Harper
by Harper · Open-source grammar and spell checker that runs on your device
BenchRank score
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 Harper markets itself
A structured read of the promise, proof and page design on Harper’s captured homepage.
Homepage capture
“Hi. I'm Harper.”
- Angle: Security / trust-led
- Hero: Interactive demo
Pricing
Published plans and prices from Harper’s own pricing page.
How this score is made up
Each dimension is scored out of 100 and combined into the headline score using fixed weights.
MCP support
Whether an agent can drive the product through the Model Context Protocol, and how much setup that takes.
API quality
Public API surface: machine-readable spec, official SDKs, documented auth, errors, rate limits and versioning.
Documentation
Publicly reachable docs — coverage, freshness, code samples and machine readability.
Agent friendliness
How readable the site is to an automated client: llms.txt, structured data, server-rendered content, crawler access.
Changelog
A public, dated record of what shipped and when — the clearest signal that a product is still alive.
Marketing site structure
Whether the site answers a buyer's questions: clear positioning, the pages that matter, and accessibility.
Page speed
How fast the site loads for real visitors: Chrome UX Report 75th-percentile LCP, INP and CLS, with a Lighthouse mobile run standing in where a site has too little traffic for field data.
Operational trust
Status page and incident history, security disclosure, compliance and data-processing documentation.
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
Source availability, self-hosting, data export and open standards. Scored and shown, but not part of the composite — paid SaaS is not worse for being paid SaaS.
Maintenance
Release cadence and repository activity. Scored and shown, but not part of the composite — it is only measurable for open repositories.
This doesn’t look right — report a problem with Harper’s score
Where this comes from
The Harper pages BenchRank reads when it scores the product — its documentation, release notes, status and security pages, and its repository where there is one.
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Is this your product?
Claim Harper to manage its profile. Claiming lets you suggest edits to the descriptive fields — it never changes scores or rankings.