diff --git a/src/google/adk/tools/_function_parameter_parse_util.py b/src/google/adk/tools/_function_parameter_parse_util.py
index a8e98980d5..17621bcb68 100644
--- a/src/google/adk/tools/_function_parameter_parse_util.py
+++ b/src/google/adk/tools/_function_parameter_parse_util.py
@@ -21,6 +21,7 @@
 import types as typing_types
 from typing import _GenericAlias
 from typing import Any
+from typing import cast
 from typing import get_args
 from typing import get_origin
 from typing import Literal
@@ -118,6 +119,44 @@ def _raise_for_invalid_enum_value(param: inspect.Parameter):
       )
 
 
+def _normalize_tuple_schema_for_genai_schema(
+    json_schema: Any,
+) -> Any:
+  """Normalizes tuple schema keywords unsupported by `types.Schema`.
+
+  Pydantic emits `prefixItems` for fixed-length tuples. `types.Schema` does not
+  support `prefixItems`, so we convert tuple item definitions into
+  `items.anyOf`. We also drop `unevaluatedItems`, which is unsupported by
+  `types.Schema`.
+  """
+  if isinstance(json_schema, list):
+    return [
+        _normalize_tuple_schema_for_genai_schema(item) for item in json_schema
+    ]
+  if not isinstance(json_schema, dict):
+    return json_schema
+
+  normalized_schema = {
+      key: _normalize_tuple_schema_for_genai_schema(value)
+      for key, value in json_schema.items()
+      if key != 'unevaluatedItems'
+  }
+
+  prefix_items = normalized_schema.pop('prefixItems', None)
+  if isinstance(prefix_items, list):
+    if len(prefix_items) == 1:
+      normalized_schema['items'] = prefix_items[0]
+    elif prefix_items:
+      normalized_schema['items'] = {'anyOf': prefix_items}
+
+  # Pydantic can emit `items: false` for tuple schemas, which is unsupported by
+  # `types.Schema`.
+  if normalized_schema.get('items') is False:
+    normalized_schema.pop('items')
+
+  return normalized_schema
+
+
 def _generate_json_schema_for_parameter(
     param: inspect.Parameter,
 ) -> dict[str, Any]:
@@ -131,7 +170,10 @@ def _generate_json_schema_for_parameter(
   json_schema_dict = _add_unevaluated_items_to_fixed_len_tuple_schema(
       json_schema_dict
   )
-  return json_schema_dict
+  return cast(
+      dict[str, Any],
+      _normalize_tuple_schema_for_genai_schema(json_schema_dict),
+  )
 
 
 def _is_builtin_primitive_or_compound(
diff --git a/tests/unittests/tools/test_function_parameter_parse_util.py b/tests/unittests/tools/test_function_parameter_parse_util.py
new file mode 100644
index 0000000000..fc441d41df
--- /dev/null
+++ b/tests/unittests/tools/test_function_parameter_parse_util.py
@@ -0,0 +1,94 @@
+# Copyright 2026 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+
+from __future__ import annotations
+
+from typing import Any
+
+from google.adk.tools import _function_parameter_parse_util
+
+
+def test_normalize_strips_prefixItems() -> None:
+  schema: dict[str, Any] = {
+      "type": "array",
+      "prefixItems": [{"type": "string"}, {"type": "number"}],
+      "minItems": 2,
+      "maxItems": 2,
+      "unevaluatedItems": False,
+  }
+  normalized = (
+      _function_parameter_parse_util._normalize_tuple_schema_for_genai_schema(
+          schema
+      )
+  )
+  assert "prefixItems" not in normalized
+  assert "unevaluatedItems" not in normalized
+  assert normalized["items"] == {
+      "anyOf": [{"type": "string"}, {"type": "number"}]
+  }
+
+
+def test_normalize_strips_unevaluatedItems() -> None:
+  schema: dict[str, Any] = {
+      "type": "object",
+      "properties": {
+          "field1": {"type": "string"},
+      },
+      "unevaluatedItems": False,
+  }
+  normalized = (
+      _function_parameter_parse_util._normalize_tuple_schema_for_genai_schema(
+          schema
+      )
+  )
+  assert "unevaluatedItems" not in normalized
+  assert normalized["properties"] == {"field1": {"type": "string"}}
+
+
+def test_normalize_handles_items_false() -> None:
+  schema: dict[str, Any] = {
+      "type": "array",
+      "prefixItems": [{"type": "string"}],
+      "items": False,
+  }
+  normalized = (
+      _function_parameter_parse_util._normalize_tuple_schema_for_genai_schema(
+          schema
+      )
+  )
+  assert "items" in normalized
+  assert normalized["items"] == {"type": "string"}
+  assert normalized.get("items") is not False
+
+
+def test_normalize_handles_nested_schemas() -> None:
+  schema: dict[str, Any] = {
+      "type": "object",
+      "properties": {
+          "field1": {
+              "type": "array",
+              "prefixItems": [{"type": "string"}],
+              "unevaluatedItems": False,
+          }
+      },
+  }
+  normalized = (
+      _function_parameter_parse_util._normalize_tuple_schema_for_genai_schema(
+          schema
+      )
+  )
+  assert "unevaluatedItems" not in normalized["properties"]["field1"]
+  assert "prefixItems" not in normalized["properties"]["field1"]
+  assert normalized["properties"]["field1"]["items"] == {"type": "string"}