fix(litellm): strip embedded thought_signature from tool call id

When a Gemini thinking model exposes its thought_signature only embedded in
the tool call id via the __thought__ separator,
_message_to_generate_content_response extracted the signature onto
part.thought_signature but assigned the full, unsplit id (including the
__thought__<signature> suffix) to part.function_call.id.

That corrupted id is persisted to session history and, on later turns, replayed
to the model as a literal tool_call_id, which OpenAI-compatible endpoints reject
(422 'Tool Call ID required on tool calls'). Because the malformed id is now
part of the event history, every subsequent turn on the thread fails
identically, permanently breaking the conversation.

Strip the embedded suffix so function_call.id is the clean, original id, while
still preserving the signature separately on part.thought_signature. The
extra_content and provider_specific_fields paths are unchanged. Adds regression
and unit tests.

Fixes #6454

Author: saakshigupta2002

src/google/adk/models/lite_llm.py

@@ -987,6 +987,33 @@ def _extract_thought_signature_from_tool_call(
     if len(parts) == 2:
       return _decode_thought_signature(parts[1])
 
+
+def _strip_thought_signature_from_tool_call_id(
+    tool_call_id: Optional[str],
+) -> Optional[str]:
+  """Strips an embedded thought_signature suffix from a tool call ID.
+
+  Some Gemini thinking model paths embed the thought_signature in the tool
+  call ID via the ``__thought__`` separator (see
+  ``_extract_thought_signature_from_tool_call``). The signature is surfaced
+  separately on ``part.thought_signature``, so the ID assigned to
+  ``function_call.id`` must be the clean, original ID without the suffix.
+  Otherwise the corrupted ID is persisted to session history and later
+  replayed to the model as a ``tool_call_id``, which downstream
+  OpenAI-compatible endpoints reject.
+
+  Args:
+    tool_call_id: The raw tool call ID, which may contain an embedded
+      thought_signature.
+
+  Returns:
+    The tool call ID with any ``__thought__`` suffix removed, or the value
+    unchanged when no separator is present.
+  """
+  if not tool_call_id:
+    return tool_call_id
+  return tool_call_id.split(_THOUGHT_SIGNATURE_SEPARATOR, 1)[0]
+
   return None
 
 
@@ -2182,7 +2209,12 @@ def _message_to_generate_content_response(
             name=tool_call.function.name,
             args=_parse_tool_call_arguments(tool_call.function.arguments),
         )
-        part.function_call.id = tool_call.id
+        # Strip any embedded thought_signature suffix so the persisted
+        # function_call.id stays the clean, original tool call ID. The
+        # signature is preserved separately on part.thought_signature.
+        part.function_call.id = _strip_thought_signature_from_tool_call_id(
+            tool_call.id
+        )
         if thought_signature:
           part.thought_signature = thought_signature
         parts.append(part)

tests/unittests/models/test_litellm.py

@@ -57,6 +57,7 @@
 from google.adk.models.lite_llm import _safe_json_serialize
 from google.adk.models.lite_llm import _schema_to_dict
 from google.adk.models.lite_llm import _split_message_content_and_tool_calls
+from google.adk.models.lite_llm import _strip_thought_signature_from_tool_call_id
 from google.adk.models.lite_llm import _THOUGHT_SIGNATURE_SEPARATOR
 from google.adk.models.lite_llm import _to_litellm_response_format
 from google.adk.models.lite_llm import _to_litellm_role
@@ -2937,6 +2938,67 @@ def test_message_to_generate_content_response_no_thought_signature():
   assert fc_part.thought_signature is None
 
 
+def test_message_to_generate_content_response_strips_thought_signature_from_id():
+  """function_call.id is cleaned when the signature is embedded in the ID.
+
+  Regression test: when the thought_signature is only available embedded in
+  the tool call ID (the __thought__ fallback), the original code assigned the
+  entire unsplit ID to function_call.id, corrupting session history and
+  breaking every subsequent turn once the malformed ID is replayed as a
+  tool_call_id.
+  """
+  sig_b64 = base64.b64encode(b"embedded_sig").decode("utf-8")
+  embedded_id = f"call_789{_THOUGHT_SIGNATURE_SEPARATOR}{sig_b64}"
+  message = ChatCompletionAssistantMessage(
+      role="assistant",
+      content=None,
+      tool_calls=[
+          ChatCompletionMessageToolCall(
+              type="function",
+              id=embedded_id,
+              function=Function(
+                  name="test_function",
+                  arguments='{"test_arg": "test_value"}',
+              ),
+          )
+      ],
+  )
+
+  response = _message_to_generate_content_response(message)
+  fc_part = response.content.parts[0]
+  # The ID is stripped back to the clean, original tool call ID ...
+  assert fc_part.function_call.id == "call_789"
+  assert _THOUGHT_SIGNATURE_SEPARATOR not in fc_part.function_call.id
+  # ... and the signature is still preserved separately.
+  assert fc_part.thought_signature == b"embedded_sig"
+
+
+def test_strip_thought_signature_from_tool_call_id_removes_suffix():
+  """The embedded __thought__ suffix is removed from the ID."""
+  sig_b64 = base64.b64encode(b"embedded_sig").decode("utf-8")
+  tool_call_id = f"call_789{_THOUGHT_SIGNATURE_SEPARATOR}{sig_b64}"
+  assert _strip_thought_signature_from_tool_call_id(tool_call_id) == "call_789"
+
+
+def test_strip_thought_signature_from_tool_call_id_leaves_plain_id():
+  """A plain ID with no separator is returned unchanged."""
+  assert _strip_thought_signature_from_tool_call_id("call_plain") == "call_plain"
+
+
+@pytest.mark.parametrize("value", [None, ""])
+def test_strip_thought_signature_from_tool_call_id_handles_empty(value):
+  """None and empty IDs are returned unchanged."""
+  assert _strip_thought_signature_from_tool_call_id(value) == value
+
+
+def test_strip_thought_signature_from_tool_call_id_splits_once():
+  """Only the first separator is used, mirroring signature extraction."""
+  tool_call_id = (
+      f"call_1{_THOUGHT_SIGNATURE_SEPARATOR}sig{_THOUGHT_SIGNATURE_SEPARATOR}x"
+  )
+  assert _strip_thought_signature_from_tool_call_id(tool_call_id) == "call_1"
+
+
 @pytest.mark.asyncio
 async def test_content_to_message_param_preserves_thought_signature():
   """thought_signature on Part is emitted on both tool call metadata paths."""