Thunder Writer

Write your novel on real book pages, with a quiet AI editor in the margin.

Thunder Writer is an idea processor, not a word processor. A word processor waits for you to finish a thought and then helps you format it. Thunder Writer reads along while you draft and, now and then, offers what a good editor would: a typo fixed, a sharper phrase, a detail that doesn't match your opening chapters or your series notes, a real-world detail that fits the scene. One or two ideas at a time, never a wall of red ink. You decide what stays.

It runs entirely in your browser. There is no account to create and no Thunder Writer server holding your book.

Why an idea processor?

Most writing tools help with the last mile: spelling, formatting, page numbers. The hard part of a novel comes earlier. You have to keep 90,000 words of people, places and promises straight, and you have to keep the momentum going while you do it.

Thunder Writer puts its effort there:

• It reads more than the sentence under your cursor. Suggestions draw on the scene around it, the opening of your book and reference files you add, like a series bible, an earlier book or a poem that sets the mood. A short draft (up to about 2,500 words) is sent whole. For a longer book it sees the opening pages and a few pages around where you're writing, not every chapter, so keep names and facts it should check against in a reference file.
• It respects your attention. It waits until you pause, keeps at most two suggestions open, and leaves you alone for a while after each one.
• It never edits on its own. Every suggestion is a card you can take or leave. Nothing changes in your manuscript until you press Accept.

What writing with Thunder Writer feels like

You open your novel and it's laid out the way it will be printed: a 6 × 9 trade paperback, Palatino, with Chapter 12 starting at the top of a fresh page. You write for twenty minutes. The margin stays empty.

You stop to think. A few seconds later a small card appears in the margin: Spelling: "recieved". You hover over it and it opens to show the sentence, before and after. You click Accept. The word is fixed, and Cmd+Z would bring it back if you wanted.

A second card is a Context note. Your series bible, added as a reference file, calls the lighthouse keeper's daughter Maren, but in this scene she's Marin. You click the card, the passage lights up on the page, and you fix it yourself. Mark Done.

Later, a Trivia card, marked "Possibly relevant", mentions a recent news story about the coastline your book is set on, with links to its sources. You don't need it today, so you hide it and keep going.

When the chapter is done you press Preview Book and turn the pages of your book, with running heads and page numbers, the way a reader would see them.

(The names above are an illustration of the kinds of cards you'll see, not a recording.)

What you get

An AI editor that stays out of your way

Five kinds of help	Grammar and spelling, style, context and continuity (checked against the scene you're in, your opening chapters and your reference files), story ideas, and trivia.
One or two at a time	At most two open suggestions by default. You can allow up to five in Settings.
You stay in control	Every card has Accept, Decline, Mark Done and Hide. Accepting an edit is one undoable change.
Pinned to your words	Hover a card to see the full explanation and a before/after preview. Click it to highlight the passage it's about. A card whose passage you've since rewritten says so instead of changing the wrong text.
Quiet by design	It asks only after you pause typing (4 seconds by default), waits at least 45 seconds between requests, and backs off further after errors.
Collapse when you're cruising	Hide Suggestions folds the pane into a slim rail. Turn off automatic suggestions entirely and press Generate Suggestions only when you want a second opinion.
Current-events trivia, with sources	Every so often (at most once every 10 minutes by default), the AI may look up one recent real-world item connected to your book's themes or setting. The card links its sources, labelled with each site's address, so you can check them. If nothing relevant turns up, you see nothing.
Reference files	Add notes, an earlier manuscript, a series bible or a poem (text, Markdown, HTML, JSON or RTF) and suggestions take them into account.

Real book pages

• Write at a real trim size. Mass-market paperback, trade paperback (5 × 8, 5.25 × 8, 5.5 × 8.5, 6 × 9), Royal, or a double-spaced 8.5 × 11 manuscript, each with sensible margins, font and spacing. You can change the font, size and line spacing yourself.
• See where the pages fall as you type, so you have a feel for your page count long before you format anything.
• Chapters start on a new page, and they keep doing so when you change the font or trim size.

Book Preview, with page turns

Press Preview Book in the toolbar to see your manuscript typeset as a printed book:

• Two-page spreads, with chapters opening on right-hand pages, and running heads and page numbers in place.
• Facing pages are balanced to end on the same line, and you won't find a heading orphaned at the foot of a page, a scene break stranded at the bottom, or a chapter that ends with a lonely line or two.
• On a narrow window it shows one page at a time.
• Pages turn with an animated 3D page flip. Use the arrow keys, Page Up/Down, the side buttons, a click on a page's outer edge, a drag or swipe, or your trackpad.
• Jump anywhere with the slider along the bottom (hovering shows the chapter), or use Contents, Go to page and All pages thumbnails.
• Headers & footers sets the running heads (your name, once you enter it in Headers & footers, on the left page and the title on the right by default, or the chapter title, with short versions for long titles), a footer line, where the page number goes, the first page number, and what chapter openings and blank pages show. It warns you when a heading is too long for the page. Your Word and print exports use the same running heads and page numbers. Word does show heads on the blank pages it inserts before right-hand chapters.
• Print / PDF prints the book at its exact trim size.
• Paper & ink shows your pages on white, cream or groundwood paper, with a note on what each paper and ink (black and white, standard or premium color) is for.

