diff --git a/src/Rxn/Framework/Codegen/PolyparityExporter.php b/src/Rxn/Framework/Codegen/PolyparityExporter.php
index 7fe93d4..e974748 100644
--- a/src/Rxn/Framework/Codegen/PolyparityExporter.php
+++ b/src/Rxn/Framework/Codegen/PolyparityExporter.php
@@ -11,6 +11,7 @@
 use Rxn\Framework\Http\Attribute\Required;
 use Rxn\Framework\Http\Attribute\Url;
 use Rxn\Framework\Http\Binding\RequestDto;
+use Rxn\Framework\Http\Binding\Validates;
 
 /**
  * Emit a polyparity YAML spec from a Rxn `RequestDto`. The same
@@ -212,6 +213,15 @@ private function refuse(string $attrName, string $propName): ?string
                 . 'Add a mapping or document the DTO as PHP-only.',
             );
         }
+
+        if (is_subclass_of($attrName, Validates::class)) {
+            throw new \RuntimeException(
+                "PolyparityExporter: $attrName on property '$propName' is a custom validator. "
+                . 'Refusing to emit silently-divergent spec. '
+                . 'Add a polyparity mapping or keep this DTO PHP-only.'
+            );
+        }
+
         return null;
     }
 
diff --git a/src/Rxn/Framework/Tests/Codegen/PolyparityExporterTest.php b/src/Rxn/Framework/Tests/Codegen/PolyparityExporterTest.php
index 2e7689b..52bcb8d 100644
--- a/src/Rxn/Framework/Tests/Codegen/PolyparityExporterTest.php
+++ b/src/Rxn/Framework/Tests/Codegen/PolyparityExporterTest.php
@@ -183,6 +183,14 @@ public function testRefusesUnsupportedAttribute(): void
         $exporter->emit(Fixture\UnsupportedDto::class);
     }
 
+
+    public function testRefusesCustomValidatesAttribute(): void
+    {
+        $exporter = new PolyparityExporter();
+        $this->expectException(\RuntimeException::class);
+        $this->expectExceptionMessageMatches('/CustomValidatesAttribute.*custom validator/i');
+        $exporter->emit(Fixture\CustomUnsupportedDto::class);
+    }
     public function testRejectsNonRequestDto(): void
     {
         $exporter = new PolyparityExporter();