErrorDataReceived += (sender, args) => _logger.TransportError(EndpointName, args.Data ?? "(no data)");
 
-            if (!_process.Start())
+            bool processStarted;
+#if NET
+            startInfo.StandardInputEncoding = Encoding.UTF8;
+            processStarted = _process.Start();
+#else
+            // netstandard2.0 lacks ProcessStartInfo.StandardInputEncoding. Instead, it always
+            // uses Console.InputEncoding, so we need to temporarily change Console.InputEncoding
+            // if it's not already UTF-8.
+            Encoding oldStdinEncoding = Console.InputEncoding;
+            if (oldStdinEncoding.CodePage == Encoding.UTF8.CodePage)
+            {
+                processStarted = _process.Start();
+            }
+            else
+            {
+                try
+                {
+                    Console.InputEncoding = Encoding.UTF8;
+                    processStarted = _process.Start();
+                }
+                finally
+                {
+                    Console.InputEncoding = oldStdinEncoding;
+                }
+            }
+#endif
+
+            if (!processStarted)
             {
                 _logger.TransportProcessStartFailed(EndpointName);
                 throw new McpTransportException("Failed to start MCP server process");
             }
+
             _logger.TransportProcessStarted(EndpointName, _process.Id);
             _processStarted = true;
             _process.BeginErrorReadLine();
@@ -132,8 +163,9 @@ public override async Task SendMessageAsync(IJsonRpcMessage message, Cancellatio
 
         try
         {
+            _logger.TransportSendingMessage(EndpointName, id);
+
             var json = JsonSerializer.Serialize(message, _jsonOptions.GetTypeInfo<IJsonRpcMessage>());
-            _logger.TransportSendingMessage(EndpointName, id, json);
 
             // Write the message followed by a newline
             await _process!.StandardInput.WriteLineAsync(json.AsMemory(), cancellationToken).ConfigureAwait(false);

src/ModelContextProtocol/Protocol/Transport/StdioServerTransport.cs

@@ -7,6 +7,7 @@
 using Microsoft.Extensions.Logging;
 using Microsoft.Extensions.Logging.Abstractions;
 using Microsoft.Extensions.Options;
+using System.Text;
 
 namespace ModelContextProtocol.Protocol.Transport;
 
@@ -15,12 +16,14 @@ namespace ModelContextProtocol.Protocol.Transport;
 /// </summary>
 public sealed class StdioServerTransport : TransportBase, IServerTransport
 {
+    private static readonly byte[] s_newLineBytes = Encoding.UTF8.GetBytes(Environment.NewLine);
+    
     private readonly string _serverName;
     private readonly ILogger _logger;
 
     private readonly JsonSerializerOptions _jsonOptions = McpJsonUtilities.DefaultOptions;
-    private readonly TextReader _stdin = Console.In;
-    private readonly TextWriter _stdout = Console.Out;
+    private readonly StreamReader _stdin = new StreamReader(Console.OpenStandardInput(), Encoding.UTF8);
+    private readonly Stream _stdout = new BufferedStream(Console.OpenStandardOutput());
 
     private Task? _readTask;
     private CancellationTokenSource? _shutdownCts;
@@ -114,11 +117,11 @@ public override async Task SendMessageAsync(IJsonRpcMessage message, Cancellatio
 
         try
         {
-            var json = JsonSerializer.Serialize(message, _jsonOptions.GetTypeInfo<IJsonRpcMessage>());
-            _logger.TransportSendingMessage(EndpointName, id, json);
+            _logger.TransportSendingMessage(EndpointName, id);
 
-            await _stdout.WriteLineAsync(json.AsMemory(), cancellationToken).ConfigureAwait(false);
-            await _stdout.FlushAsync(cancellationToken).ConfigureAwait(false);
+            await JsonSerializer.SerializeAsync(_stdout, message, _jsonOptions.GetTypeInfo<IJsonRpcMessage>(), cancellationToken);
+            await _stdout.WriteAsync(s_newLineBytes, cancellationToken);
+            await _stdout.FlushAsync(cancellationToken);
 
             _logger.TransportSentMessage(EndpointName, id);
         }
@@ -239,6 +242,9 @@ private async Task CleanupAsync(CancellationToken cancellationToken)
             _logger.TransportReadTaskCleanedUp(EndpointName);
         }
 
+        _stdin.Dispose();
+        _stdout.Dispose();
+
         SetConnected(false);
         _logger.TransportCleanedUp(EndpointName);
     }