diff --git a/src/lib/exporter/streamingDecoder.ts b/src/lib/exporter/streamingDecoder.ts
index 24b9844..c9a9597 100644
--- a/src/lib/exporter/streamingDecoder.ts
+++ b/src/lib/exporter/streamingDecoder.ts
@@ -246,14 +246,13 @@ export class StreamingVideoDecoder {
 		const hintedDurationSec = Math.max(containerDurationSec, streamDurationSec, 0);
 		const scanEndSec =
 			hintedDurationSec > 0 ? hintedDurationSec + 0.5 : SCAN_UNBOUNDED_FALLBACK_SEC;
-		let maxPacketEndUs = 0;
+		let maxPacketTimestampUs = 0;
 		const scanReader = this.demuxer.read("video", 0, scanEndSec).getReader();
 		try {
 			while (true) {
 				const { done, value } = await scanReader.read();
 				if (done || !value) break;
-				const endUs = value.timestamp + (value.duration ?? 0);
-				if (endUs > maxPacketEndUs) maxPacketEndUs = endUs;
+				if (value.timestamp > maxPacketTimestampUs) maxPacketTimestampUs = value.timestamp;
 			}
 		} finally {
 			try {
@@ -262,7 +261,12 @@ export class StreamingVideoDecoder {
 				/* already closed */
 			}
 		}
-		const scannedDuration = maxPacketEndUs / 1_000_000;
+		// Use last frame's timestamp + one frame duration. The `duration` field on the last
+		// packet in a WebM container is frequently inflated by the demuxer to match the
+		// container's declared end, causing validateDuration to see no divergence and
+		// propagate a duration that the decoder can never actually reach.
+		const typicalFrameDurationUs = Math.ceil(1_000_000 / frameRate);
+		const scannedDuration = (maxPacketTimestampUs + typicalFrameDurationUs) / 1_000_000;
 		const validatedDuration = validateDuration(mediaInfo.duration, scannedDuration);
 
 		this.metadata = {
diff --git a/tests/fixtures/sample-inflated-duration.webm b/tests/fixtures/sample-inflated-duration.webm
new file mode 100644
index 0000000..6322840
Binary files /dev/null and b/tests/fixtures/sample-inflated-duration.webm differ