Chess engines have come a long way since the earliest chess-playing computer programs. Once only capable of beating hobbyists, the best chess engines today play at a superhuman level that challenges even world champions.
If you want to test your skills against the toughest chess AI around, playing against a modern chess engine is just the ticket. And conveniently, many of these engines are free to use.
In this guide, we‘ll cover what exactly chess engines are, how they work, and the benefits of playing against them. Then we‘ll highlight ten of the most challenging chess engines to play against—ones that will put your tactical and positional understanding to the test.
What Are Chess Engines and How Do They Work?
A chess engine is software that analyzes chess positions and calculates the best moves by looking many moves ahead. Today‘s top engines evaluate up to 60 million positions per second.
Engines leverage brute force pattern recognition and raw computing power to play at a superhuman level. They excel at tactics and long-term positional planning in ways even the best humans cannot match.
Most chess engines today use a combination of approaches:
- Alpha-beta pruning: Evaluates positions in a game tree by sorting and eliminating poor moves. Greatly reduces the number of variations needed to examine.
- Heuristic evaluation: Applies point values and positional rules to evaluate board positions.
- Opening books: Libraries of strong opening moves contributed by grandmasters and past games.
- Endgame tables: Precomputed endgame solutions for theoretical wins/draws.
- Neural networks: Machine learning to improve position evaluation over time based on results.
By blending these techniques, modern engines overcome bot weaknesses and play at a skill level unimaginable only 30 years ago.
Benefits of Playing Against Chess Engines
Pitting your skills against the toughest chess engines accelerates learning. Playing engines helps you:
- Sharpen tactics: Engines rarely fall for simple traps and force you to find non-obvious solutions.
- Improve positional understanding: Their appraisal of even subtle advantages teaches you how to squeeze wins from better positions.
- Enhance calculation: Matching an engine move-for-move pushes you to see further ahead.
- Perfect openings: Engines punish weak openings while demonstrating effective ones for you to adopt.
- Refine endgame technique: Tablebase-augmented engines exemplify flawless endgame play to a draw or win.
- Identify weaknesses: Engines exploit holes in your game, showing you exactly where you need to improve.
Now let‘s look at some of the specific chess engines that pose the greatest challenge to players.
1. Stockfish
Stockfish is arguably the world‘s strongest open-source chess engine. As an open-source project supported by a community of volunteers, Stockfish does not use neural networks. Instead it relies on traditional alpha-beta search with highly optimized modern heuristics.
Still, Stockfish dominates computer chess competitions. It has won every Top Chess Engine Championship (TCEC) since season 6. The current release Stockfish 15 searches over 60 moves ahead at depths exceeding 70 plies. Matches between it and former champions like AlphaZero estimate its Elo rating to exceed 3500.
With frequent updates and running efficiently even on modest hardware, Stockfish is an ideal engine for player study and training. It excels at punishing even slight inaccuracies which in turn teaches how to find moves that withstand the deepest possible scrutiny.
2. Leela Chess Zero
Leela Chess Zero (LCZero) is an open-source, neural network-based engine inspired by DeepMind‘s AlphaGo Zero. It uses deep reinforcement learning entirely from games against itself, with no human data, openings, or endgame tables.
This self-play, tabula rasa approach makes Leela an independent, ever-evolving opponent. The latest networks have reached over 3000 Elo at longer time controls. Leela has defeated Stockfish in drawn matches and won games in TCEC Cups with odds.
Leela poses a unique challenge by evaluating positions differently than traditional engines. Its alien play can shock players by undervaluing material or seeing moves human GMs would dismiss. Facing off against this neural newcomer teaches you to re-examine positional preconceptions.
3. Komodo
Created by Don Dailey and Mark Lefler, Komodo frequently wins computer chess tournaments despite its age. The reigning five-time Computer Chess Champion, Komodo combines traditional Stockfish-derived search with its own deep neural network evaluation.
Komodo‘s veteran status means it boasts one of the largest opening books of any engine. And its nuanced positional understanding lets it often outplay opponents by small margins. No weaknesses and a style resembling AlphaZero make even Komodo‘s "slow" setting a challenge for talented humans.
Using Komodo teaches you the danger of tiny inaccuracies. Against Komodo, one slip often snowballs as it leverages small advantages into victory.
4. Shredder
Shredder is a German engine dating to 1992 that has claimed numerous Computer Chess World Championship titles. Despite its age relative to other modern engines, Shredder remains competitive by integrating deep neural network evaluation.
