Cothyra

Chats fade. Your book stays.

Ask an AI a hundred questions and you end up with a hundred dead threads. What you understood is scattered across conversations you will never open again.

Here the conversation has a subject: a chapter you are writing. You discuss it, argue with it, refine it — and what is left when you close the tab is not a transcript. It is your book.

For self-learners and lifelong students. What you read should add up to something.

Open the app See pricing

Start free — every account begins with 500 free AI credits, no card required. Reading and writing never cost anything.

The difference is what you keep

Chatting with an AI

You ask, it answers, you close the tab.
Insights sit in threads you cannot search or cite.
Ask again next month and start from nothing.
Nothing accumulates. Every session begins at zero.

Working here

The chat is attached to a page you own.
Every good answer lands in the text, where it belongs.
Come back in a month and read what you built.
It compounds. Each session starts from the last one.

Self-teaching is slow because understanding evaporates faster than you can collect it. An assistant that writes into your pages instead of past them is the whole idea.

What you get

An interactive AI book

Math, tables, diagrams, code and citations render as you type. Ask the AI for a change and it lands in the chapter as a reviewable diff — one you can undo like your own typing. The editor is free, always: credits are only for AI.

src/07-quadric-surfaces.md

Transcript of the demonstration

Reader: Contours don't do it for me. Can you put an actual 3D surface here — one I can turn with the mouse?
read src/07-quadric-surfaces.md
Assistant: Yes — a js-render block. Plain canvas rather than a plotting library, so the chapter stays light.
edit src/07-quadric-surfaces.md — adds an interactive 3D figure — a js-render canvas block the reader can drag to turn
Assistant: It's in the chapter now — drag left and right to spin it, up and down to tilt. Say the word and I'll put a slider on the coefficient.

The saddle point

At the origin the surface $z = x^{2} - y^{2}$ is neither a maximum nor a minimum: it curves up along the $x$ axis and down along the $y$ axis, so every neighbourhood of the origin holds points both higher and lower than it.

Contour lines turn that into a family of hyperbolas. The shape is easier to believe when you can turn it over.

.cdemo__panes, .cdemo__player { display: none; } .cdemo__replay { display: none; } .cdemo__transcript { position: static; width: auto; height: auto; overflow: visible; clip-path: none; white-space: normal; padding: 1.25rem 1.5rem; } /* Without a script the phone is an empty prop, and it has no transcript of its own to fall back to. */ .cdemo-frame--phone { display: none; }

A recorded turn: the reader asks for a surface instead of a contour plot, and the assistant writes a js-render block into the chapter. The surface is that block running — drag it.

Listen to your work, or talk it through, any time

Any chapter reads itself aloud, with formulas spoken the way a lecturer says them. Ask a question by voice while you listen, and the answer is read back. It all works on a phone — the same editor and chat, not a stripped-down reader.

src/03-heat-equation.md

Transcript of the demonstration

Separation of variables.
We look for solutions of the heat equation — u sub t equals alpha u sub x x — that split into a part depending only on position and a part depending only on time: u of x and t equals X of x times T of t.
Substituting, and dividing by alpha X T, gives:
T prime over alpha T equals X double prime over X equals minus lambda.
The left side depends only on t, the right only on x — so both are the same constant.

Separation of variables

We look for solutions of the heat equation $u_{t} = α u_{xx}$ that split into a part depending only on position and a part depending only on time, $u (x, t) = X (x) T (t)$ .

Substituting and dividing by $α X T$ gives

\frac{T ^{'}}{α T} = \frac{X ^{''}}{X} = - λ

The left side depends only on $t$ , the right only on $x$ , so both are the same constant.

The chapter reading itself, alongside the narration script the book generates — which is where a formula becomes something a person says out loud.

All the features, in the User Guide →

On a phone, it is the same app

Not a reader with the writing taken out. The chapter, the conversation and the AI's edits are all here — one pane at a time, on the carousel the app really uses below 1024 pixels. The figure it writes takes a thumb as happily as a mouse.

Quadric Surfaces

Start free. Pay as you go beyond that.

Every account starts with 500 free AI credits — no subscription, no card. When you need more, buy a credit pack and spend it on AI calls. Your library, your reading, and your writing are always free.

Free to start

500 credits

one-time signup grant · no card required

Chat, edit, research and listen — your signup credits work with nearly every model and feature. Only a handful of premium frontier models require purchased credits.

Starter

$10 USD

1,000 credits

$10 of credits. Try it out.

Regular

Best value

$20 USD

2,200 credits

$20 of credits + $2 bonus. The everyday pack.

Power

$50 USD

5,750 credits

$50 of credits + $7.50 bonus. For heavy editing weeks.

Get started

Pay only for what you use — no subscription, no minimums.