This checker runs in two stages, and the difference between them matters before you paste anything sensitive. The first stage is a set of rules that execute in your own browser: a dictionary of frequently mistyped words, and patterns for doubled spaces, accidentally repeated words, missing spaces after full stops and commas, and the their-there confusion that spell-checkers wave through.
The second stage is optional. Switching on advanced mode sends your text to LanguageTool, an external grammar service, and merges its findings into the same list. That gives you a far deeper check than local patterns can manage — but it means the passage leaves your machine and travels to a third party, which is a decision worth making deliberately rather than by default.
There are two limits to know about. The box holds ten thousand characters and truncates anything longer, telling you it has done so. Advanced mode has a tighter ceiling of eight thousand; above that the request is skipped and you are shown the local results with a note explaining why.
Only if you turn on advanced mode. The local rules run inside your browser and transmit nothing. Advanced mode sends the passage to LanguageTool, an external grammar service, so it can return findings the local patterns cannot produce. The choice is yours on every check, and it is off unless you enable it.
The input box holds ten thousand characters and truncates beyond that, with a message telling you it happened. Advanced mode has a lower ceiling of eight thousand characters; longer passages fall back to the local results rather than failing outright. Neither limit is a word count, so heavily punctuated text reaches them sooner than you expect.
It checks against United States English. There is no British English setting, so British spellings and punctuation conventions may be flagged as errors when they are simply the other standard. If you write in British English, expect a category of findings you should decline consistently.