AF05

The form key you passed starts with __atta:, a namespace Attaform reserves for itself.

In development this reads:

attaform Form key "**atta:checkout" uses the reserved "**atta:" namespace. Use a different prefix — "__atta:" is for library-internal synthetic keys (anonymous useForm() calls without an explicit key).

The production message appends the offending key after the code.

What happened

Anonymous useForm() calls get a generated key in the __atta: namespace so they never collide with yours. A consumer-supplied key inside that namespace could collide with a generated one, silently sharing state between two unrelated forms, so Attaform rejects it at construction.

How to fix it

Rename the key. Anything outside __atta: works: useForm({ schema, key: 'checkout' }).

injectForm covers what form keys are for and how a key resolves.