Avoid string intermediates in MCP transport read path

src/Common/EncodingUtilities.cs

@@ -20,6 +20,19 @@ public static byte[] GetUtf8Bytes(ReadOnlySpan<char> utf16)
         return bytes;
     }
 
+    /// <summary>Decodes a UTF-8 <see cref="ReadOnlySequence{T}"/> to a <see cref="string"/>.</summary>
+    public static string GetUtf8String(in ReadOnlySequence<byte> sequence)
+    {
+        if (sequence.IsEmpty)
+        {
+            return string.Empty;
+        }
+
+        return sequence.IsSingleSegment
+            ? Encoding.UTF8.GetString(sequence.First.Span)
+            : Encoding.UTF8.GetString(sequence.ToArray());
+    }
+
     /// <summary>
     /// Encodes binary data to base64-encoded UTF-8 bytes.
     /// </summary>

src/Common/Polyfills/System/Text/EncodingExtensions.cs

@@ -45,6 +45,25 @@ public static int GetBytes(this Encoding encoding, ReadOnlySpan<char> chars, Spa
             }
         }
     }
+
+    /// <summary>
+    /// Decodes all the bytes in the specified span into a string.
+    /// </summary>
+    public static string GetString(this Encoding encoding, ReadOnlySpan<byte> bytes)
+    {
+        if (bytes.IsEmpty)
+        {
+            return string.Empty;
+        }
+
+        unsafe
+        {
+            fixed (byte* bytesPtr = bytes)
+            {
+                return encoding.GetString(bytesPtr, bytes.Length);
+            }
+        }
+    }
 }
 
 #endif

src/ModelContextProtocol.Core/Client/StdioClientSessionTransport.cs

@@ -7,7 +7,7 @@ namespace ModelContextProtocol.Client;
 /// <summary>Provides the client side of a stdio-based session transport.</summary>
 internal sealed class StdioClientSessionTransport(
     StdioClientTransportOptions options, Process process, string endpointName, Queue<string> stderrRollingLog, ILoggerFactory? loggerFactory) :
-    StreamClientSessionTransport(process.StandardInput.BaseStream, process.StandardOutput.BaseStream, encoding: null, endpointName, loggerFactory)
+    StreamClientSessionTransport(process.StandardInput.BaseStream, process.StandardOutput.BaseStream, endpointName, loggerFactory)
 {
     private readonly StdioClientTransportOptions _options = options;
     private readonly Process _process = process;

src/ModelContextProtocol.Core/Client/StdioClientTransport.cs

@@ -31,6 +31,8 @@ public sealed partial class StdioClientTransport : IClientTransport
     private static readonly object s_consoleEncodingLock = new();
 #endif
 
+    private static readonly UTF8Encoding s_noBomUtf8Encoding = new(encoderShouldEmitUTF8Identifier: false);
+
     private readonly StdioClientTransportOptions _options;
     private readonly ILoggerFactory? _loggerFactory;
 
@@ -85,10 +87,10 @@ public async Task<ITransport> ConnectAsync(CancellationToken cancellationToken =
                 UseShellExecute = false,
                 CreateNoWindow = true,
                 WorkingDirectory = _options.WorkingDirectory ?? Environment.CurrentDirectory,
-                StandardOutputEncoding = StreamClientSessionTransport.NoBomUtf8Encoding,
-                StandardErrorEncoding = StreamClientSessionTransport.NoBomUtf8Encoding,
+                StandardOutputEncoding = s_noBomUtf8Encoding,
+                StandardErrorEncoding = s_noBomUtf8Encoding,
 #if NET
-                StandardInputEncoding = StreamClientSessionTransport.NoBomUtf8Encoding,
+                StandardInputEncoding = s_noBomUtf8Encoding,
 #endif
             };
 
