diff --git a/arcade/arcade/core/catalog.py b/arcade/arcade/core/catalog.py
index d47cc4d5..fd8dc21f 100644
--- a/arcade/arcade/core/catalog.py
+++ b/arcade/arcade/core/catalog.py
@@ -40,6 +40,7 @@ from arcade.core.utils import (
     does_function_return_value,
     first_or_none,
     is_string_literal,
+    is_union,
     snake_to_pascal_case,
 )
 from arcade.sdk.annotations import Inferrable
@@ -447,12 +448,24 @@ def extract_python_param_info(param: inspect.Parameter) -> ParamInfo:
     original_type = annotation.__args__[0] if get_origin(annotation) is Annotated else annotation
     field_type = original_type
 
-    # Unwrap Optional types
+    # Handle optional types
+    # Both Optional[T] and T | None are supported
     is_optional = False
-    if get_origin(field_type) is Union and type(None) in get_args(field_type):
+    if (
+        is_union(field_type)
+        and len(get_args(field_type)) == 2
+        and type(None) in get_args(field_type)
+    ):
         field_type = next(arg for arg in get_args(field_type) if arg is not type(None))
         is_optional = True
 
+    # Union types are not currently supported
+    # (other than optional, which is handled above)
+    if is_union(field_type):
+        raise ToolDefinitionError(
+            f"Parameter {param.name} is a union type. Only optional types are supported."
+        )
+
     return ParamInfo(
         name=param.name,
         default=param.default if param.default is not inspect.Parameter.empty else None,
diff --git a/arcade/arcade/core/utils.py b/arcade/arcade/core/utils.py
index 5fdb5bc6..61fd1868 100644
--- a/arcade/arcade/core/utils.py
+++ b/arcade/arcade/core/utils.py
@@ -2,7 +2,8 @@ import ast
 import inspect
 import re
 from collections.abc import Iterable
-from typing import Any, Callable, Literal, Optional, TypeVar, get_args, get_origin
+from types import UnionType
+from typing import Any, Callable, Literal, Optional, TypeVar, Union, get_args, get_origin
 
 T = TypeVar("T")
 
@@ -44,6 +45,13 @@ def is_string_literal(_type: type) -> bool:
     return get_origin(_type) is Literal and all(isinstance(arg, str) for arg in get_args(_type))
 
 
+def is_union(_type: type) -> bool:
+    """
+    Returns True if the given type is a union, i.e. a Union[T1, T2, ...] or T1 | T2 | ... etc.
+    """
+    return get_origin(_type) in {Union, UnionType}
+
+
 def does_function_return_value(func: Callable) -> bool:
     """
     Returns True if the given function returns a value, i.e. if it has a return statement with a value.
diff --git a/arcade/arcade/sdk/eval/eval.py b/arcade/arcade/sdk/eval/eval.py
index 54fe9eee..5a3c3a2b 100644
--- a/arcade/arcade/sdk/eval/eval.py
+++ b/arcade/arcade/sdk/eval/eval.py
@@ -13,7 +13,6 @@ except ImportError:
     )
 
 from arcade.client.client import Arcade, AsyncArcade
-from arcade.core.config import config
 from arcade.sdk.error import WeightError
 
 if TYPE_CHECKING:
@@ -423,6 +422,8 @@ class EvalSuite:
         """
         Initialize the client instance for the EvalSuite.
         """
+        from arcade.core.config import config
+
         if self.max_concurrent > 1:
             self._client = AsyncArcade(
                 api_key=config.api.key,
diff --git a/arcade/tests/utils/test_utils_casing.py b/arcade/tests/core/utils/test_casing.py
similarity index 100%
rename from arcade/tests/utils/test_utils_casing.py
rename to arcade/tests/core/utils/test_casing.py
diff --git a/arcade/tests/core/utils/test_is_union.py b/arcade/tests/core/utils/test_is_union.py
new file mode 100644
index 00000000..68ab581b
--- /dev/null
+++ b/arcade/tests/core/utils/test_is_union.py
@@ -0,0 +1,22 @@
+from typing import Optional, Union
+
+import pytest
+
+from arcade.core.utils import is_union
+
+
+@pytest.mark.parametrize(
+    "type_input, expected",
+    [
+        (Union[int, str], True),
+        (Optional[int], True),  # Optional[int] is equivalent to Union[int, None]
+        (int | str, True),
+        (int | None, True),  # int | None is equivalent to Optional[int]
+        (int, False),
+        (str, False),
+        (list, False),
+        (dict, False),
+    ],
+)
+def test_is_union(type_input, expected):
+    assert is_union(type_input) == expected
diff --git a/arcade/tests/tool/test_create_tool_definition_errors.py b/arcade/tests/tool/test_create_tool_definition_errors.py
index fe1b8ef2..959ea184 100644
--- a/arcade/tests/tool/test_create_tool_definition_errors.py
+++ b/arcade/tests/tool/test_create_tool_definition_errors.py
@@ -31,6 +31,11 @@ def func_with_unsupported_param(param1: complex):
     pass
 
 
+@tool(desc="A function with a union parameter (illegal)")
+def func_with_union_param(param1: str | int):
+    pass
+
+
 @tool(desc="A function with multiple context parameters (illegal)")
 def func_with_multiple_context_params(context: ToolContext, context2: ToolContext):
     pass
@@ -64,6 +69,11 @@ def func_with_multiple_context_params(context: ToolContext, context2: ToolContex
             ToolDefinitionError,
             id=func_with_unsupported_param.__name__,
         ),
+        pytest.param(
+            func_with_union_param,
+            ToolDefinitionError,
+            id=func_with_union_param.__name__,
+        ),
         pytest.param(
             func_with_multiple_context_params,
             ToolDefinitionError,