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Handle requests for capabilty, when capability not present #290

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Handle requests for capabilty, when capability not present #290

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15 changes: 13 additions & 2 deletions src/ModelContextProtocol/Shared/McpSession.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -298,7 +298,18 @@ private void HandleMessageWithId(JsonRpcMessage message, JsonRpcMessageWithId me
if (!_requestHandlers.TryGetValue(request.Method, out var handler))
{
LogNoHandlerFoundForRequest(EndpointName, request.Method);
throw new McpException($"Method '{request.Method}' is not available.", McpErrorCode.MethodNotFound);
await SendMessageAsync(new JsonRpcError
{
Id = request.Id,
JsonRpc = "2.0",
Error = new JsonRpcErrorDetail
{
Message = $"{EndpointName} lacks capability to do {request.Method}",
Code = (int) McpErrorCode.MethodNotFound,
},
RelatedTransport = request.RelatedTransport
}, cancellationToken).ConfigureAwait(false);
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What does this achieve that's not already achieved when the caller a frame or two up catches the exception and similarly sends back a JsonRpcError?

csharp-sdk/src/ModelContextProtocol/Shared/McpSession.cs

Lines 135 to 166 in e190844

catch (Exception ex)
{
// Only send responses for request errors that aren't user-initiated cancellation.
bool isUserCancellation =
ex is OperationCanceledException &&
!cancellationToken.IsCancellationRequested &&
combinedCts?.IsCancellationRequested is true;
if (!isUserCancellation && message is JsonRpcRequest request)
{
LogRequestHandlerException(EndpointName, request.Method, ex);
JsonRpcErrorDetail detail = ex is McpException mcpe ?
new()
{
Code = (int)mcpe.ErrorCode,
Message = mcpe.Message,
} :
new()
{
Code = (int)McpErrorCode.InternalError,
Message = "An error occurred.",
};
await SendMessageAsync(new JsonRpcError
{
Id = request.Id,
JsonRpc = "2.0",
Error = detail,
RelatedTransport = request.RelatedTransport,
}, cancellationToken).ConfigureAwait(false);
}

Is the sole purpose here to avoid an exception try/catch, or are you trying to achieve something else?

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I think part of the goal is to get rid of the LogRequestHandlerException warning log, but I agree that instead special-casing McpExceptions with McpErrorCode.MethodNotFound in the existing catch to emit a lower-severity log makes more sense.

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@johnkors johnkors Apr 29, 2025 •
edited
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Yeah, lower the level from err to wrn is/was the intent.

Another option is to pass something in the Exception or fine-grained exception handling, but not sure how much you like passing state and using Exceptions for control flow.

Happy to change it.

return null;
}

LogRequestHandlerCalled(EndpointName, request.Method);
Expand Down Expand Up @@ -764,4 +775,4 @@ private static TimeSpan GetElapsed(long startingTimestamp) =>

[LoggerMessage(Level = LogLevel.Trace, Message = "{EndpointName} sending message. Message: '{Message}'.")]
private partial void LogSendingMessageSensitive(string endpointName, string message);
}
}