diff --git a/Sources/MCP/Client/Client.swift b/Sources/MCP/Client/Client.swift
index 696ffd14..a2deb3dc 100644
--- a/Sources/MCP/Client/Client.swift
+++ b/Sources/MCP/Client/Client.swift
@@ -272,8 +272,7 @@ public actor Client {
         _ type: N.Type,
         handler: @escaping @Sendable (Message<N>) async throws -> Void
     ) async -> Self {
-        let handlers = notificationHandlers[N.name, default: []]
-        notificationHandlers[N.name] = handlers + [TypedNotificationHandler(handler)]
+        notificationHandlers[N.name, default: []].append(TypedNotificationHandler(handler))
         return self
     }
 
diff --git a/Sources/MCP/Server/Server.swift b/Sources/MCP/Server/Server.swift
index 6ba1e27b..f2c975af 100644
--- a/Sources/MCP/Server/Server.swift
+++ b/Sources/MCP/Server/Server.swift
@@ -276,8 +276,7 @@ public actor Server {
         _ type: N.Type,
         handler: @escaping @Sendable (Message<N>) async throws -> Void
     ) -> Self {
-        let handlers = notificationHandlers[N.name, default: []]
-        notificationHandlers[N.name] = handlers + [TypedNotificationHandler(handler)]
+        notificationHandlers[N.name, default: []].append(TypedNotificationHandler(handler))
         return self
     }