This is a formal proof validator, which assesses the correctness of Fitch-style natural deduction proofs ("Fitch proofs").
The tool also returns a (hopefully useful) error message in case the proof is not correct.
This application takes Fitch proofs as they are defined in Language, Proof and Logic, by Dave Barker-Plummer, Jon Barwise and John Etchemendy.
It is accessible here: el-sambal.github.io/fitch-proof.
If you want to build and run the application locally, then clone the repository, install Cargo if you haven't already and install wasm-pack
(to compile Rust to WebAssembly) and do:
wasm-pack build --target web
Once you have it compiled, open a server in the root directory of the repository:
python3 -m http.server 8080
And then open http://localhost:8080/ in your favorite web browser.