Avoid intermediate strings in MCP transport serialization (#1274)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>
Co-authored-by: Stephen Toub <stoub@microsoft.com>
Co-authored-by: halter73 <54385+halter73@users.noreply.github.com>

Author: 3 people

src/ModelContextProtocol.Core/Authentication/ClientOAuthProvider.cs

@@ -618,8 +618,9 @@ private async Task PerformDynamicClientRegistrationAsync(
             Scope = GetScopeParameter(protectedResourceMetadata),
         };
 
-        var requestJson = JsonSerializer.Serialize(registrationRequest, McpJsonUtilities.JsonContext.Default.DynamicClientRegistrationRequest);
-        using var requestContent = new StringContent(requestJson, Encoding.UTF8, "application/json");
+        var requestBytes = JsonSerializer.SerializeToUtf8Bytes(registrationRequest, McpJsonUtilities.JsonContext.Default.DynamicClientRegistrationRequest);
+        using var requestContent = new ByteArrayContent(requestBytes);
+        requestContent.Headers.ContentType = McpHttpClient.s_applicationJsonContentType;
 
         using var request = new HttpRequestMessage(HttpMethod.Post, authServerMetadata.RegistrationEndpoint)
         {

src/ModelContextProtocol.Core/Client/McpHttpClient.cs

@@ -1,17 +1,19 @@
 using ModelContextProtocol.Protocol;
 using System.Diagnostics;
+using System.Net.Http.Headers;
 
 #if NET
 using System.Net.Http.Json;
 #else
-using System.Text;
 using System.Text.Json;
 #endif
 
 namespace ModelContextProtocol.Client;
 
 internal class McpHttpClient(HttpClient httpClient)
 {
+    internal static readonly MediaTypeHeaderValue s_applicationJsonContentType = new("application/json") { CharSet = "utf-8" };
+
     internal virtual async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, JsonRpcMessage? message, CancellationToken cancellationToken)
     {
         Debug.Assert(request.Content is null, "The request body should only be supplied as a JsonRpcMessage");
@@ -32,11 +34,10 @@ internal virtual async Task<HttpResponseMessage> SendAsync(HttpRequestMessage re
 #if NET
         return JsonContent.Create(message, McpJsonUtilities.JsonContext.Default.JsonRpcMessage);
 #else
-        return new StringContent(
-            JsonSerializer.Serialize(message, McpJsonUtilities.JsonContext.Default.JsonRpcMessage),
-            Encoding.UTF8,
-            "application/json"
-        );
+        var bytes = JsonSerializer.SerializeToUtf8Bytes(message, McpJsonUtilities.JsonContext.Default.JsonRpcMessage);
+        var content = new ByteArrayContent(bytes);
+        content.Headers.ContentType = s_applicationJsonContentType;
+        return content;
 #endif
     }
 }

src/ModelContextProtocol.Core/Client/StreamClientSessionTransport.cs

@@ -8,10 +8,12 @@ namespace ModelContextProtocol.Client;
 /// <summary>Provides the client side of a stream-based session transport.</summary>
 internal class StreamClientSessionTransport : TransportBase
 {
+    private static readonly byte[] s_newlineBytes = "\n"u8.ToArray();
+
     internal static UTF8Encoding NoBomUtf8Encoding { get; } = new(encoderShouldEmitUTF8Identifier: false);
 
