diff --git a/src/Codegen/Constraints/UntypedBuilder.php b/src/Codegen/Constraints/UntypedBuilder.php
index 7b9454c..2c3aead 100644
--- a/src/Codegen/Constraints/UntypedBuilder.php
+++ b/src/Codegen/Constraints/UntypedBuilder.php
@@ -569,15 +569,22 @@ private function generateGenericAnyOfChecks(vec<SchemaBuilder> $schema_builders,
   }
 
   private function generateOptimizedAnyOfChecks(TOptimizedAnyOfTypes $any_of_types, HackBuilder $hb): void {
-    $types = dict[];
+    $types = vec[];
+    $constraints = dict[];
     foreach ($any_of_types['types'] as $type_name => $schema_builder) {
       $suffix = $this->generateClassName($this->suffix, 'anyOfTypes', $type_name);
       $schema_builder->setSuffix($suffix);
       $schema_builder->build();
 
-      $types[$type_name] = "{$schema_builder->getClassName()}::check<>";
+      $constraints[$type_name] = "{$schema_builder->getClassName()}::check<>";
+      $types[] = $schema_builder->getTypeInfo();
     }
 
+    $this->type_info = Typing\TypeSystem::union(
+      $types,
+      shape('disable_shape_unification' => $this->typed_schema['disableShapeUnification'] ?? false)
+    );
+
     $hb
       ->addAssignment('$key', $any_of_types['key'], HackBuilderValues::export())
       ->addAssignment(
@@ -596,7 +603,7 @@ private function generateOptimizedAnyOfChecks(TOptimizedAnyOfTypes $any_of_types
       ->ensureEmptyLine()
       ->addAssignment(
         '$types',
-        $types,
+        $constraints,
         HackBuilderValues::dict(HackBuilderKeys::export(), HackBuilderValues::literal()),
       )
       ->ensureEmptyLine();
diff --git a/tests/examples/codegen/CustomCodegenConfigValidator.php b/tests/examples/codegen/CustomCodegenConfigValidator.php
index 1c4a83f..37ed465 100644
--- a/tests/examples/codegen/CustomCodegenConfigValidator.php
+++ b/tests/examples/codegen/CustomCodegenConfigValidator.php
@@ -5,7 +5,7 @@
  * To re-generate this file run `make test`
  *
  *
- * @generated SignedSource<<a2f9712dad75f484bf20025df1b6e2f8>>
+ * @generated SignedSource<<42a2e629fbe63e9b95076f6cf672801e>>
  */
 namespace Slack\Hack\JsonSchema\Tests\Generated;
 use namespace Slack\Hack\JsonSchema;
@@ -34,7 +34,10 @@
   ...
 );
 
-type custom_codegen_config_validator_properties_devices_items_t = mixed;
+type custom_codegen_config_validator_properties_devices_items_t = shape(
+  'type' => string,
+  ...
+);
 
 type custom_codegen_config_validator_t = shape(
   'first_name' => string,
diff --git a/tests/examples/codegen/PersonSchemaValidator.php b/tests/examples/codegen/PersonSchemaValidator.php
index 688296e..28abaf5 100644
--- a/tests/examples/codegen/PersonSchemaValidator.php
+++ b/tests/examples/codegen/PersonSchemaValidator.php
@@ -5,7 +5,7 @@
  * To re-generate this file run `make test`
  *
  *
- * @generated SignedSource<<d9e951b7c4e5dd6d27e2dc5c8d3900bc>>
+ * @generated SignedSource<<ee778314572dafea42ca53253a97056b>>
  */
 namespace Slack\Hack\JsonSchema\Tests\Generated;
 use namespace Slack\Hack\JsonSchema;
@@ -34,7 +34,10 @@
   ...
 );
 
-type TPersonSchemaValidatorPropertiesDevicesItems = mixed;
+type TPersonSchemaValidatorPropertiesDevicesItems = shape(
+  'type' => string,
+  ...
+);
 
 type TPersonSchemaValidator = shape(
   'first_name' => string,
diff --git a/tests/examples/codegen/UntypedSchemaValidator.php b/tests/examples/codegen/UntypedSchemaValidator.php
index e9dfafd..1d77f66 100644
--- a/tests/examples/codegen/UntypedSchemaValidator.php
+++ b/tests/examples/codegen/UntypedSchemaValidator.php
@@ -5,7 +5,7 @@
  * To re-generate this file run `make test`
  *
  *
- * @generated SignedSource<<2c98b928d525db21c6948ef14b310a05>>
+ * @generated SignedSource<<5d0e74247bc284d369cde4fc754bc67e>>
  */
 namespace Slack\Hack\JsonSchema\Tests\Generated;
 use namespace Slack\Hack\JsonSchema;
@@ -62,7 +62,11 @@
   ?'integer' => int,
 );
 
-type TUntypedSchemaValidatorPropertiesAnyOfOptimizedEnum = mixed;
+type TUntypedSchemaValidatorPropertiesAnyOfOptimizedEnum = shape(
+  'type' => string,
+  ?'string' => string,
+  ?'integer' => int,
+);
 
 type TUntypedSchemaValidatorPropertiesOneOfNullableString = ?string;