Reckless Chess Engine

Reckless is an open source competitive chess engine, consistently performing among the top engines in major tournaments including Chess.com Computer Chess Championship (CCC) and Top Chess Engine Championship (TCEC).

Rating

Version	SPCC	CCRL Blitz	CCRL 40/15	Release Date
Reckless v0.9.0	3833 +/- 4 [#2]	3767 +/- 13 [#2]	3612 +/- 19 [#2]	Mar 1, 2026
Reckless v0.8.0	3777 +/- 4 [#4]	3765 +/- 14 [#2]	3609 +/- 11 [#3]	Aug 29, 2025
Reckless v0.7.0		3500 +/- 12 [#68]	3422 +/- 12 [#72]	Aug 23, 2024
Reckless v0.6.0		3386 +/- 15 [#95]	3317 +/- 15 [#103]	Mar 22, 2024
Reckless v0.5.0		3240 +/- 17 [#133]	3211 +/- 17 [#140]	Feb 4, 2024
Reckless v0.4.0		2934 +/- 17 [#210]	2926 +/- 16 [#222]	Dec 13, 2023
Reckless v0.3.0		2616 +/- 19 [#297]	2617 +/- 19 [#321]	Nov 6, 2023
Reckless v0.2.0		2349 +/- 18 [#406]		Oct 7, 2023
Reckless v0.1.0		2005 +/- 18 [#539]		May 16, 2023

Getting started

Precompiled binaries

You can download precompiled builds from the GitHub Releases page.

• -avx512: Fastest, requires a recent CPU with AVX-512 support
• -avx2: Fast, supported on most modern CPUs
• -generic: Compatible with virtually all CPUs, but significantly slower than AVX2 or AVX512 builds

Note

If you're unsure which binary to use, try the AVX-512 build first. If it doesn't run on your system, fall back to the AVX2 build, or the generic one as a last resort.

Building from source

To build Reckless from source, make sure you have:

• Rust 1.88.0 or a later version installed (official Rust installation guide)
• Clang installed (required for building the Fathom library used for Syzygy endgame tablebase support)

Once installed, you can build it with:

make
# ./reckless

To build without Syzygy tablebase support and Clang dependency:

make no-syzygy
# ./reckless

PGO builds

For profile-guided optimization (PGO) builds, you need to install additional tools:

rustup component add llvm-tools
cargo install cargo-pgo

Then, you can build the engine using make:

make pgo
# ./reckless

Or run the steps manually:

cargo pgo instrument
cargo pgo run -- bench
cargo pgo optimize
# ./target/x86_64-unknown-linux-gnu/release/reckless
# (the path may vary based on your system)

Usage

Reckless is not a standalone chess program but a chess engine designed for use with UCI-compatible GUIs, such as Cute Chess, En Croissant, or Nibbler.

UCI options

Reckless supports the following UCI options:

Name	Default	Description
Hash	16	Size of the transposition table in MB [1–262144]
Threads	1	Number of search threads [1–512]
MultiPV	1	Number of principal variations to display [1–218]
UCI_Chess960	false	Enable Chess960 (Fischer Random) support [false–true]
Minimal	false	Enable minimal UCI output [false–true]
MoveOverhead	100	Time in milliseconds reserved for overhead during each move [0–2000]
Clear Hash	—	Clear the transposition table
SyzygyPath	—	Path to Syzygy endgame tablebases

Custom commands

Along with the standard UCI commands, Reckless supports additional commands for testing and debugging:

Command	Description
perft <depth>	Run a perft test to count the number of leaf nodes at a given depth
bench	Run a benchmark on a set of positions to measure the engine's performance
d	Print the current board position in a human-readable format together with FEN
eval	Print the network evaluation of the current position from white's perspective
compiler	Print the compiler version, target and flags used to compile the engine
speedtest <Threads> <Hash> <Seconds>	Runs a performance test across 50 positions

Acknowledgements

• Code contributors who help develop and improve Reckless
• Hardware contributors providing compute resources
• OpenBench is the primary testing framework powered by Cute Chess
• Bullet is the NNUE trainer
• Stockfish, PlentyChess, Ethereal, Berserk, and many other open source chess engines
• Members of the Stockfish Discord server
• Chess Programming Wiki
• CCRL and all chess engine testers out there

License

This project is licensed under the GNU Affero General Public License v3.0.