Fairy-Stockfish already has many variants built in but can be extended by an even much larger variety of custom user-defined variants using a variant configuration file.

User-defined variants can be added via an INI-style configuration file and be loaded at runtime without needing to recompile the program. See the variants.ini in the repository for documentation and examples of variant configuration.

Example

Let’s say we want to replace the bishops in chess with knight+bishop compound pieces, also referred to as archbishops. The configuration could look as follows:

# we name our variant "ourtestvariant" and let it inherit all the rules from "chess"
[ourtestvariant:chess]
# now we add the archbishop and assign it "a" as an abbreviation
archbishop = a
# since bishops are obsolete now, we remove them. This isn't strictly required, but recommended.
bishop = -
# we also need to update the promotion rules accordingly
promotionPieceTypes = nrqa
# and now let's define the new starting position
startFen = rnaqkanr/pppppppp/8/8/8/8/PPPPPPPP/RNAQKANR w KQkq - 0 1

And that’s already it. We can now e.g. go to the Fairy-Stockfish playground to test it directly in the browser. Just paste the above configuration to a file and load that file in the variants.ini section. After selecting ourtestvariant you should be able to play the variant against the Fairy-Stockfish or let it play against itself.