Spelling and grammar.

Word processors have checked grammar since the mid-nineties, and most of the editors built for manuscripts still do not, which means a novelist who leaves Word for a purpose-built tool usually loses the one check that catches "could of" before an agent does. Draftline checks spelling, grammar, and a separate layer of usage and style as you type, with deterministic rules that run on your machine, and shows each finding as an underline you can fix with one click or ignore.

Captain Mara Venn watched the colony rotate beneath them while the Echo maneuvered along the edge of the security perimiter. Everything appeared calm. Almost suspiciously calm.

“They're requesting another shuttle corridor,” Lieutenant Hale reported.

Mara looked away from the planet. “Again?”

“Again. The Harmony's delegation chief says she could of submitted the request yesterday, but apparently nobody told her about the traffic restrictions.”

The tactical display chimed before Hale could continue. A contact had appeared near the system's outer marker: an enormous vessel with no transponder, no flight plan, and no apparent interest in answering FleetCom's hails.

Irregardless, the diplomatic staff continued their luncheon as if nothing unusual had happened. Somewhere aboard the Harmony, a string quartet began playing louder.

Mara leaned toward the display. “Well,” she said, watching the unknown ship turn directly toward them, “that's probably fine!!

spelling grammar usage & style
Accept the replacement from the menu, or ignore it and keep writing.
Add an invented word to your dictionary once and it is never flagged again.

The checker.

Deterministic rules
Grammar findings come from written rules, so the same sentence is flagged the same way every time and you can learn what the checker will and will not object to. It runs on your machine and nothing is sent anywhere: there is no LLM involved and no model making judgments about your book, only local processing over your own sentences, with every finding pointing at the words that produced it.
Usage and style
Beyond grammar there is a second, optional layer for the things a copy editor marks that are not errors: three periods where a typeset ellipsis belongs, the doubled exclamation mark that survived four drafts, "irregardless" offered "regardless," and regional slang offered a broader-audience alternative in case you want one. Style guidance is shown with its own underline, separately from grammar, and you can switch it off on its own.
Two dictionaries
Words you add by hand go into your own custom dictionary, which persists across every book you open. Separately, each project has a lexicon of the character names and aliases you have confirmed in the Characters Codex, so an invented surname is recognized in the book it belongs to and still flagged if it turns up in a different one. Suggestions for a misspelling are searched in a background thread, so a long chapter never stalls while you type.