feat: add model_base_url parameter for custom LLM endpoints

Mirrors x-model-api-key pattern with x-model-base-url header,
enabling OpenAI-compatible providers like ZhipuAI and Ollama.

Author: chrisreadsf

src/stagehand/_client.py

@@ -60,6 +60,7 @@ def __init__(
         browserbase_api_key: str | None = None,
         browserbase_project_id: str | None = None,
         model_api_key: str | None = None,
+        model_base_url: str | None = None,
         server: Literal["remote", "local"] = "remote",
         _local_stagehand_binary_path: str | os.PathLike[str] | None = None,
         local_host: str = "127.0.0.1",
@@ -121,6 +122,10 @@ def __init__(
             )
         self.model_api_key = model_api_key
 
+        if model_base_url is None:
+            model_base_url = os.environ.get("MODEL_BASE_URL")
+        self.model_base_url = model_base_url
+
         self._sea_server: SeaServerManager | None = None
         if server == "local":
             # We'll switch `base_url` to the started server before the first request.
@@ -195,7 +200,7 @@ def qs(self) -> Querystring:
     @property
     @override
     def auth_headers(self) -> dict[str, str]:
-        return {**self._bb_api_key_auth, **self._bb_project_id_auth, **self._llm_model_api_key_auth}
+        return {**self._bb_api_key_auth, **self._bb_project_id_auth, **self._llm_model_api_key_auth, **self._llm_model_base_url_header}
 
     @property
     def _bb_api_key_auth(self) -> dict[str, str]:
@@ -212,6 +217,11 @@ def _llm_model_api_key_auth(self) -> dict[str, str]:
         model_api_key = self.model_api_key
         return {"x-model-api-key": model_api_key}
 
+    @property
+    def _llm_model_base_url_header(self) -> dict[str, str]:
+        model_base_url = self.model_base_url
+        return {"x-model-base-url": model_base_url} if model_base_url else {}
+
     @property
     @override
     def default_headers(self) -> dict[str, str | Omit]:
@@ -348,6 +358,7 @@ def __init__(
         browserbase_api_key: str | None = None,
         browserbase_project_id: str | None = None,
         model_api_key: str | None = None,
+        model_base_url: str | None = None,
         server: Literal["remote", "local"] = "remote",
         _local_stagehand_binary_path: str | os.PathLike[str] | None = None,
         local_host: str = "127.0.0.1",
@@ -409,6 +420,10 @@ def __init__(
             )
         self.model_api_key = model_api_key
 
+        if model_base_url is None:
+            model_base_url = os.environ.get("MODEL_BASE_URL")
+        self.model_base_url = model_base_url
+
         self._sea_server: SeaServerManager | None = None
         if server == "local":
             if base_url is None:
@@ -482,7 +497,7 @@ def qs(self) -> Querystring:
     @property
     @override
     def auth_headers(self) -> dict[str, str]:
-        return {**self._bb_api_key_auth, **self._bb_project_id_auth, **self._llm_model_api_key_auth}
+        return {**self._bb_api_key_auth, **self._bb_project_id_auth, **self._llm_model_api_key_auth, **self._llm_model_base_url_header}
 
     @property
     def _bb_api_key_auth(self) -> dict[str, str]:
@@ -499,6 +514,11 @@ def _llm_model_api_key_auth(self) -> dict[str, str]:
         model_api_key = self.model_api_key
         return {"x-model-api-key": model_api_key}
 
+    @property
+    def _llm_model_base_url_header(self) -> dict[str, str]:
+        model_base_url = self.model_base_url
+        return {"x-model-base-url": model_base_url} if model_base_url else {}
+
     @property
     @override
     def default_headers(self) -> dict[str, str | Omit]: