diff --git a/packages/core/src/runtime/init.test.ts b/packages/core/src/runtime/init.test.ts
index 340456850c..2d73a7a21e 100644
--- a/packages/core/src/runtime/init.test.ts
+++ b/packages/core/src/runtime/init.test.ts
@@ -1415,6 +1415,36 @@ describe("initSandboxRuntimeModular", () => {
     expect(window.__player?.getDuration()).toBe(10);
   });
 
+  it("resumes readiness when GSAP batching starts after runtime initialization", async () => {
+    const root = document.createElement("div");
+    root.setAttribute("data-composition-id", "main");
+    root.setAttribute("data-root", "true");
+    root.setAttribute("data-start", "0");
+    root.setAttribute("data-width", "1920");
+    root.setAttribute("data-height", "1080");
+    document.body.appendChild(root);
+
+    let timelineDuration = 0;
+    const timeline = createMockTimeline(0);
+    timeline.duration = () => timelineDuration;
+    window.__timelines = { main: timeline };
+    window.__hfTimelinesBuilding = false;
+
+    initSandboxRuntimeModular();
+    expect(window.__renderReady).toBe(true);
+
+    window.__hfTimelinesBuilding = true;
+    await new Promise((resolve) => setTimeout(resolve, 0));
+    expect(window.__renderReady).toBe(false);
+
+    timelineDuration = 10;
+    window.__hfTimelinesBuilding = false;
+    window.dispatchEvent(new CustomEvent("hf-timelines-built"));
+
+    expect(window.__renderReady).toBe(true);
+    expect(window.__player?.getDuration()).toBe(10);
+  });
+
   it("waits for THREE.DefaultLoadingManager to drain before publishing render readiness", async () => {
     const root = document.createElement("div");
     root.setAttribute("data-composition-id", "main");
diff --git a/packages/core/src/runtime/init.ts b/packages/core/src/runtime/init.ts
index 24fb83cb57..b9e812a24a 100644
--- a/packages/core/src/runtime/init.ts
+++ b/packages/core/src/runtime/init.ts
@@ -2476,11 +2476,33 @@ export function initSandboxRuntimeModular(): void {
     postState(true);
   };
 
+  let timelinesBuiltListener: (() => void) | null = null;
+  const waitForTimelinesBuilt = () => {
+    if (timelinesBuiltListener) return;
+    const onTimelinesBuilt = () => {
+      window.removeEventListener("hf-timelines-built", onTimelinesBuilt);
+      timelinesBuiltListener = null;
+      maybePublishRenderReady();
+    };
+    timelinesBuiltListener = onTimelinesBuilt;
+    window.addEventListener("hf-timelines-built", onTimelinesBuilt);
+  };
+  registerRuntimeCleanup(() => {
+    if (!timelinesBuiltListener) return;
+    window.removeEventListener("hf-timelines-built", timelinesBuiltListener);
+    timelinesBuiltListener = null;
+  });
+
   maybePublishRenderReady = () => {
-    if (!externalCompositionsReady || window.__hfTimelinesBuilding) {
+    if (!externalCompositionsReady) {
       window.__renderReady = false;
       return;
     }
+    if (window.__hfTimelinesBuilding) {
+      window.__renderReady = false;
+      waitForTimelinesBuilt();
+      return;
+    }
     // Re-run discover so adapters can refresh their state from the current
     // DOM — e.g. the Three.js adapter only hooks `DefaultLoadingManager` once
     // it sees `window.THREE`, which may have loaded AFTER the initial
@@ -2499,14 +2521,6 @@ export function initSandboxRuntimeModular(): void {
   // synchronously. Wait for the "hf-timelines-built" event before the first
   // binding attempt so the transport clock receives the finished timeline
   // duration instead of permanently publishing duration=0.
-  if (window.__hfTimelinesBuilding) {
-    window.__renderReady = false;
-    const onTimelinesBuilt = () => {
-      window.removeEventListener("hf-timelines-built", onTimelinesBuilt);
-      maybePublishRenderReady();
-    };
-    window.addEventListener("hf-timelines-built", onTimelinesBuilt);
-  }
   maybePublishRenderReady();
 
   // When the bundler inlines compositions, data-composition-src is removed so