gemini-3.5-flash-lite rejects zero budget at thinking mode

[phenobarbital]

With the latest version of python-genai, gemini-3.5-flash-lite is not able to work when "thinkingBudget=0", with "thinkingBudget=None" works, but other models working fine with a budget of zero.

But, using a simple script with python httpx (no google-genai involved) I got this:

  ┌─────────────────────────────────────┬─────────────────┐
  │        gemini-3.5-flash-lite        │ Raw REST result │
  ├─────────────────────────────────────┼─────────────────┤
  │ thinkingBudget: 0                   │ 400             │
  ├─────────────────────────────────────┼─────────────────┤
  │ omitted                             │ 200 OK          │
  ├─────────────────────────────────────┼─────────────────┤
  │ -1 (dynamic)                        │ 200 OK          │
  ├─────────────────────────────────────┼─────────────────┤
  │ 512                                 │ 200 OK          │
  ├─────────────────────────────────────┼─────────────────┤
  │ (control) gemini-2.5-flash-lite + 0 │ 200 OK          │
  └─────────────────────────────────────┴─────────────────┘

I tested all other models, including 3.8 family without issues, this is the only model with this problem.

[Venkaiahbabuneelam]

Hi @phenobarbital, Thanks for reaching out us!
To help us understand the issue, could you please share a code snippet of what you've tried so far?
Thanks

[phenobarbital]

@Venkaiahbabuneelam yes, sure!

I tested directly using this snippet:

import asyncio
import json
import os
import aiohttp
from google import genai
from google.genai import types
from parrot.conf import config

API_KEY = config.get("GOOGLE_API_KEY") or os.getenv("GOOGLE_API_KEY") or os.getenv("GEMINI_API_KEY")

async def test_raw_rest(session, model, thinking_budget, label):
    url = f"https://generativelanguage.googleapis.com/v1beta/models/{model}:generateContent?key={API_KEY}"
    generation_config = {}
    if thinking_budget is not None:
        generation_config["thinkingConfig"] = {"thinkingBudget": thinking_budget}
    
    payload = {
        "contents": [{"parts": [{"text": "Say hello in 5 words."}]}],
    }
    if generation_config:
        payload["generationConfig"] = generation_config

    try:
        async with session.post(url, json=payload) as resp:
            status = resp.status
            text = await resp.text()
            try:
                data = json.loads(text)
                if status == 200:
                    cand = data.get("candidates", [{}])[0]
                    content = cand.get("content", {}).get("parts", [{}])[0].get("text", "")
                    print(f"[REST] {model} | {label} -> HTTP {status} OK: {content.strip()[:60]}")
                else:
                    err = data.get("error", {}).get("message", text[:100])
                    print(f"[REST] {model} | {label} -> HTTP {status} ERROR: {err}")
            except Exception:
                print(f"[REST] {model} | {label} -> HTTP {status} Raw: {text[:100]}")
            return status
    except Exception as e:
        print(f"[REST] {model} | {label} -> EXCEPTION: {e}")
        return None

async def test_sdk(client, model, thinking_budget, thinking_level, label):
    th_cfg = None
    if thinking_budget is not None or thinking_level is not None:
        kwargs = {}
        if thinking_budget is not None:
            kwargs["thinking_budget"] = thinking_budget
        if thinking_level is not None:
            kwargs["thinking_level"] = thinking_level
        th_cfg = types.ThinkingConfig(**kwargs)

    gen_cfg = types.GenerateContentConfig(thinking_config=th_cfg) if th_cfg else None

    try:
        resp = await client.aio.models.generate_content(
            model=model,
            contents="Say hello in 5 words.",
            config=gen_cfg,
        )
        print(f"[SDK]  {model} | {label} -> SUCCESS: {resp.text.strip()[:60] if resp.text else 'EMPTY'}")
        return 200
    except Exception as e:
        print(f"[SDK]  {model} | {label} -> EXCEPTION: {type(e).__name__}: {e}")
        return str(e)

async def main():
    print("=" * 60)
    print("TESTING GEMINI-3.5-FLASH-LITE & CONTROL MODELS")
    print(f"SDK version: {genai.__version__}")
    print("=" * 60)

