diff --git a/static/app.js b/static/app.js
index 8d606083..d3062590 100644
--- a/static/app.js
+++ b/static/app.js
@@ -2319,11 +2319,14 @@ configureHost({
     currentFilename: () => currentFilename,
 });
 
+// `esc` is here for out-of-tree plugins only: their screen.js loads as a classic
+// script and called esc() back when app.js was one too and it was an implicit
+// global. Nothing in core reads window.esc — import it from ./js/dom.js instead.
 Object.assign(window, {
     _confirmDialog, _getArrangementNamingMode, _libraryLocalFilename, _librarySongArtUrl,
     _librarySongId, _onHeaderClick, _onNamingModeChange, _trapFocusInModal,
     changeArrangement, checkPluginUpdates, clearLibFilters, clearLoop,
-    deleteSelectedLoop, exportDiagnostics, exportSettings, filterFavorites,
+    deleteSelectedLoop, esc, exportDiagnostics, exportSettings, filterFavorites,
     filterLibrary, fullRescanLibrary, goFavPage, handleSliderInput,
     hideScanBanner, importSettings, loadPlugins, loadSavedLoop,
     loadSettings, onSectionPracticeModeChange, openEditModal, persistSetting,
diff --git a/tests/browser/plugin-globals-contract.spec.ts b/tests/browser/plugin-globals-contract.spec.ts
new file mode 100644
index 00000000..0915016d
--- /dev/null
+++ b/tests/browser/plugin-globals-contract.spec.ts
@@ -0,0 +1,63 @@
+// The window globals are a THIRD-PARTY CONTRACT. Pin them.
+//
+// Out-of-tree plugins load their screen.js as a CLASSIC script and call these
+// as bare globals. Nothing in core reads most of them, so a call-graph scan,
+// ESLint's no-undef, and a grep all come back clean while the plugin breaks in
+// the field. This is the frontend twin of tests/test_plugin_context_contract.py
+// — same reasoning, same literal-list rule.
+//
+// This guard is retroactive: `esc` was an implicit global back when app.js was
+// a classic script, went module-scoped in a9fce29, and got carved into
+// js/dom.js in 14b4058. The re-export list at the bottom of app.js was rebuilt
+// without it, and the MIDI plugin's device list threw "esc is not defined" for
+// testers — reported as "MIDI Access denied", because the ReferenceError landed
+// in a try/catch meant for permission failures.
+//
+// WHY A LITERAL LIST AND NOT A DERIVED ONE. Deriving the expected set from
+// app.js would assert the code equals itself. The point is that a human has to
+// look at a diff and consciously agree to change the contract.
+
+import { test, expect } from '@playwright/test';
+
+const PLUGIN_GLOBALS = [
+  '_confirmDialog', '_getArrangementNamingMode', '_libraryLocalFilename', '_librarySongArtUrl',
+  '_librarySongId', '_onHeaderClick', '_onNamingModeChange', '_trapFocusInModal',
+  'changeArrangement', 'checkPluginUpdates', 'clearLibFilters', 'clearLoop',
+  'deleteSelectedLoop', 'esc', 'exportDiagnostics', 'exportSettings', 'filterFavorites',
+  'filterLibrary', 'fullRescanLibrary', 'goFavPage', 'handleSliderInput',
+  'hideScanBanner', 'importSettings', 'loadPlugins', 'loadSavedLoop',
+  'loadSettings', 'onSectionPracticeModeChange', 'openEditModal', 'persistSetting',
+  'pickDlcFolder', 'pinCurrentArrangementDefault', 'playSong', 'previewDiagnostics',
+  'previewEditArt', 'renderGridCards', 'renderTreeInto', 'rescanLibrary',
+  'retuneSong', 'saveCurrentLoop', 'saveSettings', 'seekBy',
+  'setAvOffsetMs', 'setFavView', 'setInstrumentPathway', 'setLibView',
+  'setLibraryProvider', 'setLoopEnd', 'setLoopStart', 'setMastery',
+  'setSpeed', 'setViz', 'showScreen', 'sortFavorites',
+  'sortLibrary', 'syncLibrarySong', 'toggleAllArtists', 'toggleAllFavoriteArtists',
+  'toggleLibFilters', 'togglePlay', 'toggleSectionPracticePopover', 'uiPrompt',
+  'updatePlugin', 'uploadSongs',
+  'filterFavTreeLetter', 'filterTreeLetter', 'goFavTreePage', 'goTreePage',
+];
+
+test('plugin-facing window globals are all callable', async ({ page }) => {
+  await page.goto('/');
+  await page.waitForSelector('.screen.active', { timeout: 10000 });
+
+  const missing = await page.evaluate(
+    (names) => names.filter((n) => typeof (window as any)[n] !== 'function'),
+    PLUGIN_GLOBALS,
+  );
+
+  expect(missing, `window globals plugins depend on are missing or not functions: ${missing.join(', ')}`).toEqual([]);
+});
+
+// The plugin call site that actually broke: esc() interpolated into a template
+// string. A global that exists but doesn't escape is its own bug.
+test('window.esc escapes HTML metacharacters', async ({ page }) => {
+  await page.goto('/');
+  await page.waitForSelector('.screen.active', { timeout: 10000 });
+
+  const escaped = await page.evaluate(() => (window as any).esc('<img src=x onerror=alert(1)>'));
+  expect(escaped).not.toContain('<img');
+  expect(escaped).toContain('&lt;');
+});