Bring your own API key AI apps: when using your own key makes sense

Many AI apps now offer two ways to pay for the AI inside them. Either the app includes it, as an allowance or credits in your plan, or you paste in an API key from your own account with an AI provider such as Anthropic or OpenAI and pay that provider directly. The second is called bring your own key, or BYOK.

Neither is better in general. Your own key makes sense in some situations and is extra work for nothing in others. This guide is about telling which situation you are in, with a way to estimate the cost before you commit.

If you are new to API keys, bring your own AI key: what it means covers the basics: what a key is, who you pay and how to keep it safe. This post assumes you know that and are deciding whether to use one.

Download PlexiiDesk free if you want to compare the options inside one app as you read. It's free for personal use on Mac and Windows.

Four reasons people use their own key

1. Cost at higher use. An included allowance is sized for typical use. If you use AI heavily, paying the provider's rates directly can work out cheaper than buying extra credits. If you use it lightly, it usually will not.

2. Control over billing. With your own key, the bill comes from the provider, under an account you control. You can set a spending limit there, see every request, and use an organisation account your finance team already pays for. Anthropic, for example, lets you set your own spend limit (opens in a new tab) in its console under Settings, then Billing.

3. Where your requests go. With your own key, an app can send requests straight from your computer to the provider, without passing through the app maker's servers. For some organisations, that shorter path is a policy requirement. It is worth checking that an app actually does this, rather than relaying your key through its own servers.

4. Choice of provider or model. Some organisations have an agreement with one AI provider. A key lets you use that provider inside the app.

Three reasons not to bother

Light use. If you ask a handful of questions a day, an included allowance probably covers it, and a key adds a bill and a setting to manage.

You do not want to look after a key. A key is a password that spends money. It needs storing safely, rotating if exposed, and adding again on each computer.

You need a feature the app only runs on its own AI. Some apps reserve features for their own service. Check before switching.

Estimating what your own key would cost

Providers charge per token, roughly a short word or part of a word, with separate prices for what the model reads (input) and what it writes (output). Anthropic's published prices (opens in a new tab), per million tokens, are:

An assistant that answers from your documents reads a lot more than it writes: your question, plus the passages it found. So input is usually the larger number.

Here is a worked example. These are assumptions for illustration, not measurements: say each question sends about 6,000 tokens of input and gets back about 600 tokens of answer, and you ask 20 questions a working day, for 21 working days a month.

To check the Sonnet line: 6,000 input tokens at US$3 per million is US$0.018, and 600 output tokens at US$15 per million is US$0.009, so US$0.027 a question.

Your numbers will differ, sometimes a lot. Long documents, long conversations and drafting long text all raise the count. Anthropic's pricing page also notes that Opus 4.7 uses a newer tokeniser than Sonnet 4.6 and earlier models, so the same text can count as more tokens. The point of the exercise is the order of magnitude: is this a few dollars a month, or a few hundred?

Then compare that with what the app includes. If your estimate is well under the allowance, stay on the allowance. If it is well over, a key is worth a look.

Checklist for any BYOK app

Before you paste a key into any app, find the answers to these:

1. Does the request go straight to the provider, or through the app's servers?
2. Where is the key stored, and is it encrypted?
3. Is the key included in backups, exports or sync? Ideally not.
4. Which features use the key, and which use the app's own AI?
5. Can you see usage, so you can compare against the provider's bill?
6. What happens when the key runs out of funds or hits your limit?

What does your work app send off your computer? has more questions in the same spirit, about usage data and sync as well as AI.

How it works in PlexiiDesk

PlexiiDesk gives you four places to run the Plexii assistant and its other AI features, chosen under Settings > AI, in AI · source. See AI settings.

• Plexii AI credits. A monthly allowance is included, listed on the pricing page. One credit is one US cent of AI use. Requests go through PlexiiDesk to Anthropic so they can be counted against your balance. Tick Automatic to switch to your own Anthropic key when credits run out. See AI credits and where your AI runs.
• Your Anthropic key. Always uses your key, never your credits. Requests go straight from your computer to Anthropic.
• Your OpenAI key. Sends requests straight to OpenAI with your key, and lets you choose which OpenAI models handle quick, typical and hardest requests.
• Local on this device. Runs on your computer with a free model downloaded once, about 2.5 GB, on computers with 8 GB of memory or more. It works offline, costs nothing per use and needs no credits or key. It is smaller than cloud AI. In PlexiiDesk's own tests, it scored 79% against 92% for cloud AI on answering questions from your desks and documents, but 17% against 94% on setting up a whole desk with AI. When a request cannot run on your computer, PlexiiDesk asks before sending that one request to cloud AI. See Run AI on this computer with Local on this device.

Under Feature by feature, you can also send single groups of features to a different source, for example tidying on your computer and writing on your key.

Keys are encrypted with your computer's secure storage and kept on that computer. They are not part of your PlexiiDesk account, your backups or your exports, so add them again on each computer you use. A few features always use your own Anthropic key, even when everything else runs on credits, among them What was I doing? and AI Expand in a mind map. The full list, and step-by-step set-up, is in Add your own API keys.

After the first AI request, AI usage on this device shows how many calls this computer has made, the exact tokens sent and received, and an estimated cost at standard rates. That is the number to put into the comparison above, after a typical week of work.

To compare credits and your own key inside one app, download Plexii free.

PlexiiDesk is free forever for personal use. See pricing. Our guide to bringing your own AI key to Plexii sets out how the options fit together, and the glossary entry on bring your own key has the short definition.

A simple way to decide

1. Use the app on its included AI for a normal week.
2. Read the usage, or estimate it with the table above.
3. If you are comfortably inside the allowance, stop there.
4. If you are often over, or your organisation needs its own provider account, add a key, set a spending limit at the provider, and check the first bill against your usage figure.
5. If privacy is the reason, compare a key with an on-device option. A key changes whose account pays and the path the request takes; an on-device model keeps the request on your computer.

Whichever you choose, the answer still needs checking against its sources. Our routine for checking an AI answer takes a couple of minutes. The AI you can check topic page collects the rest of our guides on trustworthy AI.

Common questions

What does bring your own API key mean? You paste an API key from your own account with an AI provider into an app, and the app uses it to run AI features. The provider bills you directly for what you use.

Is using my own API key cheaper? At heavy use it can be. At light use, an app's included allowance is usually cheaper and simpler. Estimate your monthly tokens and compare.

Is my API key safe in a desktop app? It depends on the app. Look for a key stored encrypted on your computer, never included in backups or exports, and requests sent straight to the provider.

How do I limit what my API key can spend? Set a spending limit in your account with the provider. Anthropic's console has one under Settings, then Billing; for other providers, look in their billing settings.

Does using my own key make AI private? It changes whose account the request uses and can shorten its path, but the provider still processes your request. For requests that never leave your computer, use a model that runs on your device.

Can I use an OpenAI key in PlexiiDesk? Yes. Choose Your OpenAI key as the AI source and requests go straight to OpenAI with your key. An OpenAI key is also used for cloud voice transcription and image generation.