mode

an app-defined string forwarded to the backend as-is (e.g. which persona/prompt-pack to use). llm-chat doesn't know or validate the values — the backend does. null (the default) means the "mode" key is left off the request body entirely, not sent as "mode": null — a backend with a closed mode allowlist can then treat "no mode" and "an unrecognized mode" differently, e.g. defaulting the former to ordinary chat and 400ing the latter.