v3.6.0: sessions.start() with cdpUrl fails to connect to already-launched browser #316

Description

After upgrading to v3.6.0, passing a cdpUrl in sessions.start() to connect to an already-launched browser no longer works reliably. The session starts but Stagehand fails to attach to the existing browser process.

Root Cause

The change in PR #308 (src/stagehand/_base_client.py) switched request body serialization from httpx's built-in json= parameter to raw content= bytes:

v3.5.0:

else:
    kwargs["json"] = json_data if is_given(json_data) else None

v3.6.0:

elif not files:
    kwargs["content"] = openapi_dumps(json_data) if is_given(json_data) and json_data is not None else None
kwargs["files"] = files

When httpx receives json=data, it automatically serializes the data and sets Content-Type: application/json. When content=bytes is used instead, httpx does not automatically set the Content-Type header. If the SDK relies on httpx to set this header (rather than setting it explicitly), the server receives the request body without a proper Content-Type: application/json, causing the launchOptions.cdpUrl field to be silently ignored or unparsed — and Stagehand never attaches to the existing browser.

Steps to Reproduce

from stagehand import AsyncStagehand

client = AsyncStagehand(server="local", model_api_key="...", local_ready_timeout_s=30.0)

session = await client.sessions.start(
    model_name="openai/gpt-4o",
    browser={
        "type": "local",
        "launchOptions": {
            "cdpUrl": "ws://127.0.0.1:9222/devtools/browser/<uuid>",
        },
    },
)
# Session starts but Stagehand launches a new browser instead of connecting to the existing one

Expected Behavior

Stagehand connects to the already-running browser at the provided cdpUrl.

Actual Behavior

Stagehand ignores the cdpUrl and either launches a new browser or fails to connect.

Workaround

Pin to v3.5.0 until this is resolved:

stagehand==3.5.0

Additional Notes

• The change in _base_client.py was introduced to support executionModel serialization with camelCase aliases (by_alias=True) for pydantic models in the execute endpoint. The fix for that feature inadvertently changed behavior for all JSON requests.
• A secondary regression was also flagged in the same PR: when files are provided alongside json_data, the JSON body is now silently dropped instead of raising a ValueError as httpx previously did.
• The same issue exists in the TypeScript SDK.

Affected version: 3.6.0
Working version: 3.5.0

PS:
Facing the same issue in TS version 3.1.0 and working on version 3.0.8