fix(core): avoid live volume probe during render

packages/core/src/runtime/init.ts

@@ -1834,6 +1834,10 @@ export function initSandboxRuntimeModular(): void {
       state.capturedTimeline,
       getSafeTimelineDurationSeconds(state.capturedTimeline, 0),
       volumeKeyframeCache,
+      {
+        allowLiveTimelineSeek: !(window as Window & { __HF_RENDER_CAPTURE_MODE?: boolean })
+          .__HF_RENDER_CAPTURE_MODE,
+      },
     );
   };
 

packages/core/src/runtime/mediaVolumeEnvelope.test.ts

@@ -3,6 +3,32 @@ import { describe, expect, it } from "vitest";
 import { probeAndCacheElementVolume } from "./mediaVolumeEnvelope";
 
 describe("probeAndCacheElementVolume", () => {
+  it("does not seek or cache when live timeline probing is disabled", () => {
+    const audio = document.createElement("audio");
+    audio.dataset.volume = "1";
+    document.body.append(audio);
+
+    let seekCount = 0;
+    const timeline = {
+      totalTime(next?: number) {
+        if (next !== undefined) {
+          seekCount += 1;
+          audio.volume = next >= 1 ? 0 : 1;
+        }
+        return 0.75;
+      },
+    };
+    const cache = new WeakMap<HTMLMediaElement, { time: number; volume: number }[]>();
+
+    probeAndCacheElementVolume(audio, timeline, 1, cache, {
+      allowLiveTimelineSeek: false,
+    });
+
+    expect(seekCount).toBe(0);
+    expect(audio.volume).toBe(1);
+    expect(cache.has(audio)).toBe(false);
+  });
+
   it("restores the timeline playhead after sampling volume automation", () => {
     const audio = document.createElement("audio");
     audio.dataset.volume = "1";

packages/core/src/runtime/mediaVolumeEnvelope.ts

@@ -129,6 +129,18 @@ export function probeElementVolumeKeyframes(
 
 export type RuntimeTimelineRef = Partial<Pick<RuntimeTimelineLike, "totalTime" | "seek">>;
 
+export interface VolumeProbeOptions {
+  /**
+   * Render/probe pages must not sample the live visual timeline during runtime
+   * initialization. The producer discovers audio automation in its own
+   * isolated pass and bakes it before frame capture, so seeking here is both
+   * redundant and capable of materializing future zero-duration GSAP state.
+   *
+   * Preview callers omit this option and retain live automation discovery.
+   */
+  allowLiveTimelineSeek?: boolean;
+}
+
 /**
  * Probe a media element and, if volume automation is detected, store the
  * keyframes in `cache`. Safe to call with a null timeline — returns early.
@@ -138,7 +150,9 @@ export function probeAndCacheElementVolume(
   timeline: RuntimeTimelineRef | null | undefined,
   compositionDuration: number,
   cache: WeakMap<HTMLMediaElement, VolumeKeyframe[]>,
+  options: VolumeProbeOptions = {},
 ): void {
+  if (options.allowLiveTimelineSeek === false) return;
   if (!timeline) return;
   if (!(mediaEl instanceof HTMLAudioElement) && !(mediaEl instanceof HTMLVideoElement)) return;
   if (compositionDuration <= 0) return;