Base URL path for API requests, leave blank if not using a proxy or service emulator.
Resolution order (first match wins):
base_url (or openai_api_base) kwarg.OPENAI_API_BASE (read by LangChain at init).OPENAI_BASE_URL (read by the underlying openai SDK client).OPENAI_BASE_URL is also inspected by LangChain only to decide whether to
default-enable stream_usage — when set, the default is left off because many
non-OpenAI endpoints do not support streaming token usage.