Bring the manuscript you already have

• Open Word (.docx and older .doc), PDF, RTF, Google Docs, Markdown, HTML or plain text. Use File › Open from computer…, drop a file onto the page, or pick a Google Doc straight from your Drive. A PDF doesn't record paragraphs, so they're rebuilt from the page layout (running heads and page numbers are left out); an older .doc comes across as its words, without italics or bold.
• Chapters are found for you. Lines like "Chapter 1", "Chapter Twelve: The Storm", "Part One", "Prologue" or "Epilogue" become chapter headings, and * * * style lines become scene breaks. Bold, italics, lists and quotes come across.
• Your file is left alone unless you say otherwise. Afterwards you get a summary: word count, chapter count, and anything that couldn't come across (such as images). In Chrome and Edge you can choose Keep saving to it: your changes then go back into that same file as you write. The file as it was is kept first, in Backups. (Saving back into a Word file rewrites it from your manuscript, so Word-only extras like comments are dropped.)

Your manuscript, on your computer

• Export gives you a Word document laid out at your trim size, with chapters on new pages, page numbers and your running heads. You can also export a PDF of your book's pages (as Preview Book shows them), a Word 97–2003 (.doc) file for older versions of Word, Markdown, plain text, a print-ready web page or a Thunder Writer backup of this manuscript.
• Keep a copy on my computer (Chrome and Edge): choose a file once, for example on your Desktop, and Thunder Writer keeps it up to date as you write, about 10 seconds after you pause.
• Cmd+S / Ctrl+S does what you expect. It saves right away, updates your desktop copy, and never opens the browser's "Save page" dialog.

Autosave, twice over

• Every change is saved in your browser about a second after you stop typing.
• Connect your own Google Drive and your manuscripts are also saved there, in a "Thunder Writer" folder, every 10 seconds or so while you write (a few seconds after you pause, at most every 10 seconds by default), so you can pick up on another computer. Before you switch computers, check that the status says it's saved to Drive. (After reloading the page you reconnect with one click.) Thunder Writer can only see the files it created and the files you choose to import. It can't see the rest of your Drive.
• The header says where your work is saved. A manuscript that isn't in a file yet (not in Google Drive, not kept on your computer) shows Not saved to a file; click it to save it to Google Drive or to a file on your computer that updates itself.
• Backups you can always get back to. As you write, Thunder Writer keeps backups of each manuscript in your browser, plus a copy just before anything replaces or deletes one. Open Backups ▾ in the writer to see them; choosing one opens it as a new manuscript, so nothing is overwritten (and Undo closes it again). In Google Drive, the previous version is kept as a .bak file in the Thunder Writer folder before Thunder Writer saves over it.

Private by design

• No accounts, no servers. Thunder Writer is a web page that runs on your machine. There is no Thunder Writer server to send your writing to.
• You bring your own AI key, from Anthropic (Claude), OpenAI, or OpenRouter, which gives you hundreds of models (Gemini, Llama, Mistral, DeepSeek and more) with one key. For suggestions, your words go from your browser directly to the provider you choose. If you connect Google Drive, your manuscripts are saved there too.
• Current-events trivia (on by default) uses web search with Claude or OpenAI (OpenRouter models answer from their own knowledge). The provider may run a web search based on your book's topics or setting, so short search queries drawn from your manuscript can reach its search service. Turn it off under Settings → Suggestions if you'd rather it didn't.
• Your keys stay in this browser. They are never uploaded, not even to your Google Drive.
• Clear all local data (in Settings, and on the home page) wipes everything Thunder Writer stored in the browser. Your Drive files are left alone.

A tiny AI bill, shown live

• The defaults are the cheapest suitable models: Claude Haiku 4.5, OpenAI's GPT-6 Luna, or Claude Haiku 5.5 on OpenRouter. On OpenRouter you can pick any model; Settings shows each one's price.
• The status bar shows your running AI cost for the session, next to your word count, writing time and suggestions accepted.
• A suggestion request to Haiku 4.5 costs about a cent or less (see the cost notes for the arithmetic). Each current-events web search adds one cent. Lengthen the cooldown or switch to on-demand suggestions to spend less.

And the small things

Light and dark themes, or follow your system setting. A running word count and writing timer. Keep as many manuscripts as you like and switch between them from the File menu.

• Focus Mode clears everything but your pages, with a smooth glide in and out. Exit Focus Mode (or Esc) brings it all back.

