How it is built
The front page says what MetaBook does. This one says how, for anyone who wants to check the claims before paying $5 for them: how a book is made, what makes rewriting a passage in place safe, where every cent goes, where your key lives, and what changed in the last release.
First · making a book
The half of the app you use first, and the one that decides whether the book is worth reading at all. It stops before any chapter has text, so you see the shape of the book — and what it will cost — before a chapter is paid for.
The language a book is written in is chosen here, before the first call, and never again. It is what the interview asks in, what the outline comes back in, and what every chapter is written in. Ten to choose from.
Level, density and size, on the same screen as the topic. Each of them may be left alone — and left alone is not a middle setting, it means ask me about this. Set one and you are shown the exact sentence it will send, and told which question you have just bought back from the interview. They freeze once a chapter has text.
What you already know, what you want out of it, how deep to go. You end the interview, from the third question on — a model that announced it had heard enough would be deciding how much more of your money to spend.
Rename, reorder, insert, remove. Editing the outline costs nothing and calls nothing — it moves titles and synopses, never text. A chapter you have not written yet says so, with its synopsis, instead of showing you a blank page.
A chapter takes minutes and arrives as it is written. Leave the app and it keeps going — the request is handed to the phone itself, and a notification says when the chapter is ready. Come back to an interrupted one and it continues from what you already paid for rather than starting again.
Two documents: a seed taken from your brief that no model ever rewrites, and a rolling summary refreshed as chapters are absorbed. It is why chapter nine still knows who it is written for, and does not explain chapter two again.
Nothing here is written until you ask for it, and nothing you have already paid for is thrown away. Text is saved as it arrives, so a force-quit mid-chapter costs you nothing but the rest of the chapter.
Then · while reading
Touch and hold any paragraph and a toolbar appears — drawn by the app rather than by the phone, so it is the same on both. Four controls: Rewrite, Ask, Edit and copy. Behind Rewrite are four named actions and a field for saying it in your own words, and each of the five states what it will cost on the control you are about to press.
Answers a question about the passage and writes nothing into the book. Asking is not editing, and one action for both would make the book unsafe to ask about. The answer is kept against the passage you asked about, the page carries a tint there, and the chapter's answers are a list — none of which is chapter text, and none of which reaches an export.
Opens the passage's own Markdown in a field. What you type is spliced in verbatim through the same verified write a rewrite goes through, and is recorded as a version like any other change — with no call made and no cost. Emptying the field deletes the passage.
Puts the passage on the clipboard as it reads: no asterisks, no hashes, no list markers. Exactly the range you highlighted, too — not the wider span a rewrite has to take, because widening a quote would be the app editing your words.
The named actions are shortcuts for the field, which is why they live behind the same door rather than beside it — and why typing in the field puts them out of reach. One instruction per call. Select more than about a third of a chapter, or use Rewrite this chapter with nothing selected, and you leave this path for a whole-chapter rewrite — roughly fifty times the cost, stated and confirmed before it runs.
In place
Rewriting a passage inside a document you are holding open is the part everyone else solves by putting the answer in a side panel. Five decisions make it safe to put it in the book instead.
Offsets are computed against a particular version of a chapter. Slicing against text that has since moved would spend your money on a passage that is no longer there.
Two bounded slices around your selection, each snapped to a block boundary, so no request hands a model half of anything. It is what makes the call cheap enough to run casually.
What comes back replaces exactly the range you selected, and nothing else. The model is never in a position to touch a byte outside it.
Identical bytes outside the range is necessary and not sufficient: an unterminated code fence inside the replacement changes how the rest of the chapter reads without altering a byte of it. So the comparison is of the parsed document, not of the text.
It restores the previous text as a new version. History that ran backwards could not say what the model did, and every passage you had already selected would become ambiguous.
Nothing in the app retries a model call — a retry spends your money on the app's problem. The one thing that retries is the write.
Also · writing for somebody else
The interview ends in a brief, and the brief has a section called Reader: who this book is for, what they already know, why they are reading it now. The outline and every chapter are written against it. Answer the interview about somebody else and the same app stops making you a book and starts making you an author.
You are the reader
You say what you want and how much of it you already know, and the book comes back pitched at that. The four actions are how you keep it there: a chapter that lands badly is a paragraph you hold, not a book you give up on.
You are the writer
You answer the same questions about them — the new hire who starts on Monday, a student, a friend who asked what it is you actually do, a parent and their new phone. The brief describes that person, the outline is proposed for them, and every chapter is written to them rather than to nobody in particular.
One call turns the interview into it, and you are shown it before an outline exists. If it describes somebody your reader is not, you say what is wrong and it is written again — one call, before a chapter has been paid for.
Same gesture, different job: simplify what will be too dense for them, expand what is too thin, add the example the abstraction needed. Ask writes nothing into the book, so you can interrogate a draft without changing it. Every version is kept — what changed it, and what it cost.
