A few months ago I learned of the existence of Harper, which describes itself as a lightweight, offline, grammar checker. Both the website and GitHub repository are a bit sparse on how to use it, but they do link to all sorts of integrations, which I think all use the Harper language server. There is also harper.js, mostly for web applications and browser extensions.

While the language server (or harper.js) is great for extensive editor use, I sometimes just want to quickly check a file for spelling and grammatical issues. As it turns out, this is already possible by using the harper-cli tool.

This CLI is not documented on the documentation website, probably, as I found out later, because it is still listed as an experimental frontend. I'm glad it exists though; it saved me from even considering how to use the LSP from the command line directly, and from installing and using one of the integrations.

Installing it

Installing it is easy! If you have Rust and Cargo installed, you can install it from source by running cargo install --locked --git https://github.com/Automattic/harper.git harper-cli.

On Windows I also found out you can install it using scoop. I was delighted to see that scoop install harper didn't just install the language server binary, but also harper-cli.

Using it

The current version (0.1.0), shows the following help page when running harper-cli --help:

A debugging tool for the Harper grammar checker

Usage: harper-cli.exe <COMMAND>

Commands:
  lint                   Lint a provided document
  parse                  Parse a provided document and print the detected symbols
  spans                  Parse a provided document and show the spans of the detected tokens
  annotate-tokens        Parse a provided document and annotate its tokens
  metadata               Get the metadata associated with a particular word
  forms                  Get all the forms of a word using the affixes
  words                  Emit a decompressed, line-separated list of the words in Harper's dictionary
  summarize-lint-record  Summarize a lint record
  config                 Print the default config with descriptions
  mine-words             Print a list of all the words in a document, sorted by frequency
  core-version           Print harper-core version
  rename-flag            Rename a flag in the dictionary and affixes
  compounds              Emit a decompressed, line-separated list of the compounds in Harper's dictionary. As long as there's either an open or hyphenated spelling
  case-variants          Emit a decompressed, line-separated list of the words in Harper's dictionary which occur in more than one lettercase variant
  nominal-phrases        Provided a sentence or phrase, emit a list of each noun phrase contained within
  help                   Print this message or the help of the given subcommand(s)

Options:
  -h, --help     Print help
  -V, --version  Print version

For my use case, the most simple command sufficed:

$ harper-cli lint .\content\posts\2025-05-15_thank_you_all_for_ten_years_of_stable_rust.md

That gave me the following result:

What a delightful way to check for flagrant spelling errors in markdown files. Thanks Harper authors!