chorcat/rukh-rm

A reward model: it reads a chess position and answers with one number, trained on 13,838 pairs of moves that a chess engine had already ranked. This is the rm stage of Rukh, a course that builds a chess language model end to end: 37,913,601 parameters over 12 layers of width 512, reading the 69 square tokens of a board.

Read how it was built: https://lab.rukh.borjaglez.com

What the number means, and what it does not

The training objective is Bradley-Terry: -log sigma(r(chosen) - r(rejected)). Only differences appear in it, so adding a constant to every score changes nothing. The scale has no units and no origin, and comparing this model's 0.8 with another training run's 1.4 is comparing two rulers with no zero. What the model is for is ordering two positions, and the number below is how often it gets that order right.

Results

Measured on 1,462 held-out pairs, split by game so that two pairs a few plies apart cannot straddle the split.

Metric	Value
Accuracy on held-out pairs	74.21 %
Accuracy over pairs an evaluation can decide	75.77 %
Preference loss	0.5232
Margin vs engine gap (Pearson)	-0.148
Margin vs engine gap, without mates (Pearson)	+0.081

Where it is right and where it is wrong

Accuracy by how far apart the engine scored the two moves. The mate band is the one to read first: it is the largest slice after the narrowest band, and it is where the model is worst.

Engine gap	Pairs	Accuracy
100-200 cp	616	76.30 %
200-400 cp	247	74.09 %
400-800 cp	101	79.21 %
800-2000 cp	14	57.14 %
mate	484	71.07 %

Read the Pairs column before the Accuracy one. A band of a dozen pairs reports whatever those dozen did: across five runs of this configuration the 800-2000 band gave 90 %, 68 %, 90 %, 50 % and 57 %, which is the same absence of data wearing five faces.

A mate is a tactical fact, and what this model reads is a static view of the position the move leads to: it cannot see one. That band is exactly what a verifiable reward answers without error, which is why the same milestone that trained this model also wrote one.

The two correlations above do not share a sign, and that is not a typo. The mate pairs carry the largest engine gap by construction and the smallest learned margin, so a third of the points bends the line the other way. One correlation over two regimes describes neither.

What it is not for

DPO does not need this model. Direct Preference Optimization has an implicit reference -- a frozen copy of the starting policy -- and trains on the pairs directly. A reward model is what PPO needs, what a learned reward is compared against, and what makes the accuracy above a number worth publishing at all.

Training

13,838 pairs, 12,376 for training and 1,462 held out, 10 epochs, batch 64, learning rate 0.0001, seed 42.

Trained from a random initialisation, and that was measured rather than assumed: starting from the project's pretrained position encoder scored about 6.5 points worse. That encoder learned to fill in masked moves; this task is ordering positions by how good they are, and what it brought did not help.

The seed also decides the split, so it decides how many mate pairs land in validation -- and the headline number moves with that. Four seeds of this exact configuration gave 72.91 %, 74.21 %, 74.71 %, 74.76 % and 75.19 %. The first two are the same seed, so 1.3 of those points are not the split: they are what this measurement repeats to. Compare two runs only if they shared a seed.

Reproducing

uv run rukh train reward --config configs/train/rm.yaml

In the course

• Built in M5 · Alineamiento: los labs, the lesson that runs every command behind this repository.
• Measured in the single results table: every stage of the course, the same suite, the same day.
• Bring it to the paths the configs read: uv run rukh pull rm.

Licence

apache-2.0. Trained on engine evaluations of public Lichess games.