The venue's WebSocket connection — internal, and never named above the facade.

Speaks wss://ws.gemini.com, the API Gemini's current documentation describes. This is not the API the host adapter uses, and the reasoning is in docs/reference/gemini/websocket-api-replacement.md. In one line: the host's api.gemini.com/v2/marketdata still answers, but it is absent from the vendor's current documentation and from four years of its changelog, so a package published for other people to depend on should not be built on it.

Protocol

Subscription is an RPC-shaped frame naming streams:

{"method":"subscribe","params":["btcusd@bookTicker","ethusd@bookTicker"],"id":1}

and the venue acknowledges with {"id":1,"status":200}.

This package subscribes to @bookTicker and nothing else. That single stream carries best bid, best ask and — where the book has traded — the last trade price, in one message per change. It delivers Core.Types.TopOfBook on every frame that parses, and a separate Core.Types.Quote only on the frames that also carry that last trade: the two are independent facts and a bid is never dressed up as a price. See handle_message/2 for the substitution that rule exists to stop.

Which is why there is no order-book machinery here

The host maintains a 182-line L2 book (gemini/l2_book.ex) whose entire purpose is to reconstruct a mid price from l2_updates deltas, because the endpoint it connects to offers no top-of-book message. This endpoint does. The book is not ported, and the moduledoc of the file that is not ported records why it existed — a mid computed from a single delta rather than the maintained book, which is the incident that created it.

A caller wanting depth calls get_order_book/2, which is a REST snapshot with the venue's own per-level timestamps. Where a differential depth frame arrives instead — a future @depth/@depthFast subscription, not one this socket requests today — it is decoded into Core.Types.OrderBookDelta by WsDecode.to_order_book_delta/2 and forwarded once, per frame. Never accumulated into a book here: see OrderBookDelta's own moduledoc for why a distinct, non-snapshot-shaped type is what keeps that from happening by construction rather than by discipline.

Event time is nanoseconds

The E field is nanoseconds since the epoch, not milliseconds. The difference is a factor of a million: read as milliseconds, a 2026 timestamp lands in the year 58,000 and every staleness check passes forever. Converted once, in WsDecode.nanosecond_time/1 — every frame handler here reaches that through one of WsDecode's decoders rather than parsing E a second time.