     private readonly TextReader _serverOutput;
-    private readonly TextWriter _serverInput;
+    private readonly Stream _serverInputStream;
     private readonly SemaphoreSlim _sendLock = new(1, 1);
     private CancellationTokenSource? _shutdownCts = new();
     private Task? _readTask;
@@ -20,12 +22,13 @@ internal class StreamClientSessionTransport : TransportBase
     /// Initializes a new instance of the <see cref="StreamClientSessionTransport"/> class.
     /// </summary>
     /// <param name="serverInput">
-    /// The text writer connected to the server's input stream.
-    /// Messages written to this writer will be sent to the server.
+    /// The server's input stream. Messages written to this stream will be sent to the server.
     /// </param>
     /// <param name="serverOutput">
-    /// The text reader connected to the server's output stream.
-    /// Messages read from this reader will be received from the server.
+    /// The server's output stream. Messages read from this stream will be received from the server.
+    /// </param>
+    /// <param name="encoding">
+    /// The encoding used for reading and writing messages from the input and output streams. Defaults to UTF-8 without BOM if null.
     /// </param>
     /// <param name="endpointName">
     /// A name that identifies this transport endpoint in logs.
@@ -37,12 +40,18 @@ internal class StreamClientSessionTransport : TransportBase
     /// This constructor starts a background task to read messages from the server output stream.
     /// The transport will be marked as connected once initialized.
     /// </remarks>
-    public StreamClientSessionTransport(
-        TextWriter serverInput, TextReader serverOutput, string endpointName, ILoggerFactory? loggerFactory)
+    public StreamClientSessionTransport(Stream serverInput, Stream serverOutput, Encoding? encoding, string endpointName, ILoggerFactory? loggerFactory)
         : base(endpointName, loggerFactory)
     {
-        _serverOutput = serverOutput;
-        _serverInput = serverInput;
+        Throw.IfNull(serverInput);
+        Throw.IfNull(serverOutput);
+
+        _serverInputStream = serverInput;
+#if NET
+        _serverOutput = new StreamReader(serverOutput, encoding ?? NoBomUtf8Encoding);
+#else
+        _serverOutput = new CancellableStreamReader(serverOutput, encoding ?? NoBomUtf8Encoding);
+#endif
 
         SetConnected();
 
@@ -57,43 +66,6 @@ public StreamClientSessionTransport(
         readTask.Start();
     }
 
-    /// <summary>
-    /// Initializes a new instance of the <see cref="StreamClientSessionTransport"/> class.
-    /// </summary>
-    /// <param name="serverInput">
-    /// The server's input stream. Messages written to this stream will be sent to the server.
-    /// </param>
-    /// <param name="serverOutput">
-    /// The server's output stream. Messages read from this stream will be received from the server.
-    /// </param>
-    /// <param name="encoding">
-    /// The encoding used for reading and writing messages from the input and output streams. Defaults to UTF-8 without BOM if null.
-    /// </param>
-    /// <param name="endpointName">
-    /// A name that identifies this transport endpoint in logs.
-    /// </param>
-    /// <param name="loggerFactory">
-    /// Optional factory for creating loggers. If null, a NullLogger is used.
-    /// </param>
-    /// <remarks>
-    /// This constructor starts a background task to read messages from the server output stream.
-    /// The transport will be marked as connected once initialized.
-    /// </remarks>
-    public StreamClientSessionTransport(Stream serverInput, Stream serverOutput, Encoding? encoding, string endpointName, ILoggerFactory? loggerFactory)
-        : this(
-            new StreamWriter(serverInput, encoding ?? NoBomUtf8Encoding),
-#if NET
-            new StreamReader(serverOutput, encoding ?? NoBomUtf8Encoding),
-#else
-            new CancellableStreamReader(serverOutput, encoding ?? NoBomUtf8Encoding),
-#endif
-            endpointName,
-            loggerFactory)
-    {
-        Throw.IfNull(serverInput);
-        Throw.IfNull(serverOutput);
-    }
-
     /// <inheritdoc/>
     public override async Task SendMessageAsync(JsonRpcMessage message, CancellationToken cancellationToken = default)
     {
@@ -103,16 +75,15 @@ public override async Task SendMessageAsync(JsonRpcMessage message, Cancellation
             id = messageWithId.Id.ToString();
         }
 
