A Cursor for prose
Margin is one document you share with Muse, an AI collaborator. It follows every change you make. It proposes edits you accept or reject. And it leaves comments in the margin, the way a real editor would.
The lighthouse had been dark for three long years, and no one in the village could say why the keeper had left.
Mara climbed the cliff path anyway, her lantern throwing shapes across the wet rock.
How it works
Four ideas make Margin feel less like prompting a bot and more like handing pages to someone who was already reading over your shoulder.
Muse reads the draft once. After that it tracks what you changed as a diff on every turn, so “what do you think of my last pass?” just works. It never re-reads a chapter it already knows. It knows what you did, not just where the text ended up.
@@ chapter-one.md @@ - The lighthouse had been dark for years. + The lighthouse had been dark for three + long years, and no one could say why. # Muse sees the delta, not the file.
Muse can’t touch your words. It offers a change as a red/green card, with the target passage highlighted blue in your text. Accept and it applies cleanly. Reject and it vanishes. And if you edit that passage yourself first, the stale suggestion steps aside instead of overwriting your new line.
…dark for three long years, and no one could say why.
Select a passage, drop a comment, and it pins to that exact span in yellow. Muse is told about it and replies in the thread. Keep typing inside the highlight and the note rides along with the sentence instead of drifting off it. You resolve threads. Muse never quietly closes one on you.
Mara climbed the cliff path anyway, her lantern throwing shapes across the wet rock.
Too sudden a jump to Mara?
One more beat of dread on the cliff first, then she earns the entrance.
You and Muse commit as different authors to a real git history behind the scenes. So months later you can see, line by line, which words were yours and which came from the collaboration, and roll back either one. No lock-in, no black box. It’s your text, versioned.
Who it's for
Margin earns its keep on work you live inside for weeks, where continuity and voice and revision matter more than a one-shot answer.
Muse holds your story bible: characters, timeline, the rules of your world. It flags when a draft drifts from them, suggests lines in your voice rather than some house style, and remembers what you established three chapters ago.
novels · short stories · scriptsPoint Muse at your notes and sources as background reading. It keeps your argument honest across a long manuscript, catches the claim you contradicted forty pages back, and flags where a section needs a citation. It won't rewrite your thinking.
books · essays · reportsDraft fast, then hand it to Muse for the tightening pass: a sharper hook, a killed cliché, a paragraph that finally lands. You see every proposed cut before it happens, so the published voice stays unmistakably yours.
articles · newsletters · docsBut I already paste into a chatbot
A chat window forgets your document the moment you close the tab, rewrites whole passages when you wanted a nudge, and hands back a wall of text you have to re-merge by hand. Margin lives on the page instead.
Under the hood
The parts you don’t see are what keep it trustworthy.
Every Muse edit is a proposal, checked against your current text at accept time. If you touched the passage first, it goes stale. It's never silently applied over your work.
Highlights, alignment, comments: none of it is written into your prose. The document Muse reads is plain markdown you could open anywhere. The annotations live beside it, not inside it.
Group chapters, notes, and sources into a project. Flag which files Muse reads as background so it keeps continuity across a whole manuscript, and mention any file by name mid-conversation.
As you write, Muse maintains its own memory: notes on your world, plus the craft feedback you’ve given it. Start a fresh chat to reset the conversation. The memory of your world and your preferences survives.
Early access · opening soon
Margin is in early development. Leave your email and you’ll be among the first writers let in. No spam, just a note when it’s ready.