AI anchors

Surround's five AI opponents are anchors: accounts whose ratings the owner pins in SurroundRatings. Each plays at a fixed strength, with a fixed rating to match, so the rank a newcomer earns against them means something. Their games are rated like any other, but they rate only the human.

The roster

How they play

The anchors run on KataGo's analysis engine with its human SL model (b18c384nbt-humanv0, KataGo 1.15+). That model was trained on human games and predicts the move a human of a given rank would play. Each query names the anchor's rank profile, so one engine process serves all five.

• Moves are sampled, not searched. For each move the anchor samples KataGo's human policy for its rank at its native temperature, skipping illegal moves (positional superko included). Turning down a superhuman search's visits doesn't make a believable 20k; sampling what a 20k would play does. A short search (64 visits) runs alongside to decide when to pass.
• Passing. The anchor passes only when the search recommends it. When its opponent has passed, it passes back if ending the game now, scored with KataGo's dead stones removed, gives up at most one point against playing on. Otherwise an engine keeps filling its own territory, which costs nothing under area scoring, and the game never ends.
• Scoring. After two passes, the anchor proposes the stones KataGo's ownership estimate marks dead: groups whose average ownership leans past 0.8 toward the opponent, on KataGo's scale of −1 to 1. It accepts its opponent's proposal if that gives the same winner as its own, and resumes play otherwise.
• After a resumption, full strength. Once a game has been resumed, the stones on the board are what counts, so the anchor drops its rank profile and plays KataGo's best move, capturing what's dead.
• Anchors don't resign. They play every game to scoring or a flag.

How their ratings work

An anchor's rating is its pin, not something it earns.

• Fixed. An anchor plays at its pinned μ with a fixed φ of 0.25. Its rating never ages, is never stored, and never shows "?".
• One-sided. Its games rate only its opponent, by the ordinary update.
• Never against each other. An anchor's ticket band is 0, and two anchors never play a rated game.
• Rank, not peak. An anchor never counts as a settled player, so games against anchors can move a human's rank but never their peak. That bounds what a repeatable trick against the engine is worth.
• Visible in the audit trail. An anchor's side of RatingUpdated is marked anchor and shows its pin before and after; replay.mjs checks it against the owner's set_anchor events.

Why pin an AI, but not humans

Freezing settled human players against newcomers was tested and failed (evidence): a human's strength moves. An engine at a fixed profile and visit count doesn't, so a fixed rating is the right model for it.

A floating rating would be worse for an AI in two ways. Sybil accounts starting at 6k could lose to the 20k anchor on purpose, inflating it for real newcomers to farm. And an AI that has played thousands of games reaches the minimum φ and becomes an anchor anyway, at whatever level newcomers' bands dragged it to.

Calibration

Anchors are pinned at their nominal ranks, but KataGo's human profiles learned Fox's ranks, not OGS's, and high-dan imitation is approximate. replay.mjs reports each anchor's points against established players next to the points expected. An anchor that keeps scoring above expectation is stronger than its pin, and the owner re-pins it with set_anchor. The rank offset shifts anchors' shown ranks along with everyone else's.

set_anchor accepts only an account that has never been rated, and once the contract is sealed it waits 48 hours like any loosening change. remove_anchor applies at once and voids the anchor's unrated games.

The anchor daemon

One process plays all five anchors. For each, it:

1. Offers session keys. It offers the matchmaker fresh session keys for the anchor's next games, signed by the anchor's wallet, keeping each private key in its own store first. An anchor with no keys on offer is busy.
2. Checks and signs terms. It checks each pairing as any player's client does (the ticket, its board, komi and clock against the ticket's hashes, its own key at its seat, band 0, the keeper's referee) and signs the terms with the anchor's wallet.
3. Plays through the keeper, like any ranked game, on the referee's clock.

A player asks the matchmaker for an anchor by name and is offered a game at once; it's theirs when their wallet signs the terms. The anchors' accounts only sign. They never send transactions: the keeper opens and settles their games, and the matchmaker rates them.

Trust

Whoever holds an anchor's wallet key could throw its games to inflate a friend's rank. That's the operator, who already holds the matchmaker key. A new anchor waits 48 hours once the contract is sealed, the same rule as for new matchmaker keys.