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Automatic function calling rejects datetime and UUID arguments #3331
Description
Environment details
- Programming language: Python
- OS: macOS
- Language runtime version: Python 3.12.13
- Package version: google-genai 3.0.0 (same on 2.24.0 and
main)
Steps to reproduce
With automatic function calling, a tool that takes a datetime or a UUID never runs. The declaration tells the model to send a date-time (or uuid) string, the model does, and google-genai rejects it.
This fakes the Gemini API, so no key is needed:
import datetime, json, uuid
import httpx
from google import genai
from google.genai import types
ran, sent = [], []
def schedule(when: datetime.datetime, job_id: uuid.UUID) -> str:
"""Schedule a job."""
ran.append(True)
return "ok"
def fake_gemini(request):
sent.append(json.loads(request.content))
args = {"when": "2026-11-10T09:30:00Z", "job_id": "12345678-1234-5678-1234-567812345678"}
part = {"functionCall": {"name": "schedule", "args": args}} if len(sent) == 1 else {"text": "done"}
return httpx.Response(200, json={"candidates": [{"content": {"role": "model", "parts": [part]}}]})
client = genai.Client(
api_key="fake",
http_options=types.HttpOptions(httpx_client=httpx.Client(transport=httpx.MockTransport(fake_gemini))),
)
client.models.generate_content(
model="gemini-2.5-flash", contents="Schedule it.", config=types.GenerateContentConfig(tools=[schedule])
)
print("declared:", sent[0]["tools"][0]["functionDeclarations"][0]["parameters_json_schema"]["properties"])
print("ran:", bool(ran))
print("sent back:", sent[1]["contents"][-1]["parts"][0]["functionResponse"]["response"])uv run --no-project --python 3.12 --with "google-genai[httpx]==3.0.0" python repro.py prints:
declared: {'when': {'format': 'date-time', 'type': 'string'}, 'job_id': {'format': 'uuid', 'type': 'string'}}
ran: False
sent back: {'error': "Failed to parse parameter when for function schedule from function call part because function call argument value 2026-11-10T09:30:00Z is not compatible with parameter annotation <class 'datetime.datetime'>."}
job_id fails the same way once when is gone. The call still completes, so nothing says the tool never ran.
The cause: the schema comes from pydantic, but convert_if_exist_pydantic_model ends with isinstance(value, annotation), and a JSON string is never a datetime or UUID. Validating with pydantic too, e.g. pydantic.TypeAdapter(annotation).validate_python(value), would make them agree.
Related, for other types: #2013 (set[str] and other generics, fix in #3295) and #2199 (IntEnum, fix in #3288). From their diffs, neither PR covers datetime or UUID.
Thanks!
Activity
fix: Parse datetime, date, time, timedelta and UUID string arguments …
Venkaiahbabuneelam commentedon Nov 11, 2026
Hi @campoy, Thanks for reaching out us!
I have successfully reproduced the issue and When using Automatic Function Calling in google-genai, any Python tool function with datetime or UUID parameters never runs (ran: False) because the SDK schema tells the Gemini model to send date-time and UUID values as JSON strings, but convert_if_exist_pydantic_model() checks is instance without parsing those strings into datet-ime or UUID objects, causing the SDK to reject the arguments with an UnknownFunctionCallArgumentError, we will file a bug internally.
Thanks
fix: Parse datetime, date, time, timedelta and UUID string arguments …