The bot

The bot is a negamax search with alpha-beta pruning, move ordering and a material and position evaluation. It is not a stockfish port and does not pretend to be: it is an opponent for a Roblox chess table.

The strength buttons

EloPresets = { 600, 1000, 1400, 1800, 2200 },
DefaultElo = 1000,

Each preset becomes a button. The number is turned into two internal parameters:

Two things follow from that table.

The blunder chance is what creates a beatable bot. Even a depth-1 search takes every free piece, and a beginner cannot beat that consistently. Playing a deliberately worse move some of the time is what makes 600 feel like 600.

Presets between the same thresholds play identically. 1000 and 1399 are the same bot. Adding 1200 to the list gives players another button, not another opponent. To create genuinely distinct strengths, pick numbers that cross the thresholds above.

Depth 4 is the ceiling

Anything at or above 1800 gets depth 4 and no blunders. 2200 on the shipped list is therefore the same as 1800, offered as a separate button because players read the number as the challenge.

A strong club player will beat it. A casual player will not.

Keeping the server responsive

The search yields periodically rather than running to completion in one go, controlled by BotYieldEvery. Larger means fewer pauses and a longer uninterrupted stretch of work; smaller means smoother but slower.

Leave it alone unless you can measure a problem. 800 is tuned for depth 4 on a typical position.

The move delay

BotMoveDelay = 0.75,

The bot always waits this long, even when it decided in a millisecond. Without it a depth-1 bot answers before the player's piece has finished sliding, which reads as a bug.

The avatar

A cosmetic NPC sits opposite. With Npc.UseFriendAvatar = true it borrows the appearance of a random friend of the player, otherwise Npc.FallbackUsername.

The game waits up to BotLoadMaxSeconds, eight seconds, and then starts with an empty chair if the avatar has not arrived. The opponent is the search; the model is decoration.

Do bot games count

Two independent settings:

The shipped combination is the sensible one: bot games show up in a player's record but do not inflate their rating or their wallet.

If you do turn RatedVsBot on, the rating change is computed against the bot's preset number as its rating, so beating 2200 moves a player more than beating 600.

Testing the engine

The rules, move generation, board and evaluation modules use no Roblox API and ship with specs:

require(game.ReplicatedStorage.uxrChessSystem.Chess._spec.Run)()

That covers move legality, castling rights, en passant, promotion and the check and mate detection. Run it after touching anything under Shared/Chess.