If you're only using it for two-factor authentication, you don't need a PIN. But when I tried to registered mine as a passkey (passwordless authentication), my browser prompted me for a PIN. I didn't have one set at the time, so it kept rejecting whatever PIN I gave it. I had to use the YubiKey Manager to set a PIN before I could register it as a a passkey.

https://www.yubico.com/support/download/yubikey-manager/

You can also manage this from within Chrome (Privacy and security -> Security -> Manage security keys).

I use YubiCo Authenticator for TOTP via my YubiKey, and have a PIN setup due to that. Quite nice really, I imagine it's the same PIN you're talking about? I've not used it as a passkey yet

Yubico sells Yubikeys where are smartcard devices loaded with several apps (keyboard emulation OTP, GPG, PIV card, and FIDO 2).

They also sell cheaper security keys, which are purpose-built for FIDO 2 only.

When someone says they are using a passkey with a Yubico device, they are talking specifically about the FIDO 2 functionality. This does not (at least currently) support import or export - partially because they want these devices to be sold in regulatory environments where hardware-bound and non-cloneable credentials are required.

Are you sure you have a YubiKey (e.g. a "5 Series"[1]) and not a YubiCo "Security Key"[2]? The latter is a less expensive device with less functionality[3], though still good for arguably the most common 2FA situations.

[1]: https://www.yubico.com/products/yubikey-5-overview/

[2]: https://www.yubico.com/products/security-key/

[3]: https://www.yubico.com/store/compare/

The Security Key models have an access PIN just the same.

It's optional and can be required by the service. Services like Microsoft that use security keys as a single factor rather than as a MFA are more likely to require it.

Something I noticed after reading this thread is that Google basically converted my YK from a 2FA security key to a Passkey. I'm so confused.

Now that is annoying, and I wish companies would not do that without asking.

Yes, you need to use `ykman` to set a PIN. This also allows some services (really only Microsoft Accounts right now) to use "passwordless".

The idea is you register 2 or 3 passwordless keys on important accounts. Keep one in the machine, one on your physical keychain, and one in a remote location.