Your own AI key

What the key is for

When a sentence you type differs from the model answer, AI checks it: grammar and meaning. Another correct way of saying it counts too.

In the reader the same key pays for Check, Explain and your questions, and for word hints if you pick one of its models in the Reading settings. An OpenAI key also draws the pictures of new cards for your own deck.

Free checks are on me, every day, for every signed-in account. With your own key you pay the provider directly, at their prices: the bill goes from them to you, I never see it and add nothing on top.

OpenAI or Anthropic

Either works. An OpenAI key starts with sk- (newer ones with sk-proj-), an Anthropic key with sk-ant-; the app tells them apart by that.

• Both: the AI check of your sentences, and in the reader checks, explanations, questions and word hints.
• OpenAI only: the pictures of new word cards beyond the free ones.
• Neither: reading a photo of a book page stays on Kurzdeutsch, within the daily limits.

Not sure which? Take OpenAI: it covers everything, and the AI check costs least on it.

Get an OpenAI key

1. Sign in at platform.openai.com. A ChatGPT subscription does not cover the API: it is billed separately.
2. In the billing settings, add credit. The API is prepaid, so $5 lasts a long time.
3. In the limits settings, set a monthly budget, a small one, say $5.
4. On the API keys page, create a new secret key, name it Kurzdeutsch and copy it. It is shown only once.

Get an Anthropic key

1. Sign in at console.anthropic.com. A Claude Pro or Max subscription does not cover the API: it is billed separately.
2. In the billing settings, buy credits. The API is prepaid here too.
3. In the limits settings, set a monthly spend limit, a small one, say $5.
4. On the API keys page, create a key, name it Kurzdeutsch and copy it. It is shown only once.

Paste it into the app

1. In the app, open ☰ → Settings.
2. In AI check, set Key to Your own.
3. Paste the key into API key and tap Check key. The app sends one tiny request to your provider, a few hundredths of a cent, and keeps the key only if it works. Under the field it says which model answered, or why the key did not work.
4. Done: from now on the AI check runs on your key.

The key stays on this device only and is not synced with your account. It leaves the phone only with your own requests, on the way to the provider, and the server stores it nowhere. On another phone or computer, paste it there too. Kurzdeutsch switches back to the free checks and leaves the key unused on the device; Remove key deletes it.

Open the settings

What it costs

One check is one short request: about 2,000 tokens in (the instructions, the sentence, your answer) and a hundred or two out.

• OpenAI key: the check runs on GPT-6 Luna, $0.0001–0.0003 each, one to three cents per hundred checks.
• Anthropic key: it runs on Claude Sonnet 5, about $0.008 each, 70–80 cents per hundred.
• A picture for a new card (OpenAI key): about 6 cents.

An answer that was already checked comes from the cache, free. If you choose another model of your provider in the Reading settings, the check runs on that model, at its price. Prices are the providers' list prices as of October 2026.

Keep it safe

Create a separate key just for Kurzdeutsch, with a spend limit. If anything looks wrong, revoke it on your provider's API keys page: it stops working at once, everywhere. Then create a new one and paste it again.