Connecting a WordPress form to Kit (formerly ConvertKit)

ConvertKit renamed itself Kit in 2024 and moved to a v4 API, and v4 API keys don't work against the old v3 API. Plugins built for v3 keep working only as long as v3 does and your old key stays valid. A current integration uses a v4 key, adds each person as a subscriber, then adds them to a tag or a Kit form.

What changed with Kit

The API moved from v3 to v4, with new endpoints and a new way of sending the key.
Keys created for v4 are refused by the v3 API, so an old plugin given a new key simply fails.
Subscribers are created or updated by email address, then attached to a tag or a form, which is how Kit starts sequences and automations.

How Mailbag Pro sends to Kit

Mailbag Pro, the paid add-on to Mailbag Contact Form, uses Kit's v4 API with a key from your own account. For each form you choose whether entries go to a Kit tag or a Kit form, map the first name and any custom fields, and optionally send only when the consent box is ticked.

A returning visitor updates their existing subscriber instead of creating a new one. If Kit refuses an address as invalid, that's final: Pro doesn't retry it, and the entry shows Kit's reason.

When the key stops working

If Kit starts answering “unauthorised” — a key deleted, or a teammate regenerating it — Pro marks the connection as failing, keeps every entry, emails you at most once a day, and sends the backlog once you paste a working key. Nothing typed into your form is lost in the meantime, because the free plugin has already saved it in WordPress.

Why an integration needs upkeep

The Kit rename is a good example of why a form integration isn't “set and forget”: the service moves, and a plugin that doesn't follow keeps failing in the same quiet way. Keeping up with changes like this is what a Mailbag Pro licence pays for.

This is Mailbag Pro, the paid add-on to the free Mailbag Contact Form plugin. See what it costs, and what happens if you ever stop paying.

Mailbag Pro and pricing