`ChatGoogleGenerativeAI` raises `TypeError` when passing `httpx.Auth` via `client_args`, no supported path for custom auth

[yann-combarnous]

Package (Required)

• langchain-google-genai
• langchain-google-vertexai
• langchain-google-community
• Other / not sure / general

Checked other resources

• I added a descriptive title to this issue
• I searched the LangChain documentation and API reference (linked above)
• I used the GitHub search to find a similar issue and didn't find it
• I am sure this is a feature request and not a bug report or question

Feature Description

Environment

• langchain-google-genai version: 4.2.1
• google-genai version: 1.64.0
• Python: 3.13.2

Description

There is no supported way to supply a custom httpx.Auth (or equivalent auth callback) to
ChatGoogleGenerativeAI when using a custom base_url (e.g. an enterprise WSO2 proxy that
requires a Bearer token).

Passing auth via client_args raises TypeError — the constructor does not accept it and does
not forward it to the underlying google-genai client.

# Raises TypeError:
chat = ChatGoogleGenerativeAI(
    model="gemini-2.0-flash",
    base_url="https://my-proxy.example.com/gemini/",
    google_api_key="unused",
    client_args={"auth": my_httpx_auth},  # TypeError
)

Current workaround

After constructing ChatGoogleGenerativeAI, directly mutate the internal google-genai
_api_client:

chat.client._api_client._httpx_client._auth = wso2_auth

This is fragile and breaks if the internal structure of ChatGoogleGenerativeAI or google-genai
changes.

Expected behavior

ChatGoogleGenerativeAI should accept and forward auth-related constructor arguments to the
underlying google-genai client, or expose a dedicated auth / request_headers parameter
that is applied to every request. This is especially important for enterprise deployments that
route via an API gateway requiring token-based authentication.

Use Case

There is no supported way to supply a custom httpx.Auth (or equivalent auth callback) to
ChatGoogleGenerativeAI when using a custom base_url (e.g. an enterprise WSO2 proxy that
requires a Bearer token).

Proposed Solution

ChatGoogleGenerativeAI should accept and forward auth-related constructor arguments to the
underlying google-genai client, or expose a dedicated auth / request_headers parameter
that is applied to every request. This is especially important for enterprise deployments that
route via an API gateway requiring token-based authentication.

Alternatives Considered

No response

Additional Context

No response