Logging Capability (#53)

* Logging Capability

* Readability

* Redundant using

---------

Co-authored-by: Stephen Toub <stoub@microsoft.com>

src/ModelContextProtocol/Server/McpServer.cs

@@ -51,6 +51,7 @@ internal sealed class McpServer : McpJsonRpcEndpoint, IMcpServer
         SetToolsHandler(options);
         SetPromptsHandler(options);
         SetResourcesHandler(options);
+        SetSetLoggingLevelHandler(options);
     }
 
     public ClientCapabilities? ClientCapabilities { get; set; }
@@ -210,4 +211,19 @@ internal sealed class McpServer : McpJsonRpcEndpoint, IMcpServer
         SetRequestHandler<ListToolsRequestParams, ListToolsResult>("tools/list", (request, ct) => listToolsHandler(new(this, request), ct));
         SetRequestHandler<CallToolRequestParams, CallToolResponse>("tools/call", (request, ct) => callToolHandler(new(this, request), ct));
     }
+
+    private void SetSetLoggingLevelHandler(McpServerOptions options)
+    {
+        if (options.Capabilities?.Logging is not { } loggingCapability)
+        {
+            return;
+        }
+
+        if (loggingCapability.SetLoggingLevelHandler is not { } setLoggingLevelHandler)
+        {
+            throw new McpServerException("Logging capability was enabled, but SetLoggingLevelHandler was not specified.");
+        }
+
+        SetRequestHandler<SetLevelRequestParams, EmptyResult>("logging/setLevel", (request, ct) => setLoggingLevelHandler(new(this, request), ct));
+    }
 }

src/ModelContextProtocol/Server/McpServerHandlers.cs

@@ -57,6 +57,11 @@ public sealed class McpServerHandlers
     /// </summary>
     public Func<RequestContext<UnsubscribeRequestParams>, CancellationToken, Task<EmptyResult>>? UnsubscribeFromResourcesHandler { get; set; }
 
+    /// <summary>
+    /// Get or sets the handler for set logging level requests.
+    /// </summary>
+    public Func<RequestContext<SetLevelRequestParams>, CancellationToken, Task<EmptyResult>>? SetLoggingLevelHandler { get; set; }
+
     /// <summary>
     /// Overwrite any handlers in McpServerOptions with non-null handlers from this instance.
     /// </summary>
@@ -125,6 +130,20 @@ public sealed class McpServerHandlers
                 };
         }
 
+        LoggingCapability? loggingCapability = options.Capabilities?.Logging;
+        if (SetLoggingLevelHandler is not null)
+        {
+            loggingCapability = loggingCapability is null ?
+                new()
+                {
+                    SetLoggingLevelHandler = SetLoggingLevelHandler,
+                } :
+                loggingCapability with
+                {
+                    SetLoggingLevelHandler = SetLoggingLevelHandler ?? loggingCapability.SetLoggingLevelHandler,
+                };
+        }
+
         options.Capabilities = options.Capabilities is null ?
             new()
             {