diff --git a/packages/core/src/plugin-type-closed-set.test.ts b/packages/core/src/plugin-type-closed-set.test.ts
index d998ad50e4..ef059cb5c7 100644
--- a/packages/core/src/plugin-type-closed-set.test.ts
+++ b/packages/core/src/plugin-type-closed-set.test.ts
@@ -66,8 +66,10 @@ describe('Plugin.type closed set — runtime parity with the spec enum (#13925)'
 
     it('a non-member is refused by PluginSchema with invalid_value at ["type"]', () => {
         // `'plugin'` / `'module'` are PACKAGE manifest types (ManifestSchema.type),
-        // never plugin types; `'ui-plugin'` is the spelling a stale describe()
-        // string still uses; the casing variant guards against a lax comparator.
+        // never plugin types; `'ui-plugin'` is the LEGACY spelling of today's
+        // `'ui'` — once live, so callers outside this repo may still send it, and
+        // the closed set must keep REFUSING it rather than grow a tolerant alias
+        // (Prime Directive #12); the casing variant guards against a lax comparator.
         for (const type of ['bogus', 'ui-plugin', 'plugin', 'module', 'Standard']) {
             const result = PluginSchema.safeParse({ type });
             expect(result.success, `PluginSchema accepted non-member '${type}'`).toBe(false);
diff --git a/packages/rest/src/plugin-type-closed-set.pin.test.ts b/packages/rest/src/plugin-type-closed-set.pin.test.ts
index 6ee1e65ec1..8315856ac8 100644
--- a/packages/rest/src/plugin-type-closed-set.pin.test.ts
+++ b/packages/rest/src/plugin-type-closed-set.pin.test.ts
@@ -67,8 +67,10 @@ describe('Plugin.type closed set — published-surface pins (#13925)', () => {
         const declared: PluginMetadata = {
             name: 'closed-set-pin-metadata',
             version: '1.0.0',
-            // @ts-expect-error — `'ui-plugin'` (a stale describe() spelling) is
-            // not a `PluginType` (#13925).
+            // @ts-expect-error — `'ui-plugin'` is the LEGACY spelling of today's
+            // `'ui'` — once live, so callers outside this repo may still send it,
+            // and the published union must keep REFUSING it rather than grow a
+            // tolerant alias (Prime Directive #12). Not a `PluginType` (#13925).
             type: 'ui-plugin',
             async init() {},
         };