CHSM8-player-base: games-only continuation (one full pass over 26M games)

CHSM8 pretrained decoder (LegumMagister/chsm8-pt) continued for exactly one pass over 26.0M unique human games (1.95B move tokens; Lichess pool, deduplicated, split seed 20260802) with no text: plain next-move prediction on the whole game (both sides). Unconditioned strong base for the player fine-tunes (see Player models).

• Architecture: Llama-style decoder, 12 layers x 960, 15 heads, SwiGLU 3520, QK-norm, factored move head; 166.2M parameters (chsm8-pt without its 44.3M cross-attention scaffold).
• Pretraining cross-attention (frozen, constant input) removed exactly: its constant output became a per-layer bias (xbias); loss at step 0 identical to the pretrained model.
• Recipe (picked by 10/30/60-min ablations, val loss): AdamW lr 5e-5, cosine to 0 by tokens, warmup 5%, batch 4 GPUs x 32 x 512, legal-move penalty 0.5, bf16.
• Val loss (fixed 40-batch val set) at 10%, 20%, … 90% of the pass: 2.9236, 2.9117, 2.9104, 2.9068, 2.8948, 2.8871, 2.8855, 2.8786, 2.8773

Strength: 1000 games vs Stockfish 17.1 (UCI_Elo 2000, 0.05 s/move)

checkpoint (share of the pass)	Elo
pct20	1889 (95% CI 1869-1907, 999 games)
pct40	1881 (95% CI 1861-1900, 1000 games)
pct60	1886 (95% CI 1867-1905, 1000 games)
pct80	1883 (95% CI 1863-1902, 999 games)
final	1890 (95% CI 1870-1909, 1000 games)

Reference: the pretrained base, chsm8-pt, scored 1837 on the same benchmark; one extra pass over games gives +53.

Player models

Each player model is this model fine-tuned on one player's own moves (full fine-tune, AdamW lr 3e-5, cosine, 2 passes over the player's unique games; the opponent's moves are context only). Games come from chess.com and lichess for active players and from PGN Mentor (classical over-the-board) for historical ones. Held out: the player's most recent games (val + test, up to 1,000 each), never trained on.

How well a fine-tune captures the player (on held-out test games, player's own moves only):

• Move-match: share of positions where the model's top move is the move the player actually played (base = this model before fine-tuning).
• Top-3: the played move is among the model's top 3.
• CE: cross-entropy on the player's moves (lower = closer to the player).
• Model Elo: the fine-tuned model's strength, 1000 games vs Stockfish 17.1 at UCI_Elo 2000, 0.05 s/move (this base model: 1890).

The model imitates the player's moves in the games available (mostly online blitz and rapid for active players); it does not reach the player's strength. Fine-tunes stay within about ±40 Elo of the base.

FIDE top 10

#	Player	FIDE (2026-09)	Model	Unique games	Move-match base → fit	Top-3	CE base → fit	Model Elo
1	Magnus Carlsen	2823	chsm8-player-carlsen	17,561	0.554 → 0.569	0.798	2.969 → 2.915	1903
2	Hikaru Nakamura	2792	chsm8-player-nakamura	19,997	0.540 → 0.572	0.797	3.039 → 2.862	1850
3	Fabiano Caruana	2784	chsm8-player-caruana	5,513	0.545 → 0.556	0.788	2.960 → 2.938	1902
4	Javokhir Sindarov	2782	chsm8-player-sindarov	1,371	0.557 → 0.567	0.794	2.959 → 2.897	1897
5	Nodirbek Abdusattorov	2780	chsm8-player-abdusattorov	5,356	0.544 → 0.549	0.782	2.967 → 2.995	1906
6	Wesley So	2770	chsm8-player-so	5,300	0.547 → 0.564	0.793	2.962 → 2.969	1910
7	Praggnanandhaa R	2763	chsm8-player-praggnanandhaa	7,467	0.543 → 0.550	0.776	3.016 → 3.008	1871
8	Vincent Keymer	2761	chsm8-player-keymer	10,906	0.538 → 0.567	0.789	3.073 → 2.980	1887
9	Wei Yi	2758	chsm8-player-wei-yi	2,869	0.552 → 0.555	0.780	3.007 → 3.015	1895
10	Alireza Firouzja	2757	chsm8-player-firouzja	19,999	0.555 → 0.559	0.785	2.962 → 2.960	1884

Bonus: historical players

Four classic players, added because they shaped chess history and are widely known. Their games are classical over-the-board games from before online chess, so there is less data and much older play.

Player	Peak FIDE	Why	Model	Unique games	Move-match base → fit	CE base → fit	Model Elo
Garry Kasparov	2851 (1999)	13th World Champion; world #1 for most of 1984–2005	chsm8-player-kasparov	1,727	0.533 → 0.539	3.153 → 3.103	1900
Bobby Fischer	2785 (1972)	11th World Champion; won the 1972 "Match of the Century" against Spassky	chsm8-player-fischer	660	0.572 → 0.567	2.969 → 2.891	1875
Anatoly Karpov	2780 (1994)	12th World Champion (1975–85); one of the most successful tournament players ever	chsm8-player-karpov	3,128	0.519 → 0.543	3.159 → 3.043	1905
Judit Polgar	2735 (2005)	strongest woman player in history; world #8 overall at her peak	chsm8-player-polgar-judit	1,459	0.530 → 0.539	3.181 → 3.099	1887

Small corpora (Fischer: 660 games) give noisier fits: Fischer's top-1 move-match drops slightly while his CE improves.

Usage

The checkpoint is a PyTorch dict (model_state_dict, cfg) for the CHSM8 decoder (factored move head over (kind, src, dst, piece, promo)). Moves are fed as factored rows, BOS first; there is no text input, and play picks the best legal move by summed factored log-probability. The model classes and the loading and play code will be released later with the project code.