@@ -173,7 +175,7 @@ public async Task<ITransport> ConnectAsync(CancellationToken cancellationToken =
                 Encoding originalInputEncoding = Console.InputEncoding;
                 try
                 {
-                    Console.InputEncoding = StreamClientSessionTransport.NoBomUtf8Encoding;
+                    Console.InputEncoding = s_noBomUtf8Encoding;
                     processStarted = process.Start();
                 }
                 finally

src/ModelContextProtocol.Core/Client/StreamClientSessionTransport.cs

@@ -1,6 +1,7 @@
 using Microsoft.Extensions.Logging;
 using ModelContextProtocol.Protocol;
-using System.Text;
+using System.Buffers;
+using System.IO.Pipelines;
 using System.Text.Json;
 
 namespace ModelContextProtocol.Client;
@@ -10,9 +11,7 @@ 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 PipeReader _serverOutputPipe;
     private readonly Stream _serverInputStream;
     private readonly SemaphoreSlim _sendLock = new(1, 1);
     private CancellationTokenSource? _shutdownCts = new();
@@ -27,9 +26,6 @@ internal class StreamClientSessionTransport : TransportBase
     /// <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>
@@ -40,18 +36,14 @@ 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(Stream serverInput, Stream serverOutput, Encoding? encoding, string endpointName, ILoggerFactory? loggerFactory)
+    public StreamClientSessionTransport(Stream serverInput, Stream serverOutput, string endpointName, ILoggerFactory? loggerFactory)
         : base(endpointName, loggerFactory)
     {
         Throw.IfNull(serverInput);
         Throw.IfNull(serverOutput);
 
         _serverInputStream = serverInput;
-#if NET
-        _serverOutput = new StreamReader(serverOutput, encoding ?? NoBomUtf8Encoding);
-#else
-        _serverOutput = new CancellableStreamReader(serverOutput, encoding ?? NoBomUtf8Encoding);
-#endif
+        _serverOutputPipe = PipeReader.Create(serverOutput);
 
         SetConnected();
 
@@ -102,24 +94,8 @@ private async Task ReadMessagesAsync(CancellationToken cancellationToken)
         try
         {
             LogTransportEnteringReadMessagesLoop(Name);
-
-            while (true)
-            {
-                if (await _serverOutput.ReadLineAsync(cancellationToken).ConfigureAwait(false) is not string line)
-                {
-                    LogTransportEndOfStream(Name);
-                    break;
-                }
-
-                if (string.IsNullOrWhiteSpace(line))
-                {
-                    continue;
-                }
-
-                LogTransportReceivedMessageSensitive(Name, line);
-
-                await ProcessMessageAsync(line, cancellationToken).ConfigureAwait(false);
-            }
+            await _serverOutputPipe.ReadLinesAsync(ProcessLineAsync, cancellationToken).ConfigureAwait(false);
+            LogTransportEndOfStream(Name);
         }
         catch (OperationCanceledException)
         {
@@ -137,25 +113,43 @@ private async Task ReadMessagesAsync(CancellationToken cancellationToken)
         }
     }
 
-    private async Task ProcessMessageAsync(string line, CancellationToken cancellationToken)
+    private async Task ProcessLineAsync(ReadOnlySequence<byte> line, CancellationToken cancellationToken)
     {
+        if (Logger.IsEnabled(LogLevel.Trace))
+        {
+            LogTransportReceivedMessageSensitive(Name, EncodingUtilities.GetUtf8String(line));
+        }
+
         try
         {
-            var message = (JsonRpcMessage?)JsonSerializer.Deserialize(line.AsSpan().Trim(), McpJsonUtilities.DefaultOptions.GetTypeInfo(typeof(JsonRpcMessage)));
-            if (message != null)
+            JsonRpcMessage? message;
+            if (line.IsSingleSegment)
+            {
+                message = JsonSerializer.Deserialize(line.First.Span, McpJsonUtilities.DefaultOptions.GetTypeInfo(typeof(JsonRpcMessage))) as JsonRpcMessage;
+            }
+            else
+            {
+                var reader = new Utf8JsonReader(line, isFinalBlock: true, state: default);
+                message = JsonSerializer.Deserialize(ref reader, McpJsonUtilities.DefaultOptions.GetTypeInfo(typeof(JsonRpcMessage))) as JsonRpcMessage;
+            }
+
+            if (message is not null)
             {
                 await WriteMessageAsync(message, cancellationToken).ConfigureAwait(false);
             }
             else
             {
-                LogTransportMessageParseUnexpectedTypeSensitive(Name, line);
+                if (Logger.IsEnabled(LogLevel.Trace))
+                {
+                    LogTransportMessageParseUnexpectedTypeSensitive(Name, EncodingUtilities.GetUtf8String(line));
+                }
             }
         }
         catch (JsonException ex)
         {
             if (Logger.IsEnabled(LogLevel.Trace))
             {
-                LogTransportMessageParseFailedSensitive(Name, line, ex);
+                LogTransportMessageParseFailedSensitive(Name, EncodingUtilities.GetUtf8String(line), ex);
             }
             else
             {

