The CCC's Mystery Engine

In last week's post on engine vs. engine events, TCEC FRC6, CCC20 Rapid Finals; Both Underway (May 2023), for the CCC side of the report I noted,

Mystery played five 100-game matches, beating Koivisto, Berserk, and Ethereal; then losing to Dragon and Lc0.

Mystery played its first CCC game in the CCC20 Rapid: Entry League (chess.com). On the CCC's Discord forum, the early discussion was brief:-

Q: Mystery engine? Or is the engine actually called Mystery? • A: Probably a new Chess.com bot.

That's also what I would have guessed, but there's more to the story. The engine, a last minute CCC20 replacement for 'Demolito : Filling out the 12th spot', qualified into the next two 'CCC20 Rapid' stages, Qualifier #1 (chess.com), and Qualifier #2 (ditto). A handful of Mystery's 'Entry League' games were recorded in Mystery chess engine enters in [CCC]?! (ditto).

Fast forward to mid-May and we find Unknown "Mystery" Engine (chess.stackexchange.com). The writer observed,

One of the strangest things is that at 'CCC20 Rapid: Qualifier #2' it performed poorly. But at the same time, in a one-on-one battle with the "Berserker", it won confidently. And now beats "Ethereal", one of the strongest engines.

Someone commented,

Andrew Grant, the tournament director of CCC and author of Ethereal, seems to be the only one who knows Mystery's true identity. He did clarify that it was not a dumbed down version of Stockfish or Dragon as that would be rather lame. It seems to be an older engine (Maybe Laser?) with NNUE added to it but no tablebase support.

A few days later, the subject came up in one of the leading computer chess forums, Mystery engine at CCC (talkchess.com). The originator of the thread used the engine's UCI log to make an educated guess:-

The most obvious candidate would be a new engine developed by Chess.com as a successor of Komodo Dragon after their purchase of KomodoChess. The [UCI] options don't seem to match any known engine found on GitHub.

Some observers say that interest in chess engine events has been declining lately. At worst, the CCC is using the Mystery engine to rekindle interest. At best, it's a marketing strategy to introduce a new engine. I'm sure that we will soon learn much more.