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.

[llalitkumarrr]

Hello @seanxiong1989 ,

Thank you for sharing this detailed report.
As you noted, this behavior is due to Claude's strict implementation of tool use and tool results pairing. We recommend that the best approach right now is to use the callback feature, which it appears you are already doing.

Could you please let us know if callback solves this issue?

[adk-bot]

This issue has been automatically marked as stale because it has not had recent activity for 7 days after a maintainer requested clarification. It will be closed if no further activity occurs within 7 days.