diff --git a/src/openai/_models.py b/src/openai/_models.py
index af71a91850..51c3be04fa 100644
--- a/src/openai/_models.py
+++ b/src/openai/_models.py
@@ -3,6 +3,7 @@
 import os
 import inspect
 from typing import TYPE_CHECKING, Any, Type, Tuple, Union, Generic, TypeVar, Callable, Optional, cast
+from weakref import WeakKeyDictionary
 from datetime import date, datetime
 from typing_extensions import (
     List,
@@ -77,6 +78,8 @@
 
 ReprArgs = Sequence[Tuple[Optional[str], Any]]
 
+_DISCRIMINATOR_CACHE: "WeakKeyDictionary[type, DiscriminatorDetails]" = WeakKeyDictionary()
+
 
 @runtime_checkable
 class _ConfigProtocol(Protocol):
@@ -593,11 +596,6 @@ def construct_type(*, value: object, type_: object, metadata: Optional[List[Any]
     return value
 
 
-@runtime_checkable
-class CachedDiscriminatorType(Protocol):
-    __discriminator__: DiscriminatorDetails
-
-
 class DiscriminatorDetails:
     field_name: str
     """The name of the discriminator field in the variant class, e.g.
@@ -640,8 +638,9 @@ def __init__(
 
 
 def _build_discriminated_union_meta(*, union: type, meta_annotations: tuple[Any, ...]) -> DiscriminatorDetails | None:
-    if isinstance(union, CachedDiscriminatorType):
-        return union.__discriminator__
+    cached_discriminator = _DISCRIMINATOR_CACHE.get(union)
+    if cached_discriminator is not None:
+        return cached_discriminator
 
     discriminator_field_name: str | None = None
 
@@ -694,7 +693,7 @@ def _build_discriminated_union_meta(*, union: type, meta_annotations: tuple[Any,
         discriminator_field=discriminator_field_name,
         discriminator_alias=discriminator_alias,
     )
-    cast(CachedDiscriminatorType, union).__discriminator__ = details
+    _DISCRIMINATOR_CACHE[union] = details
     return details
 
 
diff --git a/tests/test_models.py b/tests/test_models.py
index 410ec3bf4e..48c4748816 100644
--- a/tests/test_models.py
+++ b/tests/test_models.py
@@ -9,7 +9,7 @@
 
 from openai._utils import PropertyInfo
 from openai._compat import PYDANTIC_V1, parse_obj, model_dump, model_json
-from openai._models import BaseModel, construct_type
+from openai._models import BaseModel, construct_type, _DISCRIMINATOR_CACHE
 
 
 class BasicModel(BaseModel):
@@ -809,7 +809,7 @@ class B(BaseModel):
 
     UnionType = cast(Any, Union[A, B])
 
-    assert not hasattr(UnionType, "__discriminator__")
+    assert _DISCRIMINATOR_CACHE.get(UnionType) is None
 
     m = construct_type(
         value={"type": "b", "data": "foo"}, type_=cast(Any, Annotated[UnionType, PropertyInfo(discriminator="type")])
@@ -818,7 +818,8 @@ class B(BaseModel):
     assert m.type == "b"
     assert m.data == "foo"  # type: ignore[comparison-overlap]
 
-    discriminator = UnionType.__discriminator__
+    discriminator = _DISCRIMINATOR_CACHE.get(UnionType)
+
     assert discriminator is not None
 
     m = construct_type(
@@ -830,7 +831,7 @@ class B(BaseModel):
 
     # if the discriminator details object stays the same between invocations then
     # we hit the cache
-    assert UnionType.__discriminator__ is discriminator
+    assert _DISCRIMINATOR_CACHE.get(UnionType) is discriminator
 
 
 @pytest.mark.skipif(PYDANTIC_V1, reason="TypeAliasType is not supported in Pydantic v1")