diff --git a/packages/spf/src/playback/behaviors/dom/sync-live-seekable-range.ts b/packages/spf/src/playback/behaviors/dom/sync-live-seekable-range.ts
index 01223cca..eb7d704e 100644
--- a/packages/spf/src/playback/behaviors/dom/sync-live-seekable-range.ts
+++ b/packages/spf/src/playback/behaviors/dom/sync-live-seekable-range.ts
@@ -57,13 +57,15 @@ function syncLiveSeekableRangeSetup({
   return effect(() => {
     const mediaSource = context.mediaSource.get();
     const liveWindow = liveWindowFromState(state);
-    if (!mediaSource || mediaSource.readyState !== 'open' || !liveWindow) return;
+    if (!mediaSource || !liveWindow) return;
 
     // Re-declared as the window slides so seekable tracks the live window
     // (the full DVR range remains seekable; seek-to-live-edge starts near the edge).
-    // No try/catch: `setLiveSeekableRange` throws only on a non-'open' readyState
-    // (checked synchronously just above — no await between, so it can't change) or
-    // an invalid range (`liveWindowFor` guarantees 0 ≤ start ≤ end).
+    // No readyState check, no try/catch: `setLiveSeekableRange` throws only on a
+    // non-'open' readyState or an invalid range, and neither can occur here —
+    // `setupMediaSource` publishes `context.mediaSource` only while open, and a
+    // non-null live window means the timeline-bearing track is still `Infinity`
+    // (so `endOfStream` hasn't ended the MS); `liveWindowFor` guarantees 0 ≤ start ≤ end.
     mediaSource.setLiveSeekableRange(liveWindow.start, liveWindow.end);
   });
 }
diff --git a/packages/spf/src/playback/behaviors/dom/tests/sync-live-seekable-range.test.ts b/packages/spf/src/playback/behaviors/dom/tests/sync-live-seekable-range.test.ts
index 869398b0..915fc251 100644
--- a/packages/spf/src/playback/behaviors/dom/tests/sync-live-seekable-range.test.ts
+++ b/packages/spf/src/playback/behaviors/dom/tests/sync-live-seekable-range.test.ts
@@ -65,11 +65,23 @@ describe('syncLiveSeekableRange', () => {
     cleanup();
   });
 
-  it('does nothing until the MediaSource is open', () => {
-    const ms = fakeMediaSource('closed');
-    const cleanup = run({ presentation: makePresentation(), trackId: 'v-1', mediaSource: ms });
+  it('declares only once the MediaSource is published (open)', async () => {
+    // `setupMediaSource` publishes `context.mediaSource` only once open, so an
+    // unpublished (absent) MediaSource is the "not ready" gate — no `readyState`
+    // check needed (presence + a live window ⟹ open).
+    const ms = fakeMediaSource();
+    const state = {
+      presentation: signal<MaybeResolvedPresentation | undefined>(makePresentation()),
+      selectedVideoTrackId: signal<string | undefined>('v-1'),
+    };
+    const context = { mediaSource: signal<MediaSource | undefined>(undefined) };
+    const cleanup = syncLiveSeekableRange.setup({ state, context, config: {} }) as () => void;
 
-    expect(ms.setLiveSeekableRange).not.toHaveBeenCalled();
+    expect(ms.setLiveSeekableRange).not.toHaveBeenCalled(); // unpublished → no declaration
+
+    context.mediaSource.set(ms); // published (open)
+    await Promise.resolve(); // effect re-runs on a microtask
+    expect(ms.setLiveSeekableRange).toHaveBeenCalledWith(100, 110);
 
     cleanup();
   });