diff --git a/package.json b/package.json
index 8f363871..eb62d3ef 100644
--- a/package.json
+++ b/package.json
@@ -20,7 +20,7 @@
     "start:ff": "node tools/launch-ff.mjs",
     "start:ff:fresh": "node tools/launch-ff-fresh.mjs",
     "start:ff:clean": "web-ext run --source-dir=build_firefox_github --target=firefox-desktop",
-    "test": "vitest run",
+    "test": "node tools/sync-vendor.mjs && vitest run",
     "test:watch": "vitest",
     "test:coverage": "vitest run --coverage",
     "test:workflows": "bash tests/workflows/run.sh",
diff --git a/tools/sync-vendor.mjs b/tools/sync-vendor.mjs
index 495f4b5e..4f450cbb 100644
--- a/tools/sync-vendor.mjs
+++ b/tools/sync-vendor.mjs
@@ -11,7 +11,7 @@
 // so a reviewer can check the base by hash and read the diff in minutes.
 //
 // The generated files are gitignored. Run after `pnpm install`, or let
-// `pnpm run build` do it.
+// `pnpm run build` or `pnpm test` do it.
 
 import fs from 'node:fs';
 import path from 'node:path';
diff --git a/vitest.config.mjs b/vitest.config.mjs
index cacbeeec..ce28312c 100644
--- a/vitest.config.mjs
+++ b/vitest.config.mjs
@@ -2,8 +2,10 @@ import path from 'node:path';
 import {defineConfig} from 'vitest/config';
 
 // Libraries tools/sync-vendor.mjs copies out of node_modules unchanged (patched by pnpm at
-// install). The copies are generated, and CI runs the unit tests before it makes them, so a
-// test of code that imports one (the save's MP4 writers) gets the npm build itself.
+// install). A test of code that imports one (the save's MP4 writers) gets the npm build itself.
+// `pnpm test` also makes the copies first: in one local checkout this redirect was not
+// called (2026-11-09), and the unit tests there ran on copies from before a dependency
+// update (31 failures in hls2mp4 until a build).
 const root = import.meta.dirname;
 const VENDORED = new Map([
   ['chrome/player/modules/hls.mjs', 'node_modules/hls.js/dist/hls.mjs'],