From 0593818cd45b95d7f1de5b2a314557e464c421a0 Mon Sep 17 00:00:00 2001
From: Mikhail Dubov <mikhail@chattermill.io>
Date: Tue, 3 Feb 2026 19:51:48 +0000
Subject: [PATCH] Add client info to instrumentation callback

---
 README.md                  |  1 +
 lib/mcp/instrumentation.rb |  4 ++++
 lib/mcp/server.rb          |  4 +++-
 test/mcp/server_test.rb    | 47 ++++++++++++++++++++++++++++++++++++++
 4 files changed, 55 insertions(+), 1 deletion(-)

diff --git a/README.md b/README.md
index 6440384c..c1f2a3db 100644
--- a/README.md
+++ b/README.md
@@ -400,6 +400,7 @@ The instrumentation callback receives a hash with the following possible keys:
 - `resource_uri`: (String, optional) The URI of the resource called
 - `error`: (String, optional) Error code if a lookup failed
 - `duration`: (Float) Duration of the call in seconds
+- `client`: (Hash, optional) Client information with `name` and `version` keys, from the initialize request
 
 **Type:**
 
diff --git a/lib/mcp/instrumentation.rb b/lib/mcp/instrumentation.rb
index 40975a91..6c6d91d6 100644
--- a/lib/mcp/instrumentation.rb
+++ b/lib/mcp/instrumentation.rb
@@ -10,6 +10,10 @@ def instrument_call(method, &block)
 
         result = yield block
 
+        if respond_to?(:client) && client
+          add_instrumentation_data(client: client)
+        end
+
         result
       ensure
         end_time = Process.clock_gettime(Process::CLOCK_MONOTONIC)
diff --git a/lib/mcp/server.rb b/lib/mcp/server.rb
index e1005166..eff1a84b 100644
--- a/lib/mcp/server.rb
+++ b/lib/mcp/server.rb
@@ -41,7 +41,7 @@ def initialize(method_name)
 
     include Instrumentation
 
-    attr_accessor :description, :icons, :name, :title, :version, :website_url, :instructions, :tools, :prompts, :resources, :server_context, :configuration, :capabilities, :transport, :logging_message_notification
+    attr_accessor :description, :icons, :name, :title, :version, :website_url, :instructions, :tools, :prompts, :resources, :server_context, :configuration, :capabilities, :transport, :logging_message_notification, :client
 
     def initialize(
       description: nil,
@@ -75,6 +75,7 @@ def initialize(
       @resource_index = index_resources_by_uri(resources)
       @server_context = server_context
       @configuration = MCP.configuration.merge(configuration)
+      @client = nil
 
       validate!
 
@@ -314,6 +315,7 @@ def server_info
     end
 
     def init(request)
+      @client = request[:clientInfo] || request["clientInfo"] if request
       {
         protocolVersion: configuration.protocol_version,
         capabilities: capabilities,
diff --git a/test/mcp/server_test.rb b/test/mcp/server_test.rb
index 061987c1..9c81401d 100644
--- a/test/mcp/server_test.rb
+++ b/test/mcp/server_test.rb
@@ -159,6 +159,53 @@ class ServerTest < ActiveSupport::TestCase
       assert_instrumentation_data({ method: "initialize" })
     end
 
+    test "#handle initialize request with clientInfo includes client in instrumentation data" do
+      client_info = { name: "test_client", version: "1.0.0" }
+      request = {
+        jsonrpc: "2.0",
+        method: "initialize",
+        id: 1,
+        params: {
+          clientInfo: client_info,
+        },
+      }
+
+      @server.handle(request)
+      assert_instrumentation_data({ method: "initialize", client: client_info })
+    end
+
+    test "instrumentation data includes client info for subsequent requests after initialize" do
+      client_info = { name: "test_client", version: "1.0.0" }
+      initialize_request = {
+        jsonrpc: "2.0",
+        method: "initialize",
+        id: 1,
+        params: {
+          clientInfo: client_info,
+        },
+      }
+      @server.handle(initialize_request)
+
+      ping_request = {
+        jsonrpc: "2.0",
+        method: "ping",
+        id: 2,
+      }
+      @server.handle(ping_request)
+      assert_instrumentation_data({ method: "ping", client: client_info })
+    end
+
+    test "instrumentation data does not include client key when no clientInfo provided" do
+      request = {
+        jsonrpc: "2.0",
+        method: "ping",
+        id: 1,
+      }
+
+      @server.handle(request)
+      assert_instrumentation_data({ method: "ping" })
+    end
+
     test "#handle returns nil for notification requests" do
       request = {
         jsonrpc: "2.0",