Answer
What are Klaviyo's Extended ID and First-Party ID, and should I turn them on?
They're Klaviyo's options for keeping returning visitors recognized when browsers limit its regular cookie. Yes, start by turning on Extended ID, which is on all Klaviyo plans, free and paid. First-Party ID adds a server-set cookie that restores Klaviyo's regular cookie for people with an existing profile. Neither carries recognition or a cart to another device.
October 31, 2026
Klaviyo's Extended ID and First-Party ID are options for keeping returning visitors recognized when browsers limit its regular cookie. Yes, turn on Extended ID first, because it's available on every plan. Then check whether First-Party ID suits your setup.
Klaviyo's documentation explains how Extended ID works. When a shopper is identified, for example by clicking an email, Klaviyo links identifiers that other platforms such as ad and social tools have set in that browser. On a later visit, those identifiers can re-identify the shopper and refresh Klaviyo's cookie without a new click. The shopper needs an existing Klaviyo profile.
What's the difference?
| Extended ID | First-Party ID | |
|---|---|---|
| Availability | All Klaviyo plans, free and paid | Check your plan and setup |
| What it does | Extends recognition beyond the regular cookie | Sets __kle_id, a server-set cookie that restores __kla_id |
| Who it helps | Visitors Klaviyo can already recognize | Only people with an existing profile |
| Crosses devices | No | No |
What problem do they solve?
Klaviyo's regular cookie, __kla_id, is set by JavaScript for 2 years, but browsers can shorten that. Safari deletes script-set cookies after 7 days of browser use without interaction with the site, as How long cookies last in Safari explains. When the cookie disappears, a known customer can look like a new visitor, and Klaviyo can miss them.
A server-side cookie such as __kle_id isn't covered by WebKit's rule on script-set storage. It can then restore the regular cookie.
Where do the exceptions apply?
Klaviyo's documentation gives these limits for server-set cookies:
- Safari: the 7-day cap applies when a subdomain points to a cross-site host.
- Brave: server-set cookies last 6 months at most, and JavaScript-set ones 7 days.
- Chrome: cookies can last up to 400 days.
What won't they do?
Both options work inside one browser. They can help an email get triggered for a known shopper returning on the same device. They can't bring a cart to a phone, so a shopper who built their cart on a laptop can still reach an empty cart after clicking on their phone.
Where does reOtter fit?
reOtter's recognition is separate from these settings. It recognizes people who've already given your brand their details, without a third-party network, in a record that doesn't depend on a browser cookie surviving. Its cart recovery page restores a cart when reOtter holds it and knows who clicked. Carts are held only with analytics consent from your store's banner.
Key takeaways
- Turn on Extended ID first, since every Klaviyo plan includes it.
- First-Party ID restores Klaviyo's cookie for people with an existing profile, with exceptions in Safari and Brave.
- Neither option crosses devices or changes where a cart email button lands.
See how reOtter restores the cart → Abandoned cart recovery
Frequently asked questions
- Is Klaviyo's Extended ID free?
- Klaviyo's documentation says Extended ID is on all plans, free and paid. That makes it the sensible first step before paying for anything else. Check your account settings for the current steps.
- Does First-Party ID identify anonymous visitors?
- It doesn't. Klaviyo says it identifies only people with an existing profile. Someone who has never shared their details remains unknown.
- Does First-Party ID get around Safari's 7-day limit?
- It sits outside the limit on JavaScript-set cookies because it's set by a server. Klaviyo notes that Safari's 7-day cap still applies when a subdomain points to a cross-site host. So the setup matters as much as the feature.
- Will these options fix empty carts from email clicks?
- They won't fix the cross-device case. Both work inside one browser, and a cookie never crosses devices. An empty cart on a phone comes from the destination of the email link and where the cart was saved.