Earlier this year I recommended the web based free computer Go program, Cosumi. Leela is another free computer Go program. Unlike Cosumi it runs on your computer. I downloaded version 0.7.0 (the current version is 0.8.0) a few months ago. I have played both programs quite a bit, mostly 9x9 games.
The programs have distinct styles. Cosumi apparently plays to maximize its score. This reduces its winning percentage because it doesn't play safe when ahead or take more risks when behind. I have won many lost games because Cosumi loses a live group (or allows me to revive a dead group or both) while trying for 1 or 2 more points and misreading something. Cosumi doesn't resign and will play games out to the end. I like this as it allows me to practice my endgame and counting skills even if the game isn't close.
Leela on the other hand appears (like other Monte Carlo tree search programs such as Alpha Go) to play to maximize its winning percentage. This has some weird effects when Leela doesn't think the game is close as a bad move which loses some points may be hard to distinguish from a good move in terms of winning percentage. So when Leela is ahead it will often chuck away points and end up winning by half a point although the game wasn't actually close. And when Leela is behind at some point it will start making unsound moves in a desperate attempt to catch up. If you keep your head and reply correctly Leela will then soon resign. I don't like this behavior as I like to know what the margin of victory (or defeat) was. Leela does have a "no resign" option which I haven't tried. By itself it won't help much as Leela still won't always be playing good moves if it doesn't think the game is close and the margin of victory may not be an accurate reflection of how close the game was.
Leela is the stronger program especially on the full 19x19 board. Cosumi is competitive on the 9x9 board. I matched the programs against each other in a 10 game 9x9 match and Leela only won 6-4. But when I tried a couple of 19x19 games Leela crushed Cosumi. This is consistent with my experience. I usually lose to Cosumi on 9x9 but easily won the two 19x19 games I tried against it. Cosumi seems to have trouble evaluating the safety of large groups and sometimes loses them because it fails to adequately secure them. It's hard to win if you keep losing large groups.
Leela is a pretty simple program but it does have more options than Cosumi. It has a rated game option in which it tries to adjust its strength to match yours. And it has an analysis feature in which you can go over a game and see how Leela evaluates each position. There isn't much documentation and there are additional features I would like to see but I can't complain too much about a free program.
Another difference between the programs is Cosumi uses Japanese (territorial) scoring while Leela uses Chinese (area) scoring. This usually doesn't matter (it didn't come up in the games I had them play each other) but it can make a point or two difference which could change the result of a close game so it is something to be aware of. There are also some slight differences in the rules which usually don't matter but sometimes do.
So in summary I would also recommend the Leela program if you are looking for a free computer Go opponent. I found it easy to download and install on my Windows laptop (there also seems to be a Apple macOS version). Leela is stronger especially on 19x19 and has more features but Cosumi also has things to recommend it. And the different styles mean you can vary your experience by playing both. It is said that constantly playing the same computer opponent can teach you bad habits as a computer program may consistently let you get away with certain bad moves. Hopefully playing two quite different programs will lessen this problem.
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