-        var json = JsonSerializer.Serialize(message, McpJsonUtilities.JsonContext.Default.JsonRpcMessage);
-
-        LogTransportSendingMessageSensitive(Name, json);
+        LogTransportSendingMessageSensitive(message);
 
         using var _ = await _sendLock.LockAsync(cancellationToken).ConfigureAwait(false);
         try
         {
-            // Write the message followed by a newline using our UTF-8 writer
-            await _serverInput.WriteLineAsync(json).ConfigureAwait(false);
-            await _serverInput.FlushAsync(cancellationToken).ConfigureAwait(false);
+            var json = JsonSerializer.SerializeToUtf8Bytes(message, McpJsonUtilities.JsonContext.Default.JsonRpcMessage);
+            await _serverInputStream.WriteAsync(json, cancellationToken).ConfigureAwait(false);
+            await _serverInputStream.WriteAsync(s_newlineBytes, cancellationToken).ConfigureAwait(false);
+            await _serverInputStream.FlushAsync(cancellationToken).ConfigureAwait(false);
         }
         catch (Exception ex)
         {

src/ModelContextProtocol.Core/Server/StreamServerTransport.cs

@@ -74,9 +74,9 @@ public override async Task SendMessageAsync(JsonRpcMessage message, Cancellation
 
         try
         {
-            var json = JsonSerializer.Serialize(message, McpJsonUtilities.JsonContext.Default.JsonRpcMessage);
-            LogTransportSendingMessageSensitive(Name, json);
-            await _outputStream.WriteAsync(Encoding.UTF8.GetBytes(json), cancellationToken).ConfigureAwait(false);
+            var json = JsonSerializer.SerializeToUtf8Bytes(message, McpJsonUtilities.JsonContext.Default.JsonRpcMessage);
+            LogTransportSendingMessageSensitive(message);
+            await _outputStream.WriteAsync(json, cancellationToken).ConfigureAwait(false);
             await _outputStream.WriteAsync(s_newlineBytes, cancellationToken).ConfigureAwait(false);
             await _outputStream.FlushAsync(cancellationToken).ConfigureAwait(false);
         }

tests/ModelContextProtocol.Tests/Transport/StdioClientTransportTests.cs

@@ -1,8 +1,10 @@
 using ModelContextProtocol.Client;
 using ModelContextProtocol.Protocol;
 using ModelContextProtocol.Tests.Utils;
+using System.IO.Pipelines;
 using System.Runtime.InteropServices;
 using System.Text;
+using System.Text.Json;
 
 namespace ModelContextProtocol.Tests.Transport;
 
@@ -136,4 +138,54 @@ public async Task EscapesCliArgumentsCorrectly(string? cliArgumentValue)
         var content = Assert.IsType<TextContentBlock>(Assert.Single(result.Content));
         Assert.Equal(cliArgumentValue ?? "", content.Text);
     }
+
+    [Fact]
+    public async Task SendMessageAsync_Should_Use_LF_Not_CRLF()
+    {
+        using var serverInput = new MemoryStream();
+        Pipe serverOutputPipe = new();
+
+        var transport = new StreamClientTransport(serverInput, serverOutputPipe.Reader.AsStream(), LoggerFactory);
+        await using var sessionTransport = await transport.ConnectAsync(TestContext.Current.CancellationToken);
+
+        var message = new JsonRpcRequest { Method = "test", Id = new RequestId(44) };
+
+        await sessionTransport.SendMessageAsync(message, TestContext.Current.CancellationToken);
+
+        byte[] bytes = serverInput.ToArray();
+
+        // The output should end with exactly \n (0x0A), not \r\n (0x0D 0x0A).
+        Assert.True(bytes.Length > 1, "Output should contain message data");
+        Assert.Equal((byte)'\n', bytes[^1]);
+        Assert.NotEqual((byte)'\r', bytes[^2]);
+
+        // Also verify the JSON content is valid
+        var json = Encoding.UTF8.GetString(bytes).TrimEnd('\n');
+        var expected = JsonSerializer.Serialize(message, McpJsonUtilities.DefaultOptions);
+        Assert.Equal(expected, json);
+    }
+
+    [Fact]
+    public async Task ReadMessagesAsync_Should_Accept_CRLF_Delimited_Messages()
+    {
+        Pipe serverInputPipe = new();
+        Pipe serverOutputPipe = new();
+
+        var transport = new StreamClientTransport(serverInputPipe.Writer.AsStream(), serverOutputPipe.Reader.AsStream(), LoggerFactory);
+        await using var sessionTransport = await transport.ConnectAsync(TestContext.Current.CancellationToken);
+
+        var message = new JsonRpcRequest { Method = "test", Id = new RequestId(44) };
+        var json = JsonSerializer.Serialize(message, McpJsonUtilities.DefaultOptions);
+
+        // Write a \r\n-delimited message to the server's output (which the client reads)
+        await serverOutputPipe.Writer.WriteAsync(Encoding.UTF8.GetBytes($"{json}\r\n"), TestContext.Current.CancellationToken);
+
+        var canRead = await sessionTransport.MessageReader.WaitToReadAsync(TestContext.Current.CancellationToken);
+
+        Assert.True(canRead, "Should be able to read a \\r\\n-delimited message");
+        Assert.True(sessionTransport.MessageReader.TryPeek(out var readMessage));
+        Assert.NotNull(readMessage);
+        Assert.IsType<JsonRpcRequest>(readMessage);
+        Assert.Equal("44", ((JsonRpcRequest)readMessage).Id.ToString());
+    }
 }

