diff --git a/tests/background/webm-remux.test.ts b/tests/background/webm-remux.test.ts
index 5be8130..d86e9c9 100644
--- a/tests/background/webm-remux.test.ts
+++ b/tests/background/webm-remux.test.ts
@@ -55,13 +55,18 @@ import { tmpdir } from 'node:os';
 import type { VideoSegment } from '../../src/shared/types';
 
 const here = dirname(fileURLToPath(import.meta.url));
-const FIXTURE_PATH = resolve(here, '..', '..', 'tests', 'fixtures', 'last_30sec.webm');
+// Plan 01-08 Task 5 closeout note (2026-05-17): `tests/fixtures/last_30sec.webm`
+// is now the post-remux OUTPUT (single-EBML, validated by webm-playback.test.ts).
+// These unit tests need the pre-remux INPUT shape (3-EBML concat), so they
+// read from the preserved raw fixture instead.
+const FIXTURE_PATH = resolve(here, '..', '..', 'tests', 'fixtures', 'raw-3ebml-concat.webm');
 const FFPROBE_BIN = '/usr/bin/ffprobe';
 
-// Byte offsets of each EBML header in `tests/fixtures/last_30sec.webm`,
+// Byte offsets of each EBML header in `tests/fixtures/raw-3ebml-concat.webm`,
 // per the d13 debug session probe (Evidence/H4 byte-level EBML probe).
-// Verified at Plan 01-08 Task 2 land time against the committed
-// fixture: offsets [0, 509038, 970967]; total 1_633_459 bytes.
+// Verified at Plan 01-08 Task 2 land time against the original committed
+// fixture (now preserved as raw-3ebml-concat.webm): offsets [0, 509038, 970967];
+// total 1_633_459 bytes.
 const SEG1_START = 0;
 const SEG2_START = 509038;
 const SEG3_START = 970967;
diff --git a/tests/fixtures/last_30sec.webm b/tests/fixtures/last_30sec.webm
index a92981a..4898bcd 100644
Binary files a/tests/fixtures/last_30sec.webm and b/tests/fixtures/last_30sec.webm differ
diff --git a/tests/fixtures/raw-3ebml-concat.webm b/tests/fixtures/raw-3ebml-concat.webm
new file mode 100644
index 0000000..a92981a
Binary files /dev/null and b/tests/fixtures/raw-3ebml-concat.webm differ