Add Trace-level logging for JSON-RPC payloads in transports

- Add LogTransportSendingMessage and LogTransportSendingMessageSensitive to TransportBase
- Update StreamServerTransport to log sent messages at Trace level
- Update StreamClientSessionTransport to log sent messages at Trace level
- Update SseClientSessionTransport to log sent messages at Trace level
- Update StreamableHttpClientSessionTransport to log sent messages at Trace level
- Add tests for Trace-level logging of sent and received messages

Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

Author: Copilot

src/ModelContextProtocol.Core/Client/SseClientSessionTransport.cs

@@ -80,6 +80,15 @@ public override async Task SendMessageAsync(
             messageId = messageWithId.Id.ToString();
         }
 
+        if (_logger.IsEnabled(LogLevel.Trace))
+        {
+            LogTransportSendingMessageSensitive(Name, JsonSerializer.Serialize(message, McpJsonUtilities.JsonContext.Default.JsonRpcMessage));
+        }
+        else
+        {
+            LogTransportSendingMessage(Name, messageId);
+        }
+
         using var httpRequestMessage = new HttpRequestMessage(HttpMethod.Post, _messageEndpoint);
         StreamableHttpClientSessionTransport.CopyAdditionalHeaders(httpRequestMessage.Headers, _options.AdditionalHeaders, sessionId: null, protocolVersion: null);
         var response = await _httpClient.SendAsync(httpRequestMessage, message, cancellationToken).ConfigureAwait(false);

src/ModelContextProtocol.Core/Client/StreamClientSessionTransport.cs

@@ -105,6 +105,15 @@ public override async Task SendMessageAsync(JsonRpcMessage message, Cancellation
 
         var json = JsonSerializer.Serialize(message, McpJsonUtilities.JsonContext.Default.JsonRpcMessage);
 
+        if (Logger.IsEnabled(LogLevel.Trace))
+        {
+            LogTransportSendingMessageSensitive(Name, json);
+        }
+        else
+        {
+            LogTransportSendingMessage(Name, id);
+        }
+
         using var _ = await _sendLock.LockAsync(cancellationToken).ConfigureAwait(false);
         try
         {

src/ModelContextProtocol.Core/Client/StreamableHttpClientSessionTransport.cs

@@ -75,6 +75,21 @@ internal async Task<HttpResponseMessage> SendHttpRequestAsync(JsonRpcMessage mes
                 $"Call {nameof(McpClient)}.{nameof(McpClient.ResumeSessionAsync)} to resume existing sessions.");
         }
 
+        string messageId = "(no id)";
+        if (message is JsonRpcMessageWithId messageWithId)
+        {
+            messageId = messageWithId.Id.ToString();
+        }
+
+        if (_logger.IsEnabled(LogLevel.Trace))
+        {
+            LogTransportSendingMessageSensitive(Name, JsonSerializer.Serialize(message, McpJsonUtilities.JsonContext.Default.JsonRpcMessage));
+        }
+        else
+        {
+            LogTransportSendingMessage(Name, messageId);
+        }
+
         using var sendCts = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken, _connectionCts.Token);
         cancellationToken = sendCts.Token;
 

src/ModelContextProtocol.Core/Protocol/TransportBase.cs

@@ -166,6 +166,12 @@ protected void SetDisconnected(Exception? error = null)
     [LoggerMessage(Level = LogLevel.Error, Message = "{EndpointName} transport send failed for message ID '{MessageId}'.")]
     private protected partial void LogTransportSendFailed(string endpointName, string messageId, Exception exception);
 
+    [LoggerMessage(Level = LogLevel.Debug, Message = "{EndpointName} transport sending message with ID '{MessageId}'.")]
+    private protected partial void LogTransportSendingMessage(string endpointName, string messageId);
+
+    [LoggerMessage(Level = LogLevel.Trace, Message = "{EndpointName} transport sending message. Message: '{Message}'.")]
+    private protected partial void LogTransportSendingMessageSensitive(string endpointName, string message);
+
     [LoggerMessage(Level = LogLevel.Information, Message = "{EndpointName} transport reading messages.")]
     private protected partial void LogTransportEnteringReadMessagesLoop(string endpointName);
 

src/ModelContextProtocol.Core/Server/StreamServerTransport.cs

@@ -74,7 +74,17 @@ public override async Task SendMessageAsync(JsonRpcMessage message, Cancellation
 
