diff --git a/python/pyfory/format/tests/test_infer.py b/python/pyfory/format/tests/test_infer.py
index ea3a517c00..76f60b26b4 100644
--- a/python/pyfory/format/tests/test_infer.py
+++ b/python/pyfory/format/tests/test_infer.py
@@ -64,6 +64,25 @@ class X:
     assert result.type.id == TypeId.STRUCT
 
 
+def test_infer_field_builtin_types_not_treated_as_struct():
+    """Built-in types must NOT be routed to visit_customized (regression guard)."""
+    assert _infer_field("", int).type.id == TypeId.INT64
+    assert _infer_field("", float).type.id == TypeId.FLOAT64
+    assert _infer_field("", str).type.id == TypeId.STRING
+    assert _infer_field("", bytes).type.id == TypeId.BINARY
+    assert _infer_field("", bool).type.id == TypeId.BOOL
+
+
+def test_infer_field_nested_custom_class():
+    """Custom class nested inside a List should also be handled correctly."""
+
+    class Inner:
+        pass
+
+    result = _infer_field("", List[Inner])
+    assert result.type.id == TypeId.LIST
+
+
 def test_infer_class_schema():
     schema = infer_schema(Foo)
     assert schema.num_fields == 7
diff --git a/python/pyfory/type_util.py b/python/pyfory/type_util.py
index 2b24510272..4d995d75c3 100644
--- a/python/pyfory/type_util.py
+++ b/python/pyfory/type_util.py
@@ -249,11 +249,17 @@ def infer_field(field_name, type_, visitor: TypeVisitor, types_path=None):
         else:
             raise TypeError(f"Collection types should be {list, dict} instead of {type_}")
     else:
-        if is_function(origin) or not hasattr(origin, "__annotations__"):
+        if is_function(origin):
             return visitor.visit_other(field_name, type_, types_path=types_path)
-        else:
+
+        if origin in (list, dict, set):
+            return visitor.visit_other(field_name, type_, types_path=types_path)
+
+        if inspect.isclass(origin) and origin.__module__ not in ("builtins", "datetime"):
             return visitor.visit_customized(field_name, type_, types_path=types_path)
 
+        return visitor.visit_other(field_name, type_, types_path=types_path)
+
 
 def is_function(func):
     return inspect.isfunction(func) or is_cython_function(func)