tests/ModelContextProtocol.Tests/Transport/StdioServerTransportTests.cs

@@ -227,6 +227,54 @@ public async Task SendMessageAsync_Should_Log_At_Trace_Level()
         Assert.Contains(traceLogMessages, x => x.Message.Contains("\"method\":\"test\"") && x.Message.Contains("\"id\":44"));
     }
 
+    [Fact]
+    public async Task SendMessageAsync_Should_Use_LF_Not_CRLF()
+    {
+        using var output = new MemoryStream();
+
+        await using var transport = new StreamServerTransport(
+            new Pipe().Reader.AsStream(),
+            output,
+            loggerFactory: LoggerFactory);
+
+        var message = new JsonRpcRequest { Method = "test", Id = new RequestId(44) };
+
+        await transport.SendMessageAsync(message, TestContext.Current.CancellationToken);
+
+        byte[] bytes = output.ToArray();
+
+        // The output should end with exactly \n (0x0A), not \r\n (0x0D 0x0A).
+        Assert.True(bytes.Length > 1, "Output should contain message data");
+        Assert.Equal((byte)'\n', bytes[^1]);
+        Assert.NotEqual((byte)'\r', bytes[^2]);
+    }
+
+    [Fact]
+    public async Task ReadMessagesAsync_Should_Accept_CRLF_Delimited_Messages()
+    {
+        var message = new JsonRpcRequest { Method = "test", Id = new RequestId(44) };
+        var json = JsonSerializer.Serialize(message, McpJsonUtilities.DefaultOptions);
+
+        Pipe pipe = new();
+        using var input = pipe.Reader.AsStream();
+
+        await using var transport = new StreamServerTransport(
+            input,
+            Stream.Null,
+            loggerFactory: LoggerFactory);
+
+        // Write the message with \r\n line ending
+        await pipe.Writer.WriteAsync(Encoding.UTF8.GetBytes($"{json}\r\n"), TestContext.Current.CancellationToken);
+
+        var canRead = await transport.MessageReader.WaitToReadAsync(TestContext.Current.CancellationToken);
+
+        Assert.True(canRead, "Should be able to read a \\r\\n-delimited message");
+        Assert.True(transport.MessageReader.TryPeek(out var readMessage));
+        Assert.NotNull(readMessage);
+        Assert.IsType<JsonRpcRequest>(readMessage);
+        Assert.Equal("44", ((JsonRpcRequest)readMessage).Id.ToString());
+    }
+
     [Fact]
     public async Task ReadMessagesAsync_Should_Log_Received_At_Trace_Level()
     {