Caipora 3.8

Caipora 3.8, an original UCI chess engine from Brazil, written from scratch in Rust with its own
NNUE. About 95 Elo stronger than 3.6 (the first public release) by our own measurements.

Strength

• Gauntlet at 8+0.08, 1 thread, 300 games per opponent, against Stash versions with known CCRL
  Blitz ratings: ~3603 ± 25 CCRL Blitz estimated (Stash 33: 86.7%, Stash 37: 74.5%, all
  600 games ended normally). This is our own estimate, not an official CCRL rating.
• Lichess: caiporaBot, blitz 2893 on 2 October 2026.

Downloads

File	For
caipora-3.8-windows-x86-64-v3.exe	Windows, CPUs with AVX2 (Intel Haswell 2013+, AMD Zen / Excavator+). Fastest.
caipora-3.8-windows-x86-64.exe	Windows, any x86-64 CPU (about 20% slower)
caipora-3.8-linux-x86-64-v3	Linux, AVX2, static binary
caipora-3.8-linux-x86-64	Linux, any x86-64 CPU, static binary

The network is embedded; no extra files are needed. Bench: 5263766 for every build
(caipora bench). Checksums in SHA256SUMS.txt.

What is new since 3.6

Each change passed an SPRT on the clock (8+0.08, bounds [0, 5]) against 3.6, and each package was
confirmed against the previous version (+48.6 for 3.7 over 3.6, +9.5 for 3.8 over 3.7):

• NNUE accumulator cache per king bucket and fused updates: about 12% faster, same evaluation (+22).
• Quiet history bucketed by threats, continuation history four plies back, fail-low bonus (+20).
• Expected cut-node tracking and a richer LMR, also for captures and checks (+11).
• Capture history aware of threats on the destination square (+6).
• Non-pawn and continuation correction histories (+6).
• Time management also scaled by the best move's node fraction (+5).

Architecture unchanged otherwise: NNUE g6 (768x8 king buckets -> 1024)x2 -> 8 output buckets,
SCReLU, trained on Leela Chess Zero data (ODbL); PVS with singular extensions, Lazy SMP, pondering;
standard chess and Chess960 / DFRC.

UCI options

Hash (16 MB), Threads (1, up to 256), Move Overhead (10 ms), Ponder, UCI_Chess960,
EvalFile (<embedded>), Clear Hash.

AI usage

Caipora is written by Claude Code (Anthropic) under the direction, review and testing of its
author; every commit is co-authored by Claude. No code, constants or networks from other engines
are used. Full policy: AI_USAGE.md.

License: GPL-3.0-or-later.

Caipora 3.6

Caipora 3.6 is the first public release of Caipora, an original UCI chess engine from Brazil,
written from scratch in Rust with its own NNUE.

Strength

• Gauntlet at 8+0.08, 1 thread, 300 games per opponent, against Stash versions with known CCRL
  Blitz ratings: ~3508 ± 23 CCRL Blitz estimated (Stash 33: 80.2%, Stash 37: 62.0%, all
  600 games ended normally). This is our own estimate, not an official CCRL rating.
• Lichess: caiporaBot, blitz 2854 on 1 October 2026.

Downloads

File	For
caipora-3.6-windows-x86-64-v3.exe	Windows, CPUs with AVX2 (Intel Haswell 2013+, AMD Zen / Excavator+). Fastest.
caipora-3.6-windows-x86-64.exe	Windows, any x86-64 CPU (about 20% slower)
caipora-3.6-linux-x86-64-v3	Linux, AVX2, static binary
caipora-3.6-linux-x86-64	Linux, any x86-64 CPU, static binary

The network is embedded; no extra files are needed. Bench: 5108549 for every build
(caipora bench). Checksums in SHA256SUMS.txt.

What is new since the first versions

• NNUE g6: (768×8 king buckets → 1024)×2 → 8 output buckets, SCReLU, trained with bullet on
  Leela Chess Zero data (ODbL).
• Search: PVS, aspiration windows, singular extensions with multi-cut, null move, LMR, LMP,
  futility, SEE pruning, quiescence with TT, butterfly, continuation and correction histories,
  Lazy SMP.
• Time management scaled by best-move stability; pondering.
• Standard chess and Chess960 / DFRC.

UCI options

Hash (16 MB), Threads (1, up to 256), Move Overhead (10 ms), Ponder, UCI_Chess960,
EvalFile (<embedded>), Clear Hash.

AI usage

Caipora is written by Claude Code (Anthropic) under the direction, review and testing of its
author; every commit is co-authored by Claude. No code, constants or networks from other engines
are used. Full policy: AI_USAGE.md.

License: GPL-3.0-or-later.