    models_to_test = [
        "gemini-3.5-flash-lite",
        "gemini-2.5-flash-lite",
    ]

    async with aiohttp.ClientSession() as session:
        print("\n--- 1. RAW REST (v1beta) ---")
        for m in models_to_test:
            for tb, label in [
                (None, "omitted"),
                (0, "thinkingBudget: 0"),
                (-1, "thinkingBudget: -1"),
                (512, "thinkingBudget: 512"),
            ]:
                await test_raw_rest(session, m, tb, label)

    print("\n--- 2. GOOGLE-GENAI SDK ---")
    client = genai.Client(api_key=API_KEY)
    for m in models_to_test:
        for tb, tl, label in [
            (None, None, "omitted"),
            (0, None, "thinking_budget=0"),
            (-1, None, "thinking_budget=-1"),
            (512, None, "thinking_budget=512"),
            (None, "MINIMAL", "thinking_level='MINIMAL'"),
            (None, "LOW", "thinking_level='LOW'"),
        ]:
            await test_sdk(client, m, tb, tl, label)

if __name__ == "__main__":
    asyncio.run(main())

When is called with plain aiohttp calls I got this:

--- 1. RAW REST (v1beta) ---
[REST] gemini-3.5-flash-lite | omitted -> HTTP 200 OK: Hello, how are you today?
[REST] gemini-3.5-flash-lite | thinkingBudget: 0 -> HTTP 400 ERROR: Request contains an invalid argument.
[REST] gemini-3.5-flash-lite | thinkingBudget: -1 -> HTTP 200 OK: Hi, nice to meet you.
[REST] gemini-3.5-flash-lite | thinkingBudget: 512 -> HTTP 200 OK: Hello there, how are you?
[REST] gemini-2.5-flash-lite | omitted -> HTTP 200 OK: Hello there, how are you?
[REST] gemini-2.5-flash-lite | thinkingBudget: 0 -> HTTP 200 OK: Hello, it's great to see you.
[REST] gemini-2.5-flash-lite | thinkingBudget: -1 -> HTTP 200 OK: Hello! Wishing you a nice day.
[REST] gemini-2.5-flash-lite | thinkingBudget: 512 -> HTTP 200 OK: Hello, how are you doing?

but with google-genai (version: 2.18.1) I got:

[SDK]  gemini-3.5-flash-lite | omitted -> SUCCESS: Hello there, how are you?
[INFO] 2026-09-04 12:34:58,055 google_genai.models(models.py:8757) :: AFC is enabled with max remote calls: 10.
[SDK]  gemini-3.5-flash-lite | thinking_budget=0 -> EXCEPTION: ClientError: 400 INVALID_ARGUMENT. {'error': {'code': 400, 'message': 'Request contains an invalid argument.', 'status': 'INVALID_ARGUMENT'}}

the unrecognized thinkingBudget=0 only affects gemini-3.5-flash-lite, any other flash models (e.g. 3.8) or older (like 3.1 or 2.5) are not affected by this issue.

I was using thinkingBudget=0 on any model but gemini-3.5-flash-lite.

as conclusion:

Key Inconsistencies for the Incident Report

1. SDK Docstring vs Reality:
   The google-genai SDK docstring (types.py:13606) states:
   │ "Starting from Gemini 3.5 models, the old thinking_budget will no longer be supported and will result in a user error if set. Instead, users should use the thinking_level parameter to control the maximum thinking depth."
   However, thinking_budget is not completely unsupported on 3.5: -1 and 512 succeed without errors. Only 0 is rejected.
2. No Mechanism to Disable Thinking:
   types.ThinkingLevel only defines MINIMAL, LOW, MEDIUM, and HIGH. It does not provide an OFF or NONE level. Since thinking_budget=0 raises a 400 error, developers migrating from 2.5 have no way to turn thinking off.
3. Opaque Error Message:
   Pro models explicitly explain invalid budget configurations (e.g. "Budget 0 is invalid. This model only works in thinking mode"). gemini-3.5-flash-lite only outputs the generic message: "Request contains an invalid argument.")