feat: expose SKIP_THOUGHT_SIGNATURE_VALIDATOR constant

Gemini thinking models require a thought_signature on generated parts, and the
backend rejects replayed parts that lack one. Callers who synthesize
conversation history must set b'skip_thought_signature_validator' on the
fabricated part to bypass validation, and several ADK consumers hardcode that
byte-string independently. Expose it once so they can depend on a single
constant.

Co-authored-by: George Weale <gweale@google.com>
PiperOrigin-RevId: 941277709

Author: GWeale

src/google/adk/utils/content_utils.py

@@ -16,6 +16,14 @@
 
 from google.genai import types
 
+SKIP_THOUGHT_SIGNATURE_VALIDATOR: bytes = b'skip_thought_signature_validator'
+"""Placeholder ``Part.thought_signature`` that bypasses backend validation.
+
+Set it on a part you synthesize yourself (a model turn or tool call/response
+the model never produced) so the Gemini backend accepts the fabricated part
+instead of rejecting it for a missing signature.
+"""
+
 
 def is_audio_part(part: types.Part) -> bool:
   return (

tests/unittests/utils/test_content_utils.py

@@ -0,0 +1,32 @@
+# 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 google.adk.utils.content_utils import SKIP_THOUGHT_SIGNATURE_VALIDATOR
+from google.genai import types
+
+
+def test_skip_thought_signature_validator_wire_value():
+  # The backend recognizes this exact byte string to bypass validation;
+  # changing it would break every replayed synthetic part.
+  assert SKIP_THOUGHT_SIGNATURE_VALIDATOR == b'skip_thought_signature_validator'
+
+
+def test_skip_thought_signature_validator_assignable_to_part():
+  part = types.Part(
+      text='injected',
+      thought_signature=SKIP_THOUGHT_SIGNATURE_VALIDATOR,
+  )
+  assert part.thought_signature == SKIP_THOUGHT_SIGNATURE_VALIDATOR