diff --git a/burr/core/action.py b/burr/core/action.py
index eb6fceff..92144b07 100644
--- a/burr/core/action.py
+++ b/burr/core/action.py
@@ -24,6 +24,16 @@
 import textwrap
 import types
 import typing
+
+# types.UnionType was added in Python 3.10 to represent PEP 604 `X | Y`
+# syntax. Burr supports Python >= 3.9, so on 3.9 we fall back to a sentinel
+# type that keeps Union annotations well-formed. No PEP 604 union can ever
+# exist on 3.9, so isinstance() checks against it simply never match.
+if sys.version_info >= (3, 10):
+    _UnionType = types.UnionType
+else:  # pragma: no cover - exercised on Python 3.9 CI only
+    class _UnionType:  # type: ignore[no-redef]
+        """Placeholder for ``types.UnionType`` on Python < 3.10."""
 from collections.abc import AsyncIterator
 from typing import (
     TYPE_CHECKING,
@@ -1511,7 +1521,7 @@ def pydantic(
         writes: List[str],
         state_input_type: Type["BaseModel"],
         state_output_type: Type["BaseModel"],
-        stream_type: Union[Type["BaseModel"], Type[dict]],
+        stream_type: Union[Type["BaseModel"], Type[dict], _UnionType],
         tags: Optional[List[str]] = None,
     ) -> Callable:
         """Creates a streaming action that uses pydantic models.
diff --git a/burr/integrations/pydantic.py b/burr/integrations/pydantic.py
index 300cbb6a..3ef2a2ce 100644
--- a/burr/integrations/pydantic.py
+++ b/burr/integrations/pydantic.py
@@ -41,6 +41,7 @@
 from burr.core.action import (
     FunctionBasedAction,
     FunctionBasedStreamingAction,
+    _UnionType,
     bind,
     derive_inputs_from_fn,
 )
@@ -269,7 +270,7 @@ async def async_action_function(state: State, **kwargs) -> State:
     return decorator
 
 
-PartialType = Union[Type[pydantic.BaseModel], Type[dict]]
+PartialType = Union[Type[pydantic.BaseModel], Type[dict], _UnionType]
 
 PydanticStreamingActionFunctionSync = Callable[
     ..., Generator[Tuple[Union[pydantic.BaseModel, dict], Optional[pydantic.BaseModel]], None, None]
@@ -290,7 +291,7 @@ async def async_action_function(state: State, **kwargs) -> State:
 
 def _validate_and_extract_signature_types_streaming(
     fn: PydanticStreamingActionFunction,
-    stream_type: Optional[Union[Type[pydantic.BaseModel], Type[dict]]],
+    stream_type: Optional[Union[Type[pydantic.BaseModel], Type[dict], _UnionType]],
     state_input_type: Optional[Type[pydantic.BaseModel]] = None,
     state_output_type: Optional[Type[pydantic.BaseModel]] = None,
 ) -> Tuple[