How to Let Users Bring Their Own API Key

To let users bring their own API key, you need to decide one thing first: whether you store the key at all. Everything else follows from that. Session-only keys are the safest and the most annoying, encrypted storage is the usual answer and carries real custody obligations, and a proxy with your own key is not actually bring-your-own-key however it is marketed. Pick deliberately, because retrofitting a different choice later means asking every user to rotate.

Why Let Users Bring Their Own API Key

Two honest reasons and one bad one.

The good reasons: it moves the variable cost of inference off your balance sheet, which lets you charge a flat price without modelling usage, and it satisfies users who want their data going to their own provider account under their own terms. For a developer-facing tool, the second reason often matters more than the first.

The bad reason is using it to avoid thinking about unit economics. If you do not know what an AI feature costs per user, BYOK does not answer the question, it just moves the surprise to someone else and makes churn harder to explain.

The Storage Decision

Most products land on encrypted at rest because anything running in the background, scheduled reports, webhooks, retries, needs the key when the user is not present. Accept what that means: you are holding credentials that can spend someone else's money.

Storing It Properly

The rules are short and non-negotiable. Encrypt with a key from a managed service or a secret your application loads at runtime, never a constant in the repository. Store only the ciphertext plus enough plaintext to identify the key to its owner, which means the last four characters and nothing more. Never return the key to the client after it is saved, not even to the user who supplied it: render the masked form and offer replacement instead.

// Storing: encrypt, keep only a display hint.
async function saveUserKey(userId, apiKey) {
  const valid = await probeKey(apiKey);          // see below
  if (!valid.ok) return { error: valid.reason };

  await db.userKeys.upsert({
    userId,
    ciphertext: await kms.encrypt(apiKey),       // managed key, not a constant
    last4: apiKey.slice(-4),                     // the ONLY plaintext kept
    provider: valid.provider,
    verifiedAt: new Date(),
  });
  return { ok: true, hint: '...' + apiKey.slice(-4) };
}

// Reading: decrypt late, never log, never widen the scope.
async function withUserKey(userId, fn) {
  const row = await db.userKeys.findOne({ userId });
  if (!row) throw new MissingKeyError();
  const key = await kms.decrypt(row.ciphertext);
  try {
    return await fn(key);
  } finally {
    // no cleanup ceremony needed, but do not assign it anywhere outer
  }
}

The OWASP secrets management cheat sheet is the reference worth reading before you design this, particularly on key rotation and on keeping secrets out of environment dumps.

Validate the Key Before You Trust It

Users paste the wrong thing constantly: an expired key, a key for a different provider, a key with no billing attached, or the whole line including the variable name. Validate at save time with the cheapest call the provider offers, typically a models list endpoint, which costs nothing and confirms both that the key is live and which provider it belongs to.

Re-validate lazily rather than on a schedule. A key that worked yesterday can be revoked today, so treat an authentication failure during normal use as a signal to mark the key invalid and prompt the user, rather than retrying it into a rate limit.

The Leak Paths That Actually Cause Incidents

Nobody loses keys through their encryption. They lose them through the boring edges.

1. Logs. A request logger that serialises headers or bodies will capture the key the first time you debug something at three in the morning. Redact at the logger, not at the call site, because the call site will be forgotten.

2. Error messages. Provider SDKs sometimes include the request configuration in thrown errors. If that propagates into your error tracker or, worse, into an API response, the key is now in a third-party system with its own retention policy.

3. Support. The moment a human asks a user to paste their key into a chat to help debug, you have created an unencrypted copy in a ticketing system. Decide the policy before it comes up, and make it never.

4. Client-side exposure. If the browser needs the key, it is in the browser, and no amount of obfuscation changes that. Either accept session-only storage or keep all provider calls server-side.

Worth reading alongside what happens when an API bill goes wrong, since with BYOK that bill lands on a user who did not choose your retry logic.

The Part of the UI That Decides Whether This Works

BYOK fails on adoption far more often than on engineering. A user who cannot find their provider's key page, or who does not know a payment method is required before a key will work, abandons the setup screen and never returns. Most of the fix is copy rather than code.

Link directly to the provider's key creation page rather than to their homepage. State the prerequisite plainly, because on most providers a key without billing attached authenticates fine and then fails on the first real call, which is a confusing failure to debug from the outside. Show the expected prefix so a user who pastes the wrong credential finds out immediately. And validate on blur rather than on submit, so the error appears while they still have the provider's tab open.

One more thing worth building early: a visible connection status with the date it was last verified. Users forget which key they connected and whether it is still live, and a status line answers most support messages before they are sent.

Testing It Without Real Keys

Your test suite must never carry a live provider key, and your staging environment should not either. Point both at a fake provider: a small server that speaks the same shapes and returns canned responses, plus deliberate failures for the cases that matter, an invalid key, a revoked key, a rate limit and a timeout.

Those four failures are the ones that actually happen in production and the ones nobody tests, because with a working key in hand they are awkward to trigger. Simulating them is a morning's work and it is where BYOK bugs concentrate.

What Changes Operationally

Rate limits become the user's, which means one user's exhausted quota is now a support ticket rather than a capacity problem. Surface provider rate limit errors as themselves rather than as a generic failure, and keep your own limits in place anyway so a runaway loop cannot burn through someone's monthly budget in an afternoon.

Key ownership also needs a home in your data model. A key belongs to an account and not a session, it needs an audit trail of who added and removed it, and on team plans you need to decide whether one key serves the workspace or each member brings their own. That decision belongs with how you model accounts generally, and changing it later is a migration.

Deletion deserves its own line. When a user disconnects a key or closes their account, the ciphertext goes immediately, not on a nightly job. Say so in your privacy policy, and make it true. The rest of the build sits on the usual foundations.

Frequently Asked Questions

Can I support BYOK and my own key together?

Yes, and it is a common shape: a free tier on the user's key, a paid tier on yours. Keep the code paths identical apart from key selection, otherwise the less-used path quietly rots and breaks for whichever set of users you tested least.

Should I let users see their saved key?

No. Show the last four characters and a replace option. A reveal feature adds no capability the user does not already have, since they can retrieve or regenerate it from the provider, while adding a straightforward path to exfiltration.

What if the user's key has access to more than my app needs?

Usually it does, and you cannot fix that from your side. Where the provider supports scoped or project-restricted keys, say so in the UI and link the instructions. Asking for the narrowest workable key is the one meaningful mitigation available to you.

Does BYOK remove my liability for what the model outputs?

It does not. Whose key paid for the tokens is a billing detail. Your product still shaped the prompt and presented the output, so your terms and your safeguards carry the same weight they did before.