From 4dc2a8eb4a69d6a8890be175ab6f94c1e74604e9 Mon Sep 17 00:00:00 2001
From: Koichi ITO <koic.ito@gmail.com>
Date: Mon, 16 Feb 2026 16:08:16 +0900
Subject: [PATCH] Fix `logging/setLevel` to return empty hash per MCP
 specification

This PR fixes `logging/setLevel` to return empty hash per MCP specification.
The MCP specification requires `logging/setLevel` to return an empty result
on success, not nil or the `LoggingMessageNotification` object.

Specification reference:
https://modelcontextprotocol.io/specification/2025-11-25/server/utilities/logging

Fixes #229.
---
 lib/mcp/server.rb       |  2 ++
 test/mcp/server_test.rb | 18 ++++++++++++++++++
 2 files changed, 20 insertions(+)

diff --git a/lib/mcp/server.rb b/lib/mcp/server.rb
index 122cf99..d3cef70 100644
--- a/lib/mcp/server.rb
+++ b/lib/mcp/server.rb
@@ -362,6 +362,8 @@ def configure_logging_level(request)
       end
 
       @logging_message_notification = logging_message_notification
+
+      {}
     end
 
     def list_tools(request)
diff --git a/test/mcp/server_test.rb b/test/mcp/server_test.rb
index 37eeb77..3a059f0 100644
--- a/test/mcp/server_test.rb
+++ b/test/mcp/server_test.rb
@@ -810,6 +810,24 @@ class Example < Tool
       assert_instrumentation_data({ method: "resources/templates/list" })
     end
 
+    test "#configure_logging_level returns empty hash on success" do
+      response = @server.handle(
+        {
+          jsonrpc: "2.0",
+          id: 1,
+          method: "logging/setLevel",
+          params: {
+            level: "info",
+          },
+        },
+      )
+
+      assert_equal "2.0", response[:jsonrpc]
+      assert_equal 1, response[:id]
+      assert_empty(response[:result])
+      refute response.key?(:error)
+    end
+
     test "#configure_logging_level returns an error object when invalid log level is provided" do
       server = Server.new(
         tools: [TestTool],