diff --git a/nodes.py b/nodes.py
index a981abb8e..f24bc4a5e 100644
--- a/nodes.py
+++ b/nodes.py
@@ -747,6 +747,13 @@ class SaveImage:
                 digits = 0
             return (digits, prefix)
 
+        def compute_vars(input):
+            input = input.replace("%width%", str(images[0].shape[1]))
+            input = input.replace("%height%", str(images[0].shape[0]))
+            return input
+
+        filename_prefix = compute_vars(filename_prefix)
+
         subfolder = os.path.dirname(os.path.normpath(filename_prefix))
         filename = os.path.basename(os.path.normpath(filename_prefix))
 
diff --git a/server.py b/server.py
index e4f688cf7..e71289cdf 100644
--- a/server.py
+++ b/server.py
@@ -29,6 +29,8 @@ async def cache_control(request: web.Request, handler):
 
 class PromptServer():
     def __init__(self, loop):
+        PromptServer.instance = self
+
         mimetypes.init(); 
         mimetypes.types_map['.js'] = 'application/javascript; charset=utf-8'
         self.prompt_queue = None
diff --git a/web/scripts/api.js b/web/scripts/api.js
index b90b1c656..2b90c2abc 100644
--- a/web/scripts/api.js
+++ b/web/scripts/api.js
@@ -1,8 +1,15 @@
 class ComfyApi extends EventTarget {
+	#registered = new Set();
+
 	constructor() {
 		super();
 	}
 
+	addEventListener(type, callback, options) {
+		super.addEventListener(type, callback, options);
+		this.#registered.add(type);
+	}
+
 	/**
 	 * Poll status  for colab and other things that don't support websockets.
 	 */
@@ -82,7 +89,11 @@ class ComfyApi extends EventTarget {
 						this.dispatchEvent(new CustomEvent("executed", { detail: msg.data }));
 						break;
 					default:
-						throw new Error("Unknown message type");
+						if (this.#registered.has(msg.type)) {
+							this.dispatchEvent(new CustomEvent(msg.type, { detail: msg.data }));
+						} else {
+							throw new Error("Unknown message type");
+						}
 				}
 			} catch (error) {
 				console.warn("Unhandled message:", event.data);