`FunctionDeclaration` doesn't support dicts

[pp-eyushin]

Describe the Bug:
Python methods with dicts in signature raises issues when FunctionTool tries to come up with schema.
Complex types as list/dict are handled in fallback branch relying on pydantic.TypeAdapter.json_schema(), that emits additionalProperties for typed dicts.

Follow-up for #4798

Steps to Reproduce:
Please provide a numbered list of steps to reproduce the behavior:

import asyncio
import os
from datetime import datetime, timezone

from google.adk import Agent, Runner
from google.adk.sessions import InMemorySessionService
from google.genai import types as genai_types

APP_NAME = "repro"
USER_ID = "u1"
MODEL = "gemini-2.5-flash"


async def get_utc_timestamp(
    options: dict[str, str] | None = None,
    # options: list[str] | None = None,
) -> str:
    _ = options
    return datetime.now(timezone.utc).isoformat()


async def main() -> None:
    agent = Agent(
        name="time_agent",
        model=MODEL,
        instruction="You are a helpful assistant. Use tools when asked for the time.",
        tools=[get_utc_timestamp],
    )

    session_service = InMemorySessionService()
    session = await session_service.create_session(app_name=APP_NAME, user_id=USER_ID)
    runner = Runner(app_name=APP_NAME, agent=agent, session_service=session_service)

    user_msg = genai_types.Content(
        role="user",
        parts=[genai_types.Part(text="What's the current UTC timestamp?")],
    )

    async for event in runner.run_async(
        user_id=USER_ID, session_id=session.id, new_message=user_msg
    ):
        if event.content and event.content.parts and event.is_final_response():
            for part in event.content.parts:
                print(part.text)


if __name__ == "__main__":
    os.environ.setdefault("GOOGLE_API_KEY", "<your key here>")
    asyncio.run(main())

Expected Behavior:
dict should be recognized in Schema.

Observed Behavior:

google.genai.errors.ClientError: 400 INVALID_ARGUMENT. {'error': {'code': 400, 'message': 'Invalid JSON payload received. Unknown name "additional_properties" at \'tools[0].function_declarations[0].parameters.properties[0].value.any_of[0]\': Cannot find field.', 'status': 'INVALID_ARGUMENT', 'details': [{'@type': 'type.googleapis.com/google.rpc.BadRequest', 'fieldViolations': [{'field': 'tools[0].function_declarations[0].parameters.properties[0].value.any_of[0]', 'description': 'Invalid JSON payload received. Unknown name "additional_properties" at \'tools[0].function_declarations[0].parameters.properties[0].value.any_of[0]\': Cannot find field.'}]}]}}

Environment Details:

• ADK Library Version (pip show google-adk): 1.31.0
• Desktop OS: macOS
• Python Version (python -V): Python 3.11.14

Model Information:

• Are you using LiteLLM: No
• Which model is being used: gemini-2.5-flash

---

How often has this issue occurred?:

• Always (100%)

[perrymanuk]

Confirming this reproduces in a production setting (google-adk==2.0.0a3, google-genai==1.72.0, Python 3.14) with a few extra data points that may help narrow it down:

Model-dependent strictness

The failure only surfaces on gemini-2.5-flash and gemini-3.1-flash. The identical agent + tools + schema ran against gemini-3.1-pro-preview for weeks without issue — pro was tolerating the unknown additional_properties field silently.

In our case the bug was latent after the ADK 2.x upgrade but only started 400-ing once we downgraded our orchestrator from pro to flash for cost reasons. That means other users may be sitting on this without realising, and any model-tier downshift in a multi-agent app is likely to surface it.

Also triggered by Optional[List[...]]

Not just dicts — Optional[List[str]] parameters hit the same fallback branch and produce the same snake_case leak. Our failing schemas included:

• attendees: Optional[List[str]] (calendar event tool)
• related_refs: Optional[List[str]]
• metadata: Optional[Dict[str, Any]]
• metadata_merge: Optional[Dict[str, Any]]
• related_info: Optional[Dict[str, Any]]

All five shapes produced the same Unknown name "additional_properties" at '...any_of[0]' 400 when routed to flash.

Workaround for anyone blocked

A small before_model_callback that recursively strips the non-standard key from every outgoing llm_request.config.tools[*].function_declarations[*].parameters is working reliably as a holdover. The camelCase additionalProperties (standards-compliant) is left untouched; only the snake_case one is removed:

https://github.com/perrymanuk/radbot/blob/main/radbot/callbacks/sanitize_tool_schemas.py

Wired onto the root agent + every sub-agent via before_model_callback so the scrub runs on every LLM call. Idempotent and becomes a harmless no-op once the upstream fix lands.

Note on #5052

FWIW #5052 appears to explicitly emit the snake_case additional_properties key in the dict branch, which is the opposite direction from what the Gemini API accepts (it requires camelCase additionalProperties per standard JSON Schema). May be worth double-checking that PR's output against the REST validator before merging — the repro in this issue would presumably still 400 after #5052 lands.