diff --git a/backend/instance_url.ts b/backend/instance_url.ts
index 50e81ca..b453028 100644
--- a/backend/instance_url.ts
+++ b/backend/instance_url.ts
@@ -1,4 +1,5 @@
 import { isWebContainer, HostURL } from "@webcontainer/env";
+import { env } from "process";
 
 export function getInstanceUrl(port: number) {
   if (isWebContainer()) {
@@ -6,5 +7,9 @@ export function getInstanceUrl(port: number) {
     // This is why we need to convert it here.
     return HostURL.parse(`https://localhost:${port}`).href;
   }
+  if (env["CODESPACE_NAME"]) {
+    return `${env["CODESPACE_NAME"]}-${port}.app.github.dev`;
+  }
+
   return `http://localhost:${port}`;
 }
diff --git a/sim/webxdc.ts b/sim/webxdc.ts
index a8d8b02..55678fb 100644
--- a/sim/webxdc.ts
+++ b/sim/webxdc.ts
@@ -97,6 +97,11 @@ export class DevServerTransport implements Transport {
         /--(\d+)--/.exec(document.location.href)?.[1] ||
         "error in webxdc simulator"
       );
+    } else if (location.host.endsWith(".app.github.dev")) {
+      return (
+        /-(\d+)\.app\.github\.dev/.exec(document.location.href)?.[1] ||
+        "error in webxdc simulator"
+      );
     } else {
       return document.location.port;
     }