What is server-sent events (sse)?
SSE is a long-lived HTTP response carrying named events rather than a single body. Twin uses it in three places: a live run streams meta, then frames, then a result; a parked run streams a screencast you can watch while you drive it; and a job streams status transitions as an alternative to polling.
Why it matters
It is a plain HTTP response, so it works through the same client, the same key and the same proxy as everything else — no second protocol and no socket to manage. Billing settles once on the terminal frame or on disconnect, so a dropped stream is not a free run or a double charge.