Codex deep diagnosis surfaced two real issues in our MCTS pipeline:
1. mcts.pyx::search() hardcoded prepare_simulation_batch(state, traverser, 1)
instead of respecting MctsConfig.parallel_simulations. Standalone evals
(eval_checkpoint, evaluate_with_mcts sequential path, eval_worker) all
use this entry point, so all eval-time MCTS was running 1 sim per batch
regardless of the configured 64. Training was unaffected because it
goes through interleaved_self_play._run_search_jobs which respects the
config. Fix uses min(config.parallel_simulations, sims - completed).
2. use_rollout_value defaulted to True (config.py) but was never set in
the YAML. With this, _expand_with_prior returns the heuristic rollout
value and discards network_value, so the network value head is trained
from final game scores but its outputs are never fed back into MCTS
backups. This explains why mcts/value_prediction_error stays high
despite training -- learning the value head produces no behavioral
change because MCTS never reads it.
Now setting use_rollout_value=false in default.yaml so the network value
head closes the loop. Combined with the existing Dirichlet root noise +
heuristic rollout removal, this should give the network's value learning
actual leverage on action selection.
Also: updated test_search_visit_counts_match_with_parallel_simulations
to test the correct invariant (legal-action set match + total visit
count near n_sims) rather than literal visit-count equality, which was
only true under the previous bug.
Tests: 19/19 passing.
50-iter sweep on default.yaml with stronger MCTS exploration:
- c_puct: 3.0 -> 5.0 (UCB weight, more exploration of low-prior actions)
- root_dirichlet_epsilon: 0.25 -> 0.4 (more noise injected at root prior)
Standalone eval at iter 50 (30 games/opponent, all natural-end, timeouts=0):
- vs heuristic-balanced: W=0/30 S=-70.5 (PA 0.15)
- vs heuristic-aggressive: W=2/30 S=-65.1 (PA 0.14) [+10, +4]
- vs heuristic-cautious: W=1/30 S=-48.0 (PA 0.14) [+2]
3 natural-end wins vs prev trapfix baseline iter 44 (which had 0 natural
wins + 1 timeout-tie). Stall trap fixed remains true (timeouts=0 in c1).
Trade-off observed: more exploration -> higher variance. Score avg vs
cautious worsened (-32 -> -48), but win events appeared. For the
non-terminal-win objective, exploration win > score-avg loss.
Next: commit to long run (300 iter) with these params before tuning more.
Trap diagnosis: agent learned to stall (avoid opening expeditions, draw
from discard pile to extend deck) until max_steps timeout, then squeak by
on opponents' negative scores. All eval wins were from timeouts; agent
never won a naturally-terminating game. Self-play reinforced this because
timeout games still got a positive value target.
Fixes (no algorithm change, all MCTS hyperparameters or signal shaping):
- Dirichlet noise at root prior (AlphaZero standard, was missing):
mcts.pyx `_expand_with_prior` takes `is_root` flag; root expansion
mixes prior with Dirichlet(α). Callers in interleaved_self_play and
the internal evaluate_and_backup pass `not item.path`.
- Default config strengthens exploration on the 50-sim batched search:
c_puct 1.5 -> 3.0, virtual_loss_value 1.0 -> 5.0, plus new
root_dirichlet_alpha=0.3 / root_dirichlet_epsilon=0.25.
- Self-play timeout signal zeroed: `_finalize_context` sets v_target=0
if context.state is not terminal. Stops the network from learning
"stall = positive value".
New standalone evaluator:
- `lost-cities-ismcts eval` subcommand (eval_checkpoint.py): loads a
checkpoint, runs N games per opponent across a parallel pool, reports
win/score with 95% CIs plus per-game logging via --verbose. Defaults
cover heuristic-balanced/aggressive/cautious (rollout policy isn't in
the training-eval opponent list, so this is the natural way to compare
the trained policy against its rollout target).
Tests (19) still pass; .so rebuilt.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Key changes for ISMCTS speed and correctness:
- Cython port: HeuristicBot helpers (`heuristic_cy.pyx` + new `.pxd`) and
ISMCTS searcher (`mcts.pyx`) now run as cdef. Both share a fast
unified-action path through GameState's C interface to avoid Python
round-trips on hot rollout/tree-walk paths.
- Multi-process self-play and eval: `workers.py`, `eval_worker.py`,
`interleaved_self_play.py`, plus trainer wiring with ProcessPoolExecutor
+ spawn context. Eval inside `evaluate.py` is parallel per opponent.
- ISMCTS-specific eval (`evaluate.py`) runs MCTS at decision time so the
metric matches deploy mode; `evaluation.eval_with_mcts` flag preserves
backwards-compatible policy-only eval when needed.
- Trainer logs progress per phase (self-play start/done, eval per
opponent), and value loss is now scaled by `value_scale` so policy and
value losses sit on comparable magnitudes.
- Compact info-set key (`info_set.py`) using packed-struct format and
child-key reuse during MCTS descent to cut per-step canonicalization.
Tests: 19 ISMCTS suite passing, including parity (Cython-vs-Python
sequential, batched-vs-sequential visit counts, push/pop round-trip).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Implements a proof-of-concept single-observer IS-MCTS trainer with AlphaZero-style policy/value network, determinization, replay, self-play, CLI configs, and focused tests. Mini acceptance run reaches positive random eval while keeping play_action_rate above the Deep CFR trap threshold.
Tests: uv run python -m pytest tests/games/classic/ismcts/ -x; uv run python -m pytest tests/games/classic/test_deep_cfr_trainer.py -x; uv run lost-cities-ismcts train --config configs/ismcts/mini.yaml
coolguy