EPUB for their e-reader, Markdown for anything, FB2 — a whole book or a single chapter, out through the phone's own share sheet to wherever you send files. They need no key, no account, and no copy of MetaBook.
What leaves the phone is a file: a copy, finished at the moment you exported it. The hold-a-passage loop belongs to the app and to the device the book lives on — which is the argument for reading it through before you send it.
Money
The app is $5 once on the store, and that is the last cent we see. Every model call is billed to the provider account your key belongs to, and the app is built as though you were watching the meter — because you are.
Your key, your provider, your device
No API service, no database server, no accounts, no hosting of any kind. The app on your phone talks to your model provider directly, and to nothing else. That is a decision with a cost, and it buys you this:
What you type and what your books hold leave the device only as part of a model call, addressed to the provider your key belongs to. There is no account to make, no server of ours in the path, and no analytics anywhere in the app.
Keychain on iOS, Keystore on Android, and nowhere else in the app. It never reaches a log, an export, or a backup — so a backup file you hand to somebody cannot carry your key with it.
The key screen names the provider it is about to talk to, then poses the choice as a question: connect an OpenRouter account and approve it in the browser, or paste a key you already have — with each answer saying whether it needs an account. Both take you to the same place. You can also finish setup with no key at all and read the sample book first.
One SQLite database — every book, every version of every chapter, the outline, the memory, the ledger. Back it up whole, restore it on the other phone, or delete all of it, key included, in one action that says exactly what it removes.
And the rest of it
English, German, Spanish, French, Italian, Portuguese, Russian, Turkish, Indonesian and Vietnamese — for the app, and separately for each book.
Markdown that contains each chapter's stored source byte for byte, EPUB 3 for an e-reader, and FB2 — with a picture packaged inside the last two. Exporting spends nothing and changes nothing.
The system voice, sentence by sentence, with the phone's own play controls on the lock screen.
Display and inline formulae are part of what a chapter may contain — and they are drawn, not printed as dollar signs and backslashes.
Each chapter keeps its history — what changed, which model changed it, what it cost, what the instruction was — and the whole book's changes are one list.
A table is drawn as a grid with its header set apart, and one too wide for the page scrolls inside its own block rather than making every paragraph around it harder to read.
Below level three a heading gets quieter rather than smaller: under the body size a heading stops reading as a heading and starts reading as a caption.
The app seeds one book and marks it as its own, so there is something real to read — formulae, tables and all — before you have a key. It is on this site too.
Reading position is an offset into the chapter's own text, not a scroll position, so it survives the text being rewritten under it.
A reading theme that is designed rather than inverted, with the app around it built from the same palette.
Text is written down as it arrives. Force-quit the app mid-chapter and you come back to everything you had already paid for.
One codebase, and a rule that a feature exists on both targets or on neither. The selection toolbar is the app's own, so it behaves identically on each.
Version 0.6 · August 2026
Almost all of the release is in the reading loop. A passage can now be drawn as well as rewritten, a question you asked stays answered on the passage you asked it about, and everything that has ever been done to a book is one list.
Illustrate
Hold a paragraph, take Illustrate, and say what the picture should show — or say nothing and let the passage speak for itself. The reply comes back as an image, and the app writes it into the chapter as its own block directly under the passage. It is part of the book from then on: it rides inside an EPUB or an FB2 export, it returns if you restore a version that had it, and deleting it by hand costs nothing. Around 8¢ a picture, quoted high at 25¢ on the control before you press it.
Kept answers
Ask was the one paid action that left nothing behind — the ledger outlived the sheet and the thing it paid for did not. Now the question and its answer are kept against the passage, the page carries a tint where you asked, and tapping it opens the answer again. The chapter's answers are a list, each with what it cost. None of it is chapter text: no byte moves, and nothing reaches an export.
Bookmarks
Mark the place you are reading from the running head. The book's marks are one list in reading order, each row naming its chapter, the heading above it and the opening of the block it holds — and each is a way back into it. A mark follows its block when the text is rewritten under it, and says so plainly when the place is gone.
The inner index
The chapter list now opens out the chapter you are reading and lists the headings inside it, indented by level, with the one you are currently under marked. Taking one moves you there without reloading the chapter. Every other chapter keeps its single row, so opening the list parses nothing it does not already hold.
Book history
The version rows were always written, and were readable one chapter at a time. Now they are a single list for the whole book: what was done — written, rewritten, edited by hand, taken back — when it happened, the sentence you typed if you typed one, the model behind it and what the call cost. Every row leads back to the chapter it is about.
Dials
Three dials on the topic screen — level, density and size — each of which may be left alone. Left alone is not a middle setting: it means ask me about this. Set one and you are shown the exact sentence it will send, and told which question you have just bought back from the interview.
And one change you see only on the bill: every task now runs on a single fast model by default, which puts an eight-chapter book in English at about 23¢ to write, cover to cover. What things cost.
$5 once on Google Play, then your own OpenRouter key at cost. Read the sample book first if you like.
iPhone: coming soon — tell me when.
