diff --git a/Sources/MCP/Client/Client.swift b/Sources/MCP/Client/Client.swift
index f6ec77a..e240665 100644
--- a/Sources/MCP/Client/Client.swift
+++ b/Sources/MCP/Client/Client.swift
@@ -244,8 +244,7 @@ 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 15ad3db..077103f 100644
--- a/Sources/MCP/Server/Server.swift
+++ b/Sources/MCP/Server/Server.swift
@@ -257,8 +257,7 @@ 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
     }