Async requests use `aiohttp` when available, silently bypassing httpx-level auth and transports

[yann-combarnous]

Environment

• google-genai version: 1.64.0
• Python: 3.13.2

Description

The SDK's async request path (_async_request_once) switches to aiohttp at runtime when the
package is installed. This means any auth or transport customisation applied to the internal
_httpx_client (e.g. _httpx_client._auth = my_auth) is silently ignored for all await/
ainvoke calls.

There is no public API to supply a custom aiohttp session or per-request headers for the async
path, making it impossible to inject auth credentials (e.g. a rotating Bearer token) for async
usage without monkey-patching an internal method.

Current workaround

original = api_client._async_request_once

async def _patched(http_request, stream=False):
    http_request.headers["Authorization"] = f"Bearer {get_token()}"
    return await original(http_request, stream=stream)

api_client._async_request_once = _patched

Expected behavior

The SDK should provide a supported hook for injecting per-request headers (or a custom async
session/transport) that works consistently regardless of whether the underlying async backend is
httpx or aiohttp. For example: an async_http_options.headers dict or an auth callback
parameter on Client.init that is applied on every async request.

[yinghsienwu]

Thanks for raising the issue. We can fix it to make sure aiohttp is an optional dependency. We also fixed a bug when using custom aiohttp.ClientSession through types.HttpOptions(aiohttp_client=aiohttp.ClientSession(...)) when initializing Client (PR#2099). To answer your feature request "per request override of headers", we should already support per request HttpOptions headers merging.
https://github.com/googleapis/python-genai/blob/v1.64.0/google/genai/_api_client.py#L142-L145

In order to do that, here's an example:

client = genai.Client(project=YOUR_PROJECT', vertexai=True)
response = client.models.generate_content(
    model='gemini-2.5-flash', 
    contents='why the cloud can float?',
    config=genai.types.GenerateContentConfig(
        http_options=genai.types.HttpOptions(headers={'Authorization': f"Bearer {get_token()}"})
    )
)

To get token in each request may cause additional latency though.