• Typewriter sounds, if you like them: a key strike as you type, a softer knock when you delete. Off until you tick the box.

• Zoom the pages from 100% up to 250% with the slider at the bottom left.

• Three page views, at the right of the bar above your pages: Scroll (one whole page at a time, top to bottom), Side by side (two pages across, in Chrome and Edge) and Flip (an open book, page 1 on the right, with pages that turn). You can type in all three.

• Collapse tools, the button centered at the bottom of the toolbar, tucks every tool away except Preview Book (and the tip jar and book pick), and makes the header compact. Show tools brings them back.

• On a phone, Thunder Writer keeps it simple: Write (your pages), Read (the book, a page at a time) and File:

  • Open from Files and Save to Files use your phone's own files (on iPhone, the Files app, through the share sheet). Phones can't keep saving into the same file, so each save is a copy.
  • Save to Google Drive keeps a copy up to date automatically.
  • From File you can also open your manuscripts in Drive, import a Google Doc, or start a new manuscript.

  An iPad gets the full app.

• Picks up where you left off. Your work is saved in the browser as you type and the moment you switch away. Connected to Google Drive, it goes up to Drive as you leave too. Each manuscript reopens at the spot you were writing.

Try it

1. Open Thunder Writer in a desktop browser. Chrome or Edge gets you every feature; see Which browsers? below. A hosted version is coming soon. For now you need to run your own copy, which takes a developer setup (Node.js and a terminal): see the technical notes.
2. Add your AI key under Settings → AI provider: a Claude key, an OpenAI key or an OpenRouter key for any model. Test key checks it without spending anything.
3. Start Writing for a fresh manuscript. When you come back, the home page leads with Continue writing: your latest manuscript, when you last edited it, its word count and its latest lines. On a computer you haven't written on before, Continue Writing connects your Google Drive and lists the manuscripts you saved there. You can also Import a manuscript you already have.
4. (Optional) Connect Google Drive from the File menu for a cloud copy. There's nothing to set up: just approve Google's permission window. (Hosting your own copy? It needs a one-time Google setup by you, not your writers.)

You can write without an AI key. The suggestion pane will just point you to Settings.

Questions

Is my writing used to train AI?

Thunder Writer never receives your writing. When suggestions are on, your browser sends the relevant part of your manuscript (and your reference files) directly to Anthropic, OpenAI or OpenRouter, using your own API key. With current-events trivia on (the default), the provider may also run web searches based on your book's topics or setting; turn it off under Settings → Suggestions to stop that. What happens to your text at the provider is governed by that provider's terms and privacy policy for API customers, not by Thunder Writer. Read them before you start: Anthropic's Commercial Terms and Privacy Policy, and OpenAI's data controls for the API. With OpenRouter, the request goes on to the company that runs the model you picked; OpenRouter's privacy settings let you exclude providers that may train on your data. If you'd rather send nothing, leave the key blank. You can still write, import, preview and save.

What does it cost?

There is no Thunder Writer subscription or account. You pay your AI provider directly for what you use, at their published rates. The status bar shows the running total as you go. If a model's price isn't known to the app, the suggestion pane says the cost is unknown.

Does it work offline?

Partly. Once the page is open, you can keep writing, and every change is still saved in your browser. Suggestions, Google Drive saving and importing from Drive need a connection, and Drive catches up when you reconnect. Book Preview, Headers & footers, Word import, and saving to your computer (every format, including the desktop copy and Cmd+S / Ctrl+S when you have one) load extra code the first time you use them in a session, so they may need a connection too. Try each once while you're online if you plan to work offline. If Thunder Writer was updated while your tab was open, reload the page before using them. There is no installable offline app yet, so you need a connection to open Thunder Writer in the first place.

Which browsers?

Thunder Writer is built for current desktop browsers. So far it has been checked only in Chromium-based browsers (Chrome, Edge). Safari and Firefox should work but haven't been tested yet. Keep a copy on my computer, and the Save dialog that opens on your Desktop, need Chrome or Edge. In other browsers, saved files go to your Downloads folder. Page numbers in the print-ready web page need Chrome 131 or newer. Book Preview's Print / PDF has been checked only in Chrome.

What happens to my book if I clear my browser?

Anything stored only in the browser goes with it. So connect Google Drive, keep a desktop copy, or use Export for a backup now and then. A backup covers the one manuscript you have open, so save one for each book (Drive, when connected, keeps all of them). Reference files and your AI keys live only in this browser and are never uploaded or included in a backup, so keep your own copies of your reference files.

Will it rewrite my book?

No. It suggests. Nothing in your manuscript changes unless you press Accept on a specific card, and each accepted change can be undone.

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Setting it up, hosting your own copy, the Google Cloud and Vercel steps, the privacy model in detail and how it's built: see tech_notes.md.