Shredder offers a free 30-day trial perfect for testing your mettle against time-tested engine logic. Even in its base form, Shredder plays at over 3000 Elo, enough to defeat all but a handful of GMs. Activating NNUE evaluation pushes it close to 3200.
Shredder excels at positional constriction. It will patiently improve its pieces, restrict your counterplay, then suddenly strike once your position buckles. Shredder teaches resilience by forcing you to search for defensive resources in cramped positions.
5. Fritz
Part of the ChessBase suite, Fritz often introduces new users to chess engines thanks to its familiar Windows interface. But don‘t let the friendly appearance fool you; Fritz plays at a GM level even without added neural networks.
Fritz combines automatic analysis with guides from resident chess experts to highlight difficult ideas. Cloud access lets Fritz draw on ever-growing databases of games for sharp opening analysis informed by millions of examples.
Playing against Fritz and having your own games annotated by it provides a basecamp experience that eases players into using engines effectively. Before scaling the K2 challenge of top open-source opponents, Fritz offers a more guided Mount Everest attempt.
6. Houdini
Created by Belgian programmer Robert Houdart, Houdini dominated computer chess in the early 2010s. It claimed consecutive TCEC championships and scored wins over rivals like Rybka. While no longer the fastest engine, Houdini remains formidable especially at longer time controls.
Houdini offers multiple configurations to scale difficulty, from a moderate "Houdini 6" up to the unrelenting attacking play of post-TCEC "Houdyni 6 Pro with ABN". It also connects to the AI web service for regular tuning updates against the latest engines.
Facing the barrage of continuual attacks, sacrifices, and initiative from Houdini will stress-test both your defensive resourcefulness and counterpunching capability. Training against Houdini‘s onslaught hardens you against aggressive opponents of all stripes.
7. Critter
Critter is an open-source engine dating to 2004. Written by chess master Richard Vida, Critter pioneered many ideas in search, evaluation, and pruning adopted by later engines like Stockfish and Komodo. While not as strong or widely used today, Critter remains a crafty opponent.
Critter couples unusual positional play with furious tactical complications. Early novel moves may not come good for many exchanges, at which point combinations explode. Holding together such messy battles teaches you to meet aggression with patience and always keep threads from unraveling completely.
Against Critter you learn to soak up early pressure, navigate messy middlegames, and avoid panic as the tension ratchets move after move. Mastering the controlled chaos of a Critter game hones skills critical in human competition.
8. Rodent III
Rodent III is the final evolution of Swiss programmer Paweł Kozioł‘s chess engine series started in 2004. Rodent matches a diminutive name and authorship with big gritty punch above its weight class.
For such a tiny free engine—fitting in just a few KB without bulky opening books—Rodent plays at a respectable ~2900 CCRL Elo. It accomplishes this via a heavily optimized alpha-beta search updated in 2020 to leverage many core CPUs and NNUE. Rodent III running multicore easily defeats its distant predecessors.
Facing this fierce underdog teaches you never to underestimate an opponent based on expectations or stereotypes. Rodent III will nip with counterpunches if you lounge a single move, reminding you concentration and respect pay no matter how low the rated odds.
9. Scorpio NNue
An inaugural entrant in the still-ongoing Season 20 TCEC Premier Division, Scorpio NNue is an engine optimized specifically for neural network integration. Developer Jonathon Rosenthal himself a FIDE master drafted Scorpio as an experimental testbed for applying self-play machine learning techniques to traditional search architecture.
The result is one of the first open-source engines built ground-up for deep learning rather than adapting traditional designs. Scorpio NNue accessing the latest MLBrain test net poses stiff competition even for leading commercial engines, outpacing extreme optimization like Rodent III.
Scorpio represents both the present and future direction of chess AI. Testing yourself against Scorpio is testing yourself against the sharp cutting edge where humans and increasingly advanced algorithms meet.
Further Steps
The best chess engines today play at a superhuman level barely distinguishable from one another. But each engine offers unique personality reflecting its heritage and distinguishing search or evaluative features.
Facing off against a diversity of opponents prepares you better for human competition by introducing you to a wide gamut of plausible styles. Further steps to consider include:
- Varying time controls to simulate turn-based or rapid/blitz games
- Playing handicap matches where engines drop pieces or temps
- Using multiple engines via in-browser tools like Chess24
- Consulting Lc0 training run game databases
- Studying annotator commentary and opening books
- Capping search depth/nodes to increase chances
Whichever engines you select, playing against the best chess AIs ensures rapid improvement. Their silicon mastery encapsulates centuries of chess wisdom mixed with inexorable calculative perfection against which we flawed yet creative humans still struggle to prevail.
