Harper: The 100% Free Open-Source Grammarly Alternative (Offline & Privacy-First) ⚡

For nearly a decade, Grammarly held a near-monopoly over digital proofreading. But as software workflows evolved toward local-first architectures, heavy cloud extensions began showing severe structural weaknesses:

• 1. The Keystroke Surveillance Problem: To check your prose, Grammarly's browser extension captures your active input field and transmits the raw text over HTTPS to its cloud servers. For corporate executives, medical professionals, legal teams, and developers handling proprietary IP, this represents an unvetted data exfiltration vector.
• 2. Total Paralysis Without Internet: Traveling on an airplane, working in low-connectivity environments, or experiencing network outages renders cloud proofreading completely non-functional. Harper bundles its entire linguistic engine on-device, meaning you get instantaneous corrections anywhere in the world.
• 3. The Subscription Paywall Treadmill: Grammarly locks advanced clarity, style tone adjustments, and vocabulary suggestions behind a monthly paywall reaching up to $30/month ($144 to $360 annually). Harper is 100% free under the Apache 2.0 open-source license.
• 4. Complete Ignorance of Code and Markdown: If you write technical documentation, blog posts in Markdown, or code comments, Grammarly treats variable names like const userAuthToken, shell commands like cargo build --release, and regex patterns as catastrophic spelling errors, cluttering your screen with false positives.

Here is how Harper stacks up against Grammarly Premium across operational, performance, and security dimensions:

Harper isn't just a simple dictionary lookup; it is a full linguistic engine that evaluates syntactic relationships, common orthographic errors, tautologies, and grammatical dependencies. Automattic designed Harper around three core engineering principles:

Depending on whether you write in browser web forms, markdown note apps, developer IDEs, or automated pipelines, here is how to deploy Harper in minutes:

# Install via Rust Cargo
cargo install harper-ls

# Or install via Homebrew on macOS/Linux
brew install harper

# Install the command line tool
cargo install harper-cli

# Run a grammar check on all documentation files
harper-cli lint ./docs/*.md

# Check your README before publishing a release
harper-cli lint ./README.md

Every brand, startup, and technical domain has unique jargon—terms like Kubernetes, LLM, ARR, CAC, WebAssembly, Substack, DALL-E, Supabase. Harper lets you define local custom dictionaries so these words are never flagged as spelling mistakes.

In your project directory or home directory, Harper reads configuration settings allowing you to select your preferred English dialect and ignore specific technical rules: