diff --git a/src/google/adk/tools/openapi_tool/common/common.py b/src/google/adk/tools/openapi_tool/common/common.py
index 3b9b6b2497d..716b728716c 100644
--- a/src/google/adk/tools/openapi_tool/common/common.py
+++ b/src/google/adk/tools/openapi_tool/common/common.py
@@ -227,8 +227,13 @@ def generate_return_doc(responses: Dict[str, Response]) -> str:
 
     # Only consider 2xx responses for return type hinting.
     # Returns the 2xx response with the smallest status code number and with
-    # content defined.
-    sorted_responses = sorted(responses.items(), key=lambda item: int(item[0]))
+    # content defined. Skip non-numeric keys such as 'default' and range
+    # patterns ('2XX', '4XX'), which are valid OpenAPI response keys but cannot
+    # be parsed as an int (they are filtered out by the 2xx check below anyway).
+    sorted_responses = sorted(
+        (item for item in responses.items() if item[0].isdigit()),
+        key=lambda item: int(item[0]),
+    )
     qualified_response = next(
         filter(
             lambda r: r[0].startswith('2') and r[1].content,
diff --git a/tests/unittests/tools/openapi_tool/common/test_common.py b/tests/unittests/tools/openapi_tool/common/test_common.py
index 1dd3195071f..20bca63c88b 100644
--- a/tests/unittests/tools/openapi_tool/common/test_common.py
+++ b/tests/unittests/tools/openapi_tool/common/test_common.py
@@ -407,6 +407,38 @@ def test_generate_return_doc_contentful_response(self):
         == expected_doc
     )
 
+  def test_generate_return_doc_with_default_key_does_not_crash(self):
+    responses = {
+        '200': {
+            'description': 'Successful response',
+            'content': {'application/json': {'schema': {'type': 'string'}}},
+        },
+        'default': {'description': 'Unexpected error'},
+    }
+    expected_doc = 'Returns (str): Successful response'
+    assert (
+        PydocHelper.generate_return_doc(dict_to_responses(responses))
+        == expected_doc
+    )
+
+  def test_generate_return_doc_with_range_key_does_not_crash(self):
+    responses = {
+        '4XX': {'description': 'Client error'},
+        '200': {
+            'description': 'Successful response',
+            'content': {'application/json': {'schema': {'type': 'string'}}},
+        },
+    }
+    expected_doc = 'Returns (str): Successful response'
+    assert (
+        PydocHelper.generate_return_doc(dict_to_responses(responses))
+        == expected_doc
+    )
+
+  def test_generate_return_doc_only_non_numeric_keys_returns_empty(self):
+    responses = {'default': {'description': 'Only a default response'}}
+    assert PydocHelper.generate_return_doc(dict_to_responses(responses)) == ''
+
 
 if __name__ == '__main__':
   pytest.main([__file__])