diff --git a/bin/gstack-diff-scope b/bin/gstack-diff-scope
index 36918381c..bf1b4af84 100755
--- a/bin/gstack-diff-scope
+++ b/bin/gstack-diff-scope
@@ -75,7 +75,10 @@ while IFS= read -r f; do
 
     # Backend: everything else that's code (excluding views/components already matched)
     *.rb|*.py|*.go|*.rs|*.java|*.php|*.ex|*.exs) BACKEND=true ;;
-    *.ts|*.js) BACKEND=true ;;  # Non-component TS/JS is backend
+    # Non-component TS/JS is backend. Include ESM/CJS (.mjs/.cjs) and
+    # explicit-module TS (.mts/.cts) — #1810: these matched no category, so an
+    # ESM/CJS-only PR skipped the backend reviewer entirely.
+    *.ts|*.js|*.mjs|*.cjs|*.mts|*.cts) BACKEND=true ;;
   esac
 done <<< "$FILES"
 
diff --git a/test/diff-scope.test.ts b/test/diff-scope.test.ts
index 2130a3e57..3e80fe451 100644
--- a/test/diff-scope.test.ts
+++ b/test/diff-scope.test.ts
@@ -78,6 +78,15 @@ describe('gstack-diff-scope', () => {
     expect(scope.SCOPE_BACKEND).toBe('true');
   });
 
+  // #1810: ESM/CJS and explicit-module TS extensions matched no category, so an
+  // .mjs/.cjs/.mts/.cts-only PR skipped the backend reviewer entirely.
+  test('detects ESM/CJS/explicit-module backend files (#1810)', () => {
+    for (const f of ['server.mjs', 'worker.cjs', 'config.mts', 'legacy.cts']) {
+      const scope = runScope(createRepo([f]));
+      expect(scope.SCOPE_BACKEND).toBe('true');
+    }
+  });
+
   test('detects test files', () => {
     const dir = createRepo(['test/app.test.ts']);
     const scope = runScope(dir);