Coinbase's symbol mapping.

Its native product_id is already canonical BASE-QUOTE — BTC-USD — so the conversion is effectively identity. It is declared anyway, for two reasons.

The contract is uniform: every venue implements both directions, so nothing above the facade needs to know that one venue's conversion happens to be free.

And it is a defensive boundary. Any un-canonical form Coinbase ever returns gets normalised here rather than leaking upward — a lowercase btc-usd, or a separatorless form from an endpoint nobody has looked at lately. The cost of running the normaliser over an already-canonical string is nothing; the cost of one venue quietly emitting a form the rest of the system does not recognise is a symbol that matches no catalogue entry and collects nothing.

The quote list is ordered longest-first, and here that is precaution, not need

USDC precedes USD, as the shared convention requires. On this venue the ordering never comes into play, and it is worth being precise about why rather than repeating the general rule as though it bit here.

CanonicalPair splits on the separator when a mapping declares one, and falls back to matching a quote suffix only for a separatorless mapping. Coinbase declares sep: "-", so a string without a dash takes the :nomatch path and is returned uppercased — it is never split, and the quote list is never consulted.

The ordering is kept correct anyway. It costs nothing, and it stops being merely cosmetic the moment someone reuses this mapping for a venue that concatenates.