Most API deprecations announce themselves. You pin a version, the vendor retires it, and your next call comes back 410 Gone with a link to the migration guide. Annoying, but it's a signal — something in your monitoring goes red, someone gets paged, and the work gets scheduled.
Klaviyo does something different, and it's written plainly in their versioning policy:
If a retired revision date is passed to the request header, Klaviyo falls forward and responds to your request with the same behavior as the next oldest revision.
Read that again with an on-call hat on. Your revision: 2024-07-15 header keeps getting sent. Klaviyo keeps returning 200. Your logs, your traces, your API client's user-agent string — all still say 2024-07-15. But the code path serving you changed.
Revision 2024-07-15 reached its support end date on July 15, 2026. If you're pinned to it — or anything older — you've been served 2024-10-15 behavior since then. And 2024-10-15 itself retires on October 15, 2026, which is a sharper cliff than the one that already passed.
Why fall-forward is worse than a hard failure
The trade Klaviyo made is deliberate and defensible: don't break integrations that stopped being maintained. The cost is that "still works" and "still works correctly" stop being the same thing, and nothing in the response distinguishes them.
Klaviyo is direct about this in the same doc — relying on fall-forward is "likely to introduce unwanted, breaking changes in your app." There's a partial exception worth knowing: calls to endpoints that were removed after your pinned revision do return a 410. So endpoint removals are loud. Behavior changes to endpoints that still exist are silent. That asymmetry is the whole problem, because the July 15 cliff was entirely the second kind.
Cliff one (already fired): profile identity resolution
The breaking change in 2024-10-15 — the behavior you're getting now if you pinned 2024-07-15 — is on the Profiles API. Klaviyo's changelog describes it as updating the profile endpoints to an enhanced identity resolution processor that better follows Klaviyo's identifier priority order. It affects Create Profile and Update Profile.
Identity resolution decides which existing profile your write lands on. Klaviyo's precedence runs roughly: Klaviyo ID, then external_id, then email, then phone_number, then anonymous/device ID. When you POST a profile carrying more than one identifier, that order determines whether you update an existing record, merge two records, or create a new one.
Change the matching processor and you change the destination of a write that returns an identical 200 either way. Concretely:
1. Multi-identifier writes can land on a different profile. The classic payload from a store sync carries both a customer ID and an email:
{
"data": {
"type": "profile",
"attributes": {
"external_id": "cust_88213",
"email": "sam@example.com",
"properties": { "lifetime_value": 412.00 }
}
}
}
If external_id and email point at different existing profiles — and in real catalogs they often do — the resolution order decides the winner. Stale or reused external IDs are extremely common: guest checkouts that later register, customer records rebuilt during a platform migration, staging IDs that leaked into production, ID namespaces merged after an acquisition. When the tiebreak changes, lifetime_value: 412.00 gets written to a different human being than it did in June. Status: 200.
2. Writes that used to create now merge (and overwrite). If the new processor matches an identifier the old one didn't, a call that previously created a fresh profile now updates an existing one — and custom properties on that existing profile get overwritten by your payload. Your "new profiles created today" metric drops, which reads like a top-of-funnel problem, not an API problem.
3. Consent state can land on the wrong profile. This is the one to care about. Subscribe and unsubscribe writes resolve to a profile like any other write. A resolution change that redirects a consent write is a compliance problem wearing a 200 OK: someone who unsubscribed keeps receiving email because the suppression landed next door.
4. Your billable profile count moves. Klaviyo bills on active profiles. Duplicates and merges both shift that number, and the shift shows up on an invoice weeks after the code that caused it shipped — long past the point where anyone connects the two.
None of these throw. There is no error rate to alert on. The only observable is data that is quietly wrong.
Cliff two (October 15, 2026): the one to schedule work for
On October 15, 2026, 2024-10-15 retires and anyone pinned to it falls forward to 2025-01-15. That revision carries three changes, and they fail in different ways:
Offset pagination becomes cursor pagination on Get Messages for Flow Action and Get Action ID for Flow Message. This is the silent-truncation shape that has bitten teams on Jira, monday.com, and Contentful this year. The canonical loop:
let offset = 0;
const all = [];
while (true) {
const res = await getMessagesForFlowAction({ page_offset: offset });
all.push(...res.data);
if (res.data.length < PAGE_SIZE) break; // "short page = done"
offset += PAGE_SIZE;
}
Two independent ways this loses data under cursor pagination: the offset parameter stops steering the result set, and a non-final cursor page is not guaranteed to be full, so the short-page heuristic exits early. You get a plausible-looking partial list of flow messages. If that list feeds a content audit or a template-sync job, the missing tail is invisible — you can't miss messages you never learned existed.
The Campaigns API restructures for push notification support. 2025-01-15 changed the request and response structure across Get Campaigns, Get Campaign, Create Campaign, Update Campaign, Create Campaign Clone, and the campaign-message endpoints. Reporting code that reaches into a nested field by path gets undefined rather than an exception — and undefined propagates into a dashboard as a zero or a blank far more often than it propagates as a stack trace.
Bulk Subscribe/Unsubscribe Profiles now require the subscriptions field. This one is loud — a missing required field is a 4xx. Good. It's also the only one of the three you'll find out about on day one.
So the October cliff is a mix, and the loud member of the set is the least dangerous.
What to actually do
The audit is small and it beats reading the changelog diff by hand:
1. Find every pinned revision. Grep for the header across your services, your serverless functions, your ETL jobs, and — the one people forget — your official Klaviyo SDK version, which pins a revision internally:
grep -rn --include='*.{js,ts,py,rb,php,go,java}' -e 'revision' -e '2024-0' -e '2024-10-15' .
Check the third-party middleware too. iPaaS connectors, reverse-ETL tools, and agency-built integrations all pin revisions, and none of them will tell you when one retires.
2. Sort what you find into three buckets. Anything at 2024-07-15 or older is already being served newer behavior — that's not future work, that's a live audit of the last two weeks of profile writes. Anything at 2024-10-15 has until October 15. Anything 2025-01-15 or newer has runway.
3. For the already-fired group, look at the data, not the logs. The logs are clean by construction. Query for profiles created since July 15 that duplicate an existing external_id or email, and diff your daily new-profile count across the July 15 boundary. A step change on that date is your answer.
4. Pin deliberately, then monitor the pin. Pinning a revision is correct — it's what the header is for. What breaks teams is pinning and then never revisiting, so the pin silently becomes a lie about which behavior you're getting. A retirement date is a known future date. Put it in the calendar the day you pin.
The general shape
Fall-forward versioning is spreading — Klaviyo, monday.com, Contentful, and Xero have all shipped some flavor of it in the last year. The appeal to a vendor is obvious: nobody's integration goes dark on a Tuesday.
But it converts a hard failure into a data-quality failure, and it moves detection from your error budget to whoever eventually notices the numbers look wrong. The version header stops being a contract and becomes a preference — one the server is free to ignore once the date passes, while still echoing it back to you.
The defensive move isn't to stop pinning. It's to treat every version header in your stack as something with an expiry date attached, and to assert on the shape of what comes back rather than trusting that a 200 means the contract held.
I write about API drift — the changes that keep returning 200 while quietly changing what your integration receives. FlareCanary monitors API responses and schemas for exactly this class of change.
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