Grammar Checker
Fix grammar and spelling errors in pasted text. Runs locally via transformers.js — your draft never leaves your browser.
Why a Grammar Checker That Runs Locally
Hosted grammar tools see every sentence you check. For confidential drafts, legal documents, internal email or anything else you would not publish, that is a real concern.
This checker runs Harper, an open-source grammar checker by Automattic, compiled to WebAssembly and released under the Apache 2.0 license. Harper is rule-based, not a generative AI model: it points out a problem and offers a fix, and nothing changes until you click it, so it cannot quietly alter your meaning, names or numbers. It supports American, British, Canadian, Australian and Indian English.
We switched to Harper in October 2026. On a fixed test of 12 sentences, 8 with errors and 4 already correct, the previous AI model (grammar-synthesis-small) fixed 2 of the 8 and rewrote all 4 correct sentences, in one case turning "4.5 to 1" into "0.5". Harper flagged 7 of the 8, offered the exact fix for 5, and left the 4 correct sentences alone.
How the Grammar Checker Works
Paste your text and click Check text. The first check downloads Harper (about 16 MB, then cached by your browser) and takes a couple of seconds; later checks are near instant. Flagged words are highlighted in your text and listed below it with an explanation. Click a suggestion to apply it, or Apply all first suggestions to accept every top suggestion in one go, then read the result. Choose your English dialect in the menu: in British English, color is flagged and colour suggested.
Harper catches subject-verb agreement (she don't, we was), common confusions (their/there, then/than), spelling, capitalisation, repeated words and many style problems. Like every rule-based checker it misses things that need deeper understanding: in our test it did not flag "Me and him went to the park".
Frequently Asked Questions
What does the checking?+
Harper 2.10, an open-source grammar checker from Automattic (the company behind WordPress.com), written in Rust, compiled to WebAssembly and released under the Apache 2.0 license. It is loaded from the jsDelivr CDN (about 16 MB) and runs entirely in your browser. It is rule-based rather than a generative AI model.
Is my draft sent to a server for grammar analysis?+
No. Only the checker's code is downloaded. Your text is checked in your browser and is not sent to us or anyone else.
What kinds of errors does it catch?+
Subject-verb agreement (she don't, we was), commonly confused words (their/there/they're, then/than), spelling, capitalisation, repeated words, spacing and punctuation slips, and a range of style issues. Each issue comes with an explanation and, where possible, one or more suggested fixes.
What kinds of errors does it miss?+
Errors that need real understanding of the sentence: in our test it missed "Me and him went to the park", and it flagged "their going" only for capitalisation rather than suggesting they're. It also cannot judge tone, clarity or whether a statement is true. Read your text after fixing.
Does it work for British English, or only American?+
Both, plus Canadian, Australian and Indian English. Pick the dialect in the menu above the text box; spelling suggestions follow it (colour and theatre in British English, color and theater in American).
Can I check very long documents?+
Yes. Harper checks the whole text at once and is fast enough for long documents; the highlighted view and the issue list grow with the number of problems found.
Will it change my text without asking?+
No. Harper only suggests. Your text changes when you click a suggestion or Apply all first suggestions, and you can see each change in the text box.
How does this stack up against Grammarly or LanguageTool?+
Grammarly and LanguageTool's hosted service catch more, especially style and complex grammar, and offer rewrites, but they process your text on their servers. Harper catches the common mechanical errors, runs entirely in your browser, and costs nothing.
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