Description
Summary
I would like to suggest adding support for a single delegate in the ModelContextProtocol C# SDK to allow developers to capture structured logs of incoming requests, outgoing responses, and errors occurring during Tool execution.
Motivation
Today, the MCP Server processes and handles Tool calls internally, but there is no built-in way to observe:
- The exact JSON received in the request.
- The exact JSON sent in the response.
- Any exceptions that occurred during the Tool execution.
Having a logging delegate would provide much better visibility for developers, facilitating debugging, observability, auditing, and integration with external monitoring systems.
Proposed Solution
Introduce a single delegate that would be triggered in three situations:
- When a request is received.
- When a response is sent.
- When an error occurs.
Suggested delegate signature
public delegate void McpLogHandler(McpLogContext context);
public class McpLogContext
{
public McpStatus Status { get; set; } // RequestReceived, ResponseSent, or ErrorOccurred
public string Json { get; set; } // The raw JSON for request or response
public Exception? Exception { get; set; } // In case of errors
public string? ToolName { get; set; } // Optional, for Tool-related logs
}
public enum McpStatus
{
RequestReceived,
ResponseSent,
ErrorOccurred
}Example of usage when initializing the MCP server
builder.Services.AddMcpServer(options =>
{
options.OnLog = (context) =>
{
Console.WriteLine($"[{context.Status}]");
if (!string.IsNullOrWhiteSpace(context.ToolName))
Console.WriteLine($"Tool: {context.ToolName}");
Console.WriteLine("Payload:");
Console.WriteLine(context.Json);
if (context.Exception != null)
{
Console.WriteLine("Exception:");
Console.WriteLine(context.Exception);
}
};
});Benefits
- Full transparency of server operations.
- Easier debugging of Tool contracts and input/output.
- Integration with external log systems like Application Insights, OpenTelemetry, Datadog, etc.
- Ability to audit and trace individual operations without modifying core server logic.
- Keeps the MCP Server minimalistic for those who do not require additional logging (opt-in feature).
Final Note
Thank you very much for the great work on the MCP C# SDK.
This small addition would greatly improve the developer experience without introducing complexity to the existing usage.
It would make the SDK even more attractive and production-ready for real-world scenarios involving observability and auditing requirements.