diff --git a/test/test_subtype_identity.py b/test/test_subtype_identity.py
new file mode 100644
index 0000000..2ea1d7f
--- /dev/null
+++ b/test/test_subtype_identity.py
@@ -0,0 +1,51 @@
+"""Demonstrates a runtime polymorphism issue independent of any Rune-DSL feature.
+
+`BaseDataClass.model_config` sets `revalidate_instances='always'`. Combined
+with `@validate_call` on every generated function, this means: passing a
+subtype instance (`_Bar`) into a parameter declared as its supertype
+(`_Foo`) causes Pydantic to rebuild the value strictly from `_Foo`'s own
+schema, discarding the subtype's identity and its extra fields - before the
+function body ever runs.
+
+"""
+from pydantic import validate_call
+from pydantic import InstanceOf
+
+from rune.runtime.base_data_class import BaseDataClass
+
+
+class _Foo(BaseDataClass):
+    pass
+
+
+class _Bar(_Foo):
+    bar_attr: int = 0
+
+
+@validate_call
+def _identity_foo(foo: InstanceOf[_Foo]) -> _Foo:
+    """Mirrors a generated Rune function: declared parameter/return type is
+    the supertype, exactly as a function taking `foo Foo (1..1)` would be
+    generated."""
+    return foo
+
+
+def test_subtype_survives_plain_python_assignment():
+    """Sanity check: outside of Pydantic's validate_call, identity is preserved."""
+    bar = _Bar(bar_attr=42)
+    held = bar
+    assert isinstance(held, _Bar)
+    assert held.bar_attr == 42
+
+
+def test_subtype_identity_lost_across_validate_call_boundary():
+    """A _Bar passed into a _Foo-typed parameter should still be a _Bar on return."""
+    bar = _Bar(bar_attr=42)
+    assert isinstance(bar, _Bar)  # true before the call
+
+    result = _identity_foo(foo=bar)
+
+    # Expected under standard OOP/Liskov semantics: the identity function
+    # returns the same kind of object it was given.
+    assert isinstance(result, _Bar)
+    assert result.bar_attr == 42
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