diff --git a/DeepDrftPublic/Interop/audio/AudioPlayer.ts b/DeepDrftPublic/Interop/audio/AudioPlayer.ts
index 354389a..06ce602 100644
--- a/DeepDrftPublic/Interop/audio/AudioPlayer.ts
+++ b/DeepDrftPublic/Interop/audio/AudioPlayer.ts
@@ -229,14 +229,21 @@ export class AudioPlayer {
             const decoder = this.opusDecoder!;
             const buffers = await decoder.push(chunk);
 
+            // Duration is known up front from the sidecar — surface it as soon as the decoder reports it,
+            // NOT gated on the first decoded buffers. The C# layer locks Duration on the first chunk whose
+            // result carries a value (the `Duration == null` guard), and WebCodecs decode is async, so the
+            // earliest chunks can return zero buffers; gating duration on buffers means C# captures the
+            // initial 0 and never overwrites it — the WAV header path sets duration on chunk 1 because its
+            // header parses synchronously, which is the asymmetry this closes. Set once so a seek (which
+            // reinitialises the decoder) cannot overwrite it.
+            if (this.duration === 0 && decoder.totalDuration) {
+                this.duration = decoder.totalDuration;
+            }
+
             if (buffers.length > 0) {
                 for (const buffer of buffers) {
                     this.scheduler.addBuffer(buffer);
                 }
-                // Duration is known up front from the sidecar; set once (a seek must not overwrite it).
-                if (this.duration === 0 && decoder.totalDuration) {
-                    this.duration = decoder.totalDuration;
-                }
                 if (this.streamingStarted && this.isPlaying) {
                     this.scheduler.scheduleNewBuffers();
                 }
diff --git a/DeepDrftPublic/Interop/audio/OpusStreamDecoder.test.ts b/DeepDrftPublic/Interop/audio/OpusStreamDecoder.test.ts
index 4a4ffc2..44f716b 100644
--- a/DeepDrftPublic/Interop/audio/OpusStreamDecoder.test.ts
+++ b/DeepDrftPublic/Interop/audio/OpusStreamDecoder.test.ts
@@ -33,6 +33,7 @@
 import { parseSidecar, presentationTimeSeconds, resolveOpusByteOffset, OPUS_SAMPLE_RATE } from './OpusSidecar.js';
 import type { OpusSeekData, OpusSeekResolution } from './OpusSidecar.js';
 import { OggDemuxer, extractOpusHead, opusHeadChannelCount } from './OggDemuxer.js';
+import { OpusStreamDecoder } from './OpusStreamDecoder.js';
 
 // --- tiny inline harness (no dependencies) ---------------------------------------------------
 let passed = 0;
@@ -414,6 +415,39 @@ test('extractOpusHead returns null when no OpusHead page is present', () => {
     assertNull(extractOpusHead(tags), 'no OpusHead');
 });
 
+// --- OpusStreamDecoder.totalDuration: available from the sidecar BEFORE the first push ----------
+//
+// Defect 1 (dead Opus seekbar): the C# layer locks the UI Duration on the first chunk whose result
+// carries a value, and AudioPlayer.processOpusChunk now surfaces `decoder.totalDuration` on that first
+// chunk rather than gating it on the (async, possibly-empty-on-chunk-1) decoded buffers. The load-bearing
+// guarantee that makes this correct is that `totalDuration` is known from the sidecar IMMEDIATELY — i.e.
+// before any push and without WebCodecs. These tests pin that contract; the WebCodecs decode itself stays
+// browser-verified. The constructor only stashes the context manager (totalDuration never touches it), so a
+// null-shaped stub is safe and no AudioDecoder is constructed.
+
+const stubContextManager = {} as unknown as ConstructorParameters<typeof OpusStreamDecoder>[0];
+
+test('OpusStreamDecoder.totalDuration is the sidecar duration, available before any push', () => {
+    const sidecar = sidecarFrom({
+        setupHeader: [0x4f, 0x70, 0x75, 0x73, 0x48, 0x65, 0x61, 0x64],
+        totalByteLength: 500_000, totalDuration: 212.5, preSkip: 312,
+        points: [{ granule: 312, byteOffset: 4096 }],
+    });
+    const decoder = new OpusStreamDecoder(stubContextManager, sidecar);
+    // No push, no configure — the value the first chunk reports to C# must already be present.
+    assertEqual(decoder.totalDuration, 212.5, 'totalDuration from sidecar, pre-push');
+});
+
+test('OpusStreamDecoder.totalDuration is null when the sidecar carries no positive duration', () => {
+    const sidecar = sidecarFrom({
+        setupHeader: [0], totalByteLength: 0, totalDuration: 0, preSkip: 0, points: [],
+    });
+    const decoder = new OpusStreamDecoder(stubContextManager, sidecar);
+    // A zero/absent sidecar duration must report null (not 0) so the chunk result carries no spurious
+    // value — the WAV-header path, not a bogus Opus duration, then drives the UI.
+    assertEqual(decoder.totalDuration, null, 'no positive duration -> null');
+});
+
 function concat(arrs: Uint8Array[]): Uint8Array {
     let len = 0;
     for (const a of arrs) len += a.length;