diff --git a/packages/core-eslint/base.js b/packages/core-eslint/base.js
index 130aa20..02f839e 100644
--- a/packages/core-eslint/base.js
+++ b/packages/core-eslint/base.js
@@ -33,10 +33,9 @@ export default [
     plugins: { conformance: conformancePlugin },
     rules: {
       // Structural conformance rules (milestone iii.a).
-      // `feature-must-have-manifest` is WARN today because only auth has a manifest;
-      // flip to ERROR after blog/media/navigation/marketing-pages migrate.
+      // All 5 features now have manifests; promoted to ERROR.
       "conformance/feature-must-have-manifest": [
-        "warn",
+        "error",
         { repoRoot },
       ],
       "conformance/usecase-must-have-test-file": "error",