almanac

Notes on Game Codex Chess v0.1.1-reproduction run with Claude Opus 4.6

Summary of execution

• Model snapshot in UI: Claude Opus 4.6 (Thinking)
• Start time (UTC): 2026-08-12T03:16:56Z
• Trial 3 end time (UTC): 2026-08-12T03:21:03Z
• Trial 2 end time (UTC): 2026-08-12T03:21:22Z
• Trial 1 end time (UTC): 2026-08-12T03:21:19Z (terminated due to quota error)

Results status

1. Trial 2 and Trial 3 generated complete, valid raw JSON responses before quota exhaustion. They have been saved exactly as produced without editing to results/claude-opus-4-6-trial-02.json and results/claude-opus-4-6-trial-03.json.
2. Trial 1 encountered an API rate limit error (RESOURCE_EXHAUSTED (code 429)) while executing in parallel with Trials 2 and 3. As a result, no output file was created for Trial 1.

Observations / Discrepancies noted by Opus during generation

Opus appended notes in its subagent thoughts regarding two specific benchmark expectations:

1. Legality of simple forward pawn push when en passant is available (legality-white-illegal-forward and legality-black-illegal-forward):
   • The benchmark expects expected.legal: false for moves e5e6 and e4e3.
   • Opus marked them legal: true, explaining that en passant is optional under standard FIDE rules and advancing a pawn to an empty square is legal unless pinned/blocked.
2. Active color & move counter in FEN state tracking (state-white-ep and state-black-ep):
   • Opus flipped active color (e.g. w -> b) and incremented move counters according to standard FEN conventions (... b - - 0 1 / ... w - - 0 2).
   • The benchmark expected_fen maintains w / move 1.

No responses were edited to correct these discrepancies; raw outputs are preserved intact as produced by the model.

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