A 131-line Python engine that beats most humans at chess

What it does

Sunfish is a complete chess engine written in Python that implements the UCI protocol and plays at over 2000 Elo on Lichess. It handles all standard chess rules except the 50-move draw rule, and ships with both a terminal interface and a compressed executable under 3KB. The codebase is deliberately tiny—131 lines once comments and whitespace are stripped—making it a self-contained reference implementation rather than a framework.

The interesting bit

The project treats extreme brevity as a feature, not a constraint: the entire packed engine is smaller than many JSON configuration files, yet it still runs an MTD-bi search with piece-square-table evaluation and an experimental 1207-byte NNUE variant. That small surface area makes it a popular sandbox for testing parallel search algorithms, swapping evaluation functions, or grafting on deep learning experiments without wading through tens of thousands of lines of C++.

Key highlights

• Plays above 2000 rating on Lichess despite fitting in 131 lines of Python.
• Packs into a 2953-byte compressed shell script that speaks standard UCI.
• Uses the MTD-bi (C*) search algorithm and classic engine tricks, but relies on standard Python collections for clarity.
• Experimental NNUE version adds positional understanding with a neural network under 1.3KB.
• Runs significantly stronger under PyPy—about 250 ELO above CPython at fast time controls, according to the author’s testing.

Caveats

• Does not implement the 50-move draw rule.
• The evaluation function is deliberately primitive: piece-square tables only, with no midgame/endgame distinction.
• NNUE mode trades positional awareness for tactical weakness because the implementation is not yet fast enough.

Verdict

Grab this if you want a readable, hackable chess engine for experimentation or teaching; avoid it if you need a production-grade rules engine or a tactical monster to analyze your blitz games.