         try
         {
-            await JsonSerializer.SerializeAsync(_outputStream, message, McpJsonUtilities.DefaultOptions.GetTypeInfo(typeof(JsonRpcMessage)), cancellationToken).ConfigureAwait(false);
+            if (Logger.IsEnabled(LogLevel.Trace))
+            {
+                var json = JsonSerializer.Serialize(message, McpJsonUtilities.JsonContext.Default.JsonRpcMessage);
+                LogTransportSendingMessageSensitive(Name, json);
+                await _outputStream.WriteAsync(Encoding.UTF8.GetBytes(json), cancellationToken).ConfigureAwait(false);
+            }
+            else
+            {
+                LogTransportSendingMessage(Name, id);
+                await JsonSerializer.SerializeAsync(_outputStream, message, McpJsonUtilities.DefaultOptions.GetTypeInfo(typeof(JsonRpcMessage)), cancellationToken).ConfigureAwait(false);
+            }
             await _outputStream.WriteAsync(s_newlineBytes, cancellationToken).ConfigureAwait(false);
             await _outputStream.FlushAsync(cancellationToken).ConfigureAwait(false);
         }

tests/ModelContextProtocol.Tests/Transport/StdioServerTransportTests.cs

@@ -1,4 +1,5 @@
-using ModelContextProtocol.Protocol;
+using Microsoft.Extensions.Logging;
+using ModelContextProtocol.Protocol;
 using ModelContextProtocol.Server;
 using ModelContextProtocol.Tests.Utils;
 using System.IO.Pipelines;
@@ -193,4 +194,72 @@ public async Task SendMessageAsync_Should_Preserve_Unicode_Characters()
         Assert.True(magnifyingGlassFound, "Magnifying glass emoji not found in result");
         Assert.True(rocketFound, "Rocket emoji not found in result");
     }
+
+    [Fact]
+    public async Task SendMessageAsync_Should_Log_At_Trace_Level()
+    {
+        // Arrange
+        var mockLoggerProvider = new MockLoggerProvider();
+        using var traceLoggerFactory = Microsoft.Extensions.Logging.LoggerFactory.Create(builder =>
+        {
+            builder.AddProvider(mockLoggerProvider);
+            builder.SetMinimumLevel(LogLevel.Trace);
+        });
+        using var output = new MemoryStream();
+
+        await using var transport = new StreamServerTransport(
+            new Pipe().Reader.AsStream(),
+            output,
+            loggerFactory: traceLoggerFactory);
+
+        // Act
+        var message = new JsonRpcRequest { Method = "test", Id = new RequestId(44) };
+        await transport.SendMessageAsync(message, TestContext.Current.CancellationToken);
+
+        // Assert
+        var traceLogMessages = mockLoggerProvider.LogMessages
+            .Where(x => x.LogLevel == LogLevel.Trace && x.Message.Contains("transport sending message"))
+            .ToList();
+
+        Assert.NotEmpty(traceLogMessages);
+        Assert.Contains(traceLogMessages, x => x.Message.Contains("\"method\":\"test\"") && x.Message.Contains("\"id\":44"));
+    }
+
+    [Fact]
+    public async Task ReadMessagesAsync_Should_Log_Received_At_Trace_Level()
+    {
+        // Arrange
+        var mockLoggerProvider = new MockLoggerProvider();
+        using var traceLoggerFactory = Microsoft.Extensions.Logging.LoggerFactory.Create(builder =>
+        {
+            builder.AddProvider(mockLoggerProvider);
+            builder.SetMinimumLevel(LogLevel.Trace);
+        });
+
+        var message = new JsonRpcRequest { Method = "test", Id = new RequestId(99) };
+        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: traceLoggerFactory);
+
+        // Act
+        await pipe.Writer.WriteAsync(Encoding.UTF8.GetBytes($"{json}\n"), TestContext.Current.CancellationToken);
+
+        // Wait for the message to be processed
+        var canRead = await transport.MessageReader.WaitToReadAsync(TestContext.Current.CancellationToken);
+        Assert.True(canRead, "Nothing to read here from transport message reader");
+
+        // Assert
+        var traceLogMessages = mockLoggerProvider.LogMessages
+            .Where(x => x.LogLevel == LogLevel.Trace && x.Message.Contains("transport received message"))
+            .ToList();
+
+        Assert.NotEmpty(traceLogMessages);
+        Assert.Contains(traceLogMessages, x => x.Message.Contains("\"method\":\"test\"") && x.Message.Contains("\"id\":99"));
+    }
 }