Open-source Grammarly alternatives in 2026 — and why I built another one

Short summary

The author reviews open-source Grammarly alternatives (LanguageTool, Harper, Vale) and introduces GemType, a free browser extension that sends text directly to Google's Gemini API for LLM-powered grammar checking with no vendor server in between. The post dives into hard engineering problems — overlay rendering without corrupting editor DOM, snippet-based correction anchoring, and undo-safe fixes — that distinguish a production grammar tool from a simple API wrapper. GemType is open-source under Apache-2.0 and available for Chrome, Edge, and Firefox.

• •Reviews LanguageTool, Harper, and Vale as existing open-source grammar tools with honest trade-offs
• •Introduces GemType: free, private, LLM-powered grammar checker using Gemini API directly from the browser
• •Details five non-obvious engineering challenges including DOM-safe underlines, snippet anchoring, and undo-safe corrections

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