Flashcards in Heddy are free and unlimited forever. Conversations with a tutor are metered, in units we call Pi Turns. That split isn't a pricing trick — it's what the two things actually cost to run.

The honest economics

A flashcard review costs us essentially nothing. It's a database read and some text. You could do ten thousand of them and we would not notice.

A turn with Professor Curie is a call to a large language model, and it costs real money every single time. Not much, per turn. Enormously, at scale, if it's unlimited and free.

Most study apps resolve this by making the cheap thing scarce too — daily card limits on a free tier, decks locked behind a subscription — because scarcity converts. We've made the opposite call: meter the expensive thing honestly, and give the cheap thing away completely.

The result is that the habit-forming half of the product has no ceiling. You can review cards every day of a term, adopt the entire Atlas, and never see a limit or a prompt.

What a Pi Turn is

One new problem started with a subject professor.

Not one message. Not one minute. One problem. Ask Pi about a quadratic and work through five rungs of hints, three follow-up questions and a worked twin — that's one turn.

The unit is the problem because the problem is what costs us something, and because metering per message would produce exactly the behaviour we least want: a student compressing their confusion into one long question to save credits, instead of thinking out loud.

The three rules the meter can never break

These aren't preferences. They're written into the design as constraints, and they're the reason the metering is liveable.

1. A ladder that has started always finishes. Turns are spent at the start of a new problem, never during one. If your balance hits zero halfway through a hint ladder, the ladder completes. You will never be cut off mid-explanation, and you will never be asked to pay to see rung four.

2. No purchase prompt during a session. Not a banner, not a toast, not a subtle badge. The upgrade screen exists and is reachable — from the home screen, from the empty-allowance state, or because you went looking for it. Never from inside a conversation you're in the middle of.

3. No loss framing, anywhere. When the allowance runs out, Heddy doesn't tell you what you've lost. She dozes off — eyes shut, two small italic zs — and offers you two things: Practise cards and Remind me tomorrow. There is no price on that screen at all.

That third one is the rule people find surprising, so to be explicit: the screen you see when you run out of tutor turns contains no purchase prompt. The moment a student is most frustrated is the moment it would convert best, and that is exactly why it isn't there.

Where the meter is visible

Almost nowhere, deliberately.

There is no turn counter in the tutor room header — no pips, no number, no energy bar. It was drawn that way in an early design and removed, because a visible meter changes how you ask questions. You start rationing, hedging, saving turns for later. That's a worse conversation and a worse lesson.

The allowance surfaces in two quiet places instead: on Heddy's own home screen, and as an occasional in-stream stamp — ⚡ Energy this week: … — which tells you where you are without sitting on top of your thinking.

One confusing word, cleared up

Heddy uses the word energy for something else entirely, and the two are worth keeping apart.

  • Pi Turns — the spend allowance described above. About money and machines.
  • Your week's energy — Heddy's green/amber/red wellbeing check, the thing that decides whether this week's plan is ambitious or small. About you.

A red week doesn't cost you turns and a full balance doesn't mean you should be doing more. They're separate questions and the app treats them separately.

And when you do want more

If a student wants more tutoring than the free allowance carries, that's the point at which a grown-up comes into it — by the student's own tap, from a screen at rest. What we ask for is a guardian email, and what that email is for is narrower than you might expect: a receipt and a safeguarding contact, not a window into their conversations.

Updated 10 Sep 2026. Written by Duke Harewood, who builds the tutors at aitutors.me.