harper

Product Introduction

1. Definition: Harper is a privacy-first, open-source grammar checking language server and software suite. Technically, it is a local-first, Rust-powered natural language processing (NLP) engine that operates as a Language Server Protocol (LSP) server, a JavaScript library (harper.js), and a Rust crate (harper-core).
2. Core Value Proposition: Harper exists to provide a fast, offline, and completely private alternative to cloud-based grammar checkers like Grammarly. Its primary value is enabling error-free writing without compromising data sovereignty, ensuring user text never leaves the local device.

Main Features

1. Local-Only Processing: All grammar and spell checking computations are performed entirely on the user's device. This is achieved through a compiled Rust core engine (harper-core) that runs natively, eliminating network latency and data transmission. No telemetry, cloud API calls, or external LLM queries are involved.
2. Multi-Platform Integration Suite: Harper is architected as a core engine with numerous integration points. It is available as a system-wide desktop app for macOS, browser extensions for Chrome and Firefox, editor plugins for VS Code, Neovim, Zed, and Obsidian, and a WordPress block-editor plugin. This is facilitated by its LSP server (harper-ls) for editor-agnostic support and harper.js for web-based integrations.
3. High-Performance, Real-Time Analysis: Built in Rust for performance, Harper provides grammar and spelling suggestions with sub-10ms latency. This enables a real-time feedback loop where underlines and corrections appear almost instantly as the user types, without the lag associated with cloud-based services.

Problems Solved

1. Pain Point: The privacy risk and data ownership concerns inherent in sending all written text to third-party cloud servers for grammar checking. Harper solves the problem of unintentional data collection, model training on private writing, and potential surveillance by cloud-based writing assistants.
2. Target Audience: Privacy-conscious professionals including journalists, technical writers, developers, academics, and anyone handling sensitive documents. Specific personas include Obsidian power users, Neovim/VS Code developers writing documentation, and writers who require offline capability.
3. Use Cases: Essential for writing in secure environments (corporate, legal, journalistic), for users with limited or no internet connectivity, for developers wanting grammar checks in code comments and markdown files within their IDE, and for individuals who explicitly reject the data-harvesting business model of mainstream writing tools.

Unique Advantages

1. Differentiation: Unlike Grammarly or Microsoft Editor, Harper has zero cloud dependency and is fully open-source (Apache-2.0 license). Compared to basic built-in spellcheckers, it offers advanced, context-aware grammar checking. Its architecture as a local LSP server provides deeper, faster editor integration than most browser-based tools.
2. Key Innovation: The combination of a high-performance, locally-executed Rust core with the Language Server Protocol. This technical approach allows a single, sophisticated grammar engine to be seamlessly embedded into dozens of different writing environments—from browsers to code editors to desktop apps—while maintaining strict privacy and speed guarantees.

Frequently Asked Questions (FAQ)

1. Is Harper a completely free grammar checker? Yes, Harper is free and open-source software (FOSS) licensed under the Apache-2.0 license. There are no paid tiers, subscriptions, or feature limitations, and the source code is publicly available on GitHub.
2. How does Harper work without an internet connection? Harper's entire grammar checking engine is compiled and runs locally on your computer's CPU. It does not require an internet connection because all processing, from error detection to suggestion generation, happens on-device using its embedded linguistic rules and dictionaries.
3. Can I use Harper in Google Docs or Gmail? Harper's browser extension can work on most websites, but for Gmail specifically, you must first disable Gmail's built-in grammar checker for Harper to function correctly. Performance in complex web apps like Google Docs may vary compared to native editor integrations.
4. What languages does the Harper grammar checker support? Currently, Harper primarily supports English and its major dialects (American, British, Canadian, Australian, Indian). Support for other human languages is planned for future development.
5. Is Harper better than Grammarly for technical writing? Harper excels at catching mistakes common in technical documentation, such as improper capitalization, misspelled technical terms, and awkward phrasing. Its privacy-first, local operation makes it preferable for proprietary or sensitive technical content, though it may lack some stylistic suggestion features of cloud-based AI tools.