Please read this first

• Have you read the docs? Yes – Agents SDK docs
• Have you searched for related issues? Yes – nothing covering the cached_tokens=None → Pydantic error with LiteLLM + Gemini 2.5.

Describe the bug

When Runner.run() executes an agent whose model is a Gemini 2.5-pro instance wrapped by LiteLLM, the pipeline dies during cost-calculation with

Error in Explainer (revision): 1 validation error for InputTokensDetails
cached_tokens
  Input should be a valid integer [type=int_type, input_value=None, input_type=NoneType]
    For further information visit https://errors.pydantic.dev/2.11/v/int_type

The log shows LiteLLM repeatedly inserting cached_tokens=None into the request metadata; Pydantic 2.11’s InputTokensDetails model rejects None because the field is typed as int.
The fallback code then silently downgrades the model to o3-2025-04-16, masking the issue in production.

Debug information

Repro steps

"""
Minimal repro for cached_tokens=None crash with LiteLLM + Gemini 2.5.

Save as repro.py and run `python repro.py` (GOOGLE_API_KEY or GEMINI_API_KEY must be set).
"""

from agents import Agent, Runner
from agents.extensions.models.litellm_model import LitellmModel
import litellm, os, asyncio, json

# Suppress NULL fields – does *not* avoid the bug
litellm.drop_params = True

gemini = LitellmModel(
    model="gemini/gemini-2.5-pro-preview-05-06",
    api_key=os.getenv("GOOGLE_API_KEY") or os.getenv("GEMINI_API_KEY")
)

echo_agent = Agent(
    name="Echo",
    instructions="Return the user's message verbatim in JSON: {\"echo\": \"...\"}",
    model=gemini,
)

async def main():
    # Any prompt triggers the validation failure
    result = await Runner.run(echo_agent, [{"role": "user", "content": "ping"}])
    print(result.final_output)

asyncio.run(main())

Observed output

Error in Explainer (revision): 1 validation error for InputTokensDetails
cached_tokens
  Input should be a valid integer ...

Commenting out the Gemini/LiteLLM model and falling back to any OpenAI model (o3-2025-04-16, gpt-4o) makes the script succeed, confirming the issue is isolated to the Gemini + LiteLLM path.

Expected behavior

• LiteLLM should pass a valid integer (e.g., 0) for cached_tokens instead of None, or
• The Agents SDK should coerce None → 0 before instantiating InputTokensDetails.

Either fix would allow Gemini 2.5 to run without crashing and would eliminate silent model downgrades.