Orphaned function_call blocks from user interruption cause 400 errors

[seanxiong1989]

Description

When a user sends a new message while a tool call is still pending (or the tool errors without ADK emitting a function_response), the persisted session contains a tool_use event with no matching tool_result. Claude's API requires every tool_use to have a corresponding tool_result — without one, every subsequent turn returns a 400 error and the session is permanently broken.

Difference from existing fixes

This is distinct from the fixes in #3504 and #4027, which address orphaned function_response blocks during context truncation. This issue is about orphaned function_call blocks caused by user interruption — a different problem that ADK does not currently handle.

Current workaround

We use a before_model_callback that scans llm_request.contents and injects synthetic function_response stubs for any unmatched function_call blocks, preserving the rest of the conversation context.

Expected behavior

ADK should handle orphaned function_call blocks (from interruption or tool errors) gracefully, either by injecting synthetic responses or by stripping the orphaned calls before sending to the model API.

[llalitkumarrr]

Hello @seanxiong1989 ,

Could you please share a minimal reproducible code and the steps for reproduction? Also are you facing this issue with Claude only or is a similar problem observed with other models? Furthermore, if you are not already using the latest version, kindly try our most recent release and share your observations.

[seanxiong1989]

Hey @llalitkumarrr — reproduces on both 1.29.0 and 2.1.0. Claude-specific (Gemini doesn't enforce tool_use/tool_result pairing).

Repro (Claude on Vertex AI):

"""Minimal reproduction: orphaned function_call permanently breaks session."""

from __future__ import annotations
import asyncio

from google.adk.agents import Agent
from google.adk.events import Event
from google.adk.models.anthropic_llm import Claude
from google.adk.models.registry import LLMRegistry
from google.adk.runners import Runner
from google.adk.sessions import InMemorySessionService
from google.genai import types

LLMRegistry.register(Claude)


def get_weather(city: str) -> dict:
    """Look up weather for a city."""
    return {"city": city, "temp_f": 72, "condition": "sunny"}


agent = Agent(
    name="repro_agent",
    model="claude-sonnet-4-6",
    instruction="You are a helpful assistant. Use the get_weather tool when asked about weather.",
    tools=[get_weather],
)

session_service = InMemorySessionService()
runner = Runner(agent=agent, session_service=session_service, app_name="repro")


async def main():
    session = await session_service.create_session(app_name="repro", user_id="user1")
    sid = session.id

    # Turn 1: normal conversation
    msg1 = types.Content(role="user", parts=[types.Part(text="What is the weather in NYC?")])
    async for event in runner.run_async(user_id="user1", session_id=sid, new_message=msg1):
        pass

    # Inject orphaned function_call (simulates user interruption mid-tool)
    session = await session_service.get_session(app_name="repro", user_id="user1", session_id=sid)
    orphan = Event(
        author="repro_agent",
        invocation_id="interrupted",
        content=types.Content(
            role="model",
            parts=[types.Part(function_call=types.FunctionCall(
                id="orphan_001", name="get_weather", args={"city": "London"},
            ))],
        ),
    )
    await session_service.append_event(session, orphan)

    # Turn 2: this 400s, session is permanently broken
    msg2 = types.Content(role="user", parts=[types.Part(text="What about London?")])
    async for event in runner.run_async(user_id="user1", session_id=sid, new_message=msg2):
        pass


if __name__ == "__main__":
    asyncio.run(main())

export GOOGLE_CLOUD_PROJECT=<your-project>
export GOOGLE_CLOUD_LOCATION=<region>
python repro.py

Output:

Turn 2: ERROR: BadRequestError: messages.4: `tool_use` ids were found
without `tool_result` blocks immediately after: orphan_001.
Session is permanently broken.