Automatic function calling rejects `datetime` and `UUID` arguments

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#3,331 1 comment 0 reactions 1 assignee View on GitHub

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@Venkaiahbabuneelam is already working on this.

Since Nov 11, 2026.

  • #3333 by @copybara-service — open

Assessment

Difficulty
3/5
Estimated time
1-2 days
Newbie friendliness
75/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Active
Tech stack
python
Domain
backend-api-design

Research direction

Start by reading convert_if_exist_pydantic_model, which the issue identifies as the point rejecting JSON strings for datetime and UUID, and compare its validation with the pydantic schema generation. Check the automatic function calling tests for argument conversion and add coverage for both types. Done when valid schema-compatible strings reach the tool as converted arguments and the tests pass.

Written by the indexing model from the issue text.

Description

priority: p2 type: bug
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!

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