src/ModelContextProtocol.Core/Client/StreamClientTransport.cs

@@ -50,7 +50,6 @@ public Task<ITransport> ConnectAsync(CancellationToken cancellationToken = defau
         return Task.FromResult<ITransport>(new StreamClientSessionTransport(
             _serverInput,
             _serverOutput,
-            encoding: null,
             "Client (stream)",
             _loggerFactory));
     }

src/ModelContextProtocol.Core/Protocol/PipeReaderExtensions.cs

@@ -0,0 +1,72 @@
+using System.Buffers;
+using System.IO.Pipelines;
+
+namespace ModelContextProtocol.Protocol;
+
+/// <summary>Internal helper for reading newline-delimited UTF-8 lines from a <see cref="PipeReader"/>.</summary>
+internal static class PipeReaderExtensions
+{
+    /// <summary>
+    /// Reads newline-delimited lines from <paramref name="reader"/>, invoking
+    /// <paramref name="processLine"/> for each non-empty line, until the reader signals completion.
+    /// </summary>
+    internal static async Task ReadLinesAsync(
+        this PipeReader reader,
+        Func<ReadOnlySequence<byte>, CancellationToken, Task> processLine,
+        CancellationToken cancellationToken)
+    {
+        while (true)
+        {
+            ReadResult result = await reader.ReadAsync(cancellationToken).ConfigureAwait(false);
+            ReadOnlySequence<byte> buffer = result.Buffer;
+
+            SequencePosition? position;
+            while ((position = buffer.PositionOf((byte)'\n')) != null)
+            {
+                ReadOnlySequence<byte> line = buffer.Slice(0, position.Value);
+
+                // Trim trailing \r for Windows-style CRLF line endings.
+                if (EndsWithCarriageReturn(line))
+                {
+                    line = line.Slice(0, line.Length - 1);
+                }
+
+                if (!line.IsEmpty)
+                {
+                    await processLine(line, cancellationToken).ConfigureAwait(false);
+                }
+
+                // Advance past the '\n'.
+                buffer = buffer.Slice(buffer.GetPosition(1, position.Value));
+            }
+
+            reader.AdvanceTo(buffer.Start, buffer.End);
+
+            if (result.IsCompleted)
+            {
+                break;
+            }
+        }
+    }
+
+    private static bool EndsWithCarriageReturn(in ReadOnlySequence<byte> sequence)
+    {
+        if (sequence.IsSingleSegment)
+        {
+            ReadOnlySpan<byte> span = sequence.First.Span;
+            return span.Length > 0 && span[span.Length - 1] == (byte)'\r';
+        }
+
+        // Multi-segment: find the last non-empty segment to check its last byte.
+        ReadOnlyMemory<byte> last = default;
+        foreach (ReadOnlyMemory<byte> segment in sequence)
+        {
+            if (!segment.IsEmpty)
+            {
+                last = segment;
+            }
+        }
+
+        return !last.IsEmpty && last.Span[last.Length - 1] == (byte)'\r';
+    }
+}