diff --git a/apps/web/src/connection/desktopLocalDiscovery.expbkt3.ts b/apps/web/src/connection/desktopLocalDiscovery.expbkt3.ts
new file mode 100644
index 000000000000..9e31cf9f9ac4
--- /dev/null
+++ b/apps/web/src/connection/desktopLocalDiscovery.expbkt3.ts
@@ -0,0 +1,15 @@
+/**
+ * T3-CUSTOM(expbkt3): how long a desktop-local backend may take to describe itself.
+ *
+ * The platform source builds every registration before it hands any of them
+ * to the registry, so the primary environment is not connected until each
+ * desktop-local backend has answered. A managed build starts its bundled
+ * backend after the window opens, and a backend still starting holds every
+ * request until startup finishes. With the default 10 s request timeout, a
+ * window opened during that start waited 10 s before it even asked the
+ * remote primary for a WebSocket ticket.
+ *
+ * A ready loopback backend answers in milliseconds. One that misses this
+ * deadline is skipped for this poll and retried on the next one, 3 s later.
+ */
+export const DESKTOP_LOCAL_DESCRIPTOR_TIMEOUT_MS = 1_500;
diff --git a/apps/web/src/connection/platform.ts b/apps/web/src/connection/platform.ts
index 3cb074f54d1c..261d3c46422c 100644
--- a/apps/web/src/connection/platform.ts
+++ b/apps/web/src/connection/platform.ts
@@ -77,6 +77,8 @@ import { connectionStorageLayer } from "./storage";
 import { clientPresentationMetadata } from "./clientMetadata";
 // T3-CUSTOM(expbkt3): focus wakeups probe unless the window was away >= 60 s.
 import { type FocusWakeup, makeFocusWakeupTracker } from "./focusWakeup.expbkt3";
+// T3-CUSTOM(expbkt3): desktop-local discovery must not delay the primary.
+import { DESKTOP_LOCAL_DESCRIPTOR_TIMEOUT_MS } from "./desktopLocalDiscovery.expbkt3";
 
 let nextObservedRpcRequestId = 0;
 
@@ -411,9 +413,11 @@ const loadSecondaryConnectionRegistration = Effect.fn(
   }
   const httpBaseUrl = entry.httpBaseUrl;
   const wsBaseUrl = entry.wsBaseUrl;
-  const descriptor = yield* fetchRemoteEnvironmentDescriptor({ httpBaseUrl }).pipe(
-    Effect.mapError(mapRemoteEnvironmentError),
-  );
+  const descriptor = yield* fetchRemoteEnvironmentDescriptor({
+    httpBaseUrl,
+    // T3-CUSTOM(expbkt3): a starting local backend must not hold up the primary.
+    timeoutMs: DESKTOP_LOCAL_DESCRIPTOR_TIMEOUT_MS,
+  }).pipe(Effect.mapError(mapRemoteEnvironmentError));
   const issuedAtEpochMs = yield* Clock.currentTimeMillis;
   const access = yield* bootstrapRemoteBearerSession({
     httpBaseUrl,