The free tier and licensing

Castiel is bought once and yours to keep. There is no subscription, no advertising, and no account to sign up for before you can calculate. This chapter explains what you get for free, the one limit that applies to the free tier -- a 30-minute session timer on the four advanced modes -- and how buying the full version removes that limit. If you are just starting out, Getting started walks through the app first; this chapter covers only the pricing and the timer.

The short version: Simple and School are free forever with no timer. The four advanced modes run free for about half an hour per session, after which Castiel returns you to a basic mode. Buying the full version removes the timer everywhere.

What you pay

Castiel is paid once. It is not a subscription, and there is no recurring charge -- you pay a single low price and the app is yours. The one-time price is deliberately small: never more than about US$2 (or the equivalent in euros).

• No subscription. You are never billed monthly or yearly. One payment, kept.
• No ads. Castiel shows no advertising in any mode, free or paid.
• No account required. You do not create an account or log in to use the calculator. (Optional cloud sync is a separate, future feature and is not needed for anything described here.)

Because the price is a single small amount, the free tier is meant to let you use the calculator properly before deciding -- not to nag you into buying.

What is always free

A large part of Castiel never expires and never starts a timer:

• Simple mode and School mode -- fully free, with no time limit. School alone is a complete scientific calculator with graphing, statistics, equations, spreadsheets, and more. For most everyday and study use you never need anything beyond it.
• The paper tape -- entering, editing, recomputing, annotating, and saving or loading your .cast sessions -- works freely in every mode.
• Connectors -- currency rates, managing your own AI provider keys, the encrypted key export and import, and receipt printing -- are all free.

You can use Castiel indefinitely on the free tier, staying in Simple or School, and never meet a limit or a prompt to pay.

The 30-minute timer on advanced modes

Four modes are the advanced modes: Engineer, Financial, Programming, and Programming+. On the free tier these run for about 30 minutes per session. This is the only limit the free tier imposes.

How the timer behaves.

• It arms only when you enter an advanced mode. Switching into Engineer, Financial, Programming, or Programming+ for the first time in a session starts the countdown. Staying in Simple or School -- or using the tape and connectors -- never starts it.
• You get a warning before time runs out. A short notice appears a couple of minutes before the session ends, so the end is never a surprise.
• When the time is up, Castiel returns you to a basic mode. You are moved out of the advanced mode; Simple and School remain open and free, so your work continues -- you simply lose the advanced-mode surface for the rest of that session.
• The timer is per session, and a restart resets it. Closing and reopening Castiel begins a fresh session with a new half hour of advanced-mode time. There is no stored countdown that carries over between runs.
• Once it has started, stepping back does not pause it. Within a single session the countdown keeps running even if you return to Simple or School after visiting an advanced mode. To get a fresh half hour, restart the app.

The table below summarises which modes are affected:

You can control which modes appear at all from Calculation in Settings (the Modes shown toggles). Hiding a mode does not change its free-or-paid status; it only removes its tab.

Unlocking the full version

Buying Castiel removes the timer entirely. Once the full version is active, every advanced mode runs with no countdown, the warning never appears, and nothing changes about how you work -- the limit is simply gone. The purchase is a one-time entitlement, not a renewal.

How you unlock depends on where you got Castiel. There is no separate license key to type into the app for the store editions; the purchase is tied to the account or activation of the channel you installed from.

Because store purchases are tied to your store account rather than to a typed-in code, there is no license key for you to keep track of, lose, or re-enter -- reinstalling and signing in to the same store account restores your purchase.

Your data stays yours

The free tier is genuinely free because Castiel's model does not depend on your data.

• No advertising and no tracking-for-sale. Castiel does not show ads and does not harvest or sell your usage.
• Your work stays on your device. Your calculations, saved tapes, and settings live locally on your computer, not on a Castiel server.
• Keys are stored securely. Any AI provider or custom-API keys you enter are held in your operating system's own secure store, and the .castkeys export is encrypted. See Settings.

You can turn the automatic update check on or off from Settings > About; nothing else phones home as part of normal use.

Related chapters

• Getting started -- first run, the six modes, and finding your way around.
• Settings -- the Modes shown toggles, the About pane, and connector key export/import.
• The six modes overview -- what each mode is for and when to reach for an advanced one.