diff --git a/src/google/adk/tools/_function_parameter_parse_util.py b/src/google/adk/tools/_function_parameter_parse_util.py
index 32827d974a..6b0f26ef4a 100644
--- a/src/google/adk/tools/_function_parameter_parse_util.py
+++ b/src/google/adk/tools/_function_parameter_parse_util.py
@@ -304,14 +304,17 @@ def _parse_schema_from_parameter(
     _raise_if_schema_unsupported(variant, schema)
     return schema
   if isinstance(param.annotation, type) and issubclass(param.annotation, Enum):
+    # `schema.type` is always STRING here, so every enum value must be a
+    # string too (e.g. IntEnum members have int `.value`s otherwise).
     schema.type = types.Type.STRING
-    schema.enum = [e.value for e in param.annotation]
+    schema.enum = [str(e.value) for e in param.annotation]
     if param.default is not inspect.Parameter.empty:
       default_value = (
           param.default.value
           if isinstance(param.default, Enum)
           else param.default
       )
+      default_value = str(default_value)
       if default_value not in schema.enum:
         raise ValueError(default_value_error_msg)
       schema.default = default_value
diff --git a/tests/unittests/tools/test_build_function_declaration.py b/tests/unittests/tools/test_build_function_declaration.py
index 53968ed931..7e920e67a7 100644
--- a/tests/unittests/tools/test_build_function_declaration.py
+++ b/tests/unittests/tools/test_build_function_declaration.py
@@ -13,6 +13,7 @@
 # limitations under the License.
 
 from enum import Enum
+from enum import IntEnum
 from typing import Any
 
 from google.adk.features import FeatureName
@@ -439,6 +440,27 @@ def simple_function_with_wrong_enum(input: InputEnum = 'WRONG_ENUM'):
           func=simple_function_with_wrong_enum
       )
 
+  def test_int_enum(self):
+
+    class Level(IntEnum):
+      LOW = 1
+      HIGH = 2
+
+    def set_level(level: Level = Level.LOW):
+      return level.value
+
+    function_decl = _automatic_function_calling_util.build_function_declaration(
+        func=set_level
+    )
+
+    level_schema = function_decl.parameters.properties['level']
+    assert level_schema.type == 'STRING'
+    assert level_schema.enum == ['1', '2']
+    assert level_schema.default == '1'
+    # The declaration must be valid for types.Schema's own contract: an
+    # `enum` field on a STRING schema must contain only strings.
+    types.Schema(type=level_schema.type, enum=level_schema.enum)
+
   def test_basemodel_list(self):
     class ChildInput(BaseModel):
       input_str: str