Intermediate strings created in MCP transport

[ericstj]

When fixing #1064 I was looking at other parts of the stack and noticed there are at least a few other places where we could go directly toa/from UTF-8 but do not.

csharp-sdk/src/ModelContextProtocol.Core/Server/StreamServerTransport.cs

Lines 77 to 79 in 27b5eb8

	var json = JsonSerializer.Serialize(message, McpJsonUtilities.JsonContext.Default.JsonRpcMessage);
	LogTransportSendingMessageSensitive(Name, json);
	await _outputStream.WriteAsync(Encoding.UTF8.GetBytes(json), cancellationToken).ConfigureAwait(false);

csharp-sdk/src/ModelContextProtocol.Core/Server/StreamServerTransport.cs

Lines 100 to 124 in 27b5eb8

	var line = await _inputReader.ReadLineAsync(shutdownToken).ConfigureAwait(false);
	if (string.IsNullOrWhiteSpace(line))
	{
	if (line is null)
	{
	LogTransportEndOfStream(Name);
	break;
	}
	continue;
	}
	LogTransportReceivedMessageSensitive(Name, line);
	try
	{
	if (JsonSerializer.Deserialize(line, McpJsonUtilities.DefaultOptions.GetTypeInfo(typeof(JsonRpcMessage))) is JsonRpcMessage message)
	{
	await WriteMessageAsync(message, shutdownToken).ConfigureAwait(false);
	}
	else
	{
	LogTransportMessageParseUnexpectedTypeSensitive(Name, line);
	}
	}

csharp-sdk/src/ModelContextProtocol.Core/Client/StreamClientSessionTransport.cs

Lines 106 to 114 in 27b5eb8

	var json = JsonSerializer.Serialize(message, McpJsonUtilities.JsonContext.Default.JsonRpcMessage);
	LogTransportSendingMessageSensitive(Name, json);
	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);

csharp-sdk/src/ModelContextProtocol.Core/Client/StreamClientSessionTransport.cs

Lines 137 to 150 in 27b5eb8

	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);

We do have a some places that try to avoid strings -

csharp-sdk/src/Common/ServerSentEvents/SseEventWriterHelpers.cs

Lines 51 to 56 in 27b5eb8

	int maxByteCount = Encoding.UTF8.GetMaxByteCount(value.Length);
	Span<byte> buffer = writer.GetSpan(maxByteCount);
	Debug.Assert(buffer.Length >= maxByteCount);
	int bytesWritten = Encoding.UTF8.GetBytes(value, buffer);
	writer.Advance(bytesWritten);

If folks agree we can make a pass at these and related types to avoid encoding to strings and go straight from UTF-8 bytes on the wire to serialized objects.