Harper
by Harper · Open-source grammar and spell checker that runs on your device
36.7 — BenchRank score out of 100
Screenshots of Harper
Homepage
Overview
Harper is an open-source grammar and spell checker that runs locally on your device rather than sending text to a server. It flags issues such as improper capitalisation, misspelled words, repetition and awkward phrasing, and does not use an LLM. It ships as a language server, JavaScript library, Rust crate, browser and editor extensions, and native apps.
- Best for
- Developers and technical writers who want grammar checking that stays on their own machine.
- Pricing
- No prices are shown; the site describes Harper as free and open source under Apache-2.0.
- Runs on
- macOSBrowser extension
Strengths and trade-offs
Strengths
- All checks run locally; no cloud round-trips or telemetry
- Free and open source under Apache-2.0
- Site states suggestions in under 10ms for most documents
- Integrations for VS Code, Neovim, Zed, Obsidian, Chrome, Firefox
Trade-offs
- English only: US, UK, Canadian, Australian and Indian variants
- Desktop app is listed for macOS only
- A separate extension per editor or browser, not one install
- The FAQ flags Harper not working in Gmail
How this score is made up
Each dimension is scored out of 100 and combined into the headline score using fixed weights.
- MCP support
- 0 out of 100
- API quality
- 10 out of 100
- Documentation
- 45 out of 100
- Agent friendliness
- 67 out of 100
- Changelog
- 100 out of 100
- Marketing site structure
- 70 out of 100
- Operational trust
- 0 out of 100
Measured, but not part of the score
Useful to know, but not a mark for or against the product — so these do not affect the ranking.
- Openness
- 60 out of 100
- Maintenance
- 100 out of 100
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