diff --git a/gitnexus/scripts/materialize-vendor-grammars.cjs b/gitnexus/scripts/materialize-vendor-grammars.cjs
index b94df815d..fc9a15919 100644
--- a/gitnexus/scripts/materialize-vendor-grammars.cjs
+++ b/gitnexus/scripts/materialize-vendor-grammars.cjs
@@ -63,20 +63,31 @@ for (const name of VENDORED_GRAMMARS) {
       fs.renameSync(partial, dest);
     } catch (renameErr) {
       // Best-effort rollback: restore the previous dest from backup.
+      let restored = false;
       if (fs.existsSync(backup)) {
         try {
           fs.renameSync(backup, dest);
+          restored = true;
         } catch {
-          // If rollback also fails, the prior backup directory still exists on
-          // disk — the catch block below surfaces both errors via the warning.
+          // Rollback also failed — dest is now missing. Leave the backup in
+          // place (the catch below will NOT remove it) and surface where it is.
         }
       }
+      if (!restored && fs.existsSync(backup)) {
+        console.warn(
+          `[gitnexus] CRITICAL: could not materialize vendor/${name} AND could not restore the ` +
+            `previous node_modules/${name}. A recoverable copy remains at ${backup} — ` +
+            `restore it (e.g. \`mv ${backup} ${dest}\`) or reinstall to recover ${name}.`,
+        );
+      }
       throw renameErr;
     }
     fs.rmSync(backup, { recursive: true, force: true });
   } catch (err) {
     // Fail-soft: a single locked/inaccessible file (common on Windows) must not
     // abort the whole gitnexus install. Matches build-tree-sitter-*.cjs pattern.
+    // Only remove the scratch `partial`; never the `backup` (it may be the sole
+    // recoverable copy after a failed rollback above).
     fs.rmSync(partial, { recursive: true, force: true });
     console.warn(`[gitnexus] Could not materialize vendor/${name}: ${err.message}`);
     console.warn(