Add an A/B arm to a campaign
Authorizations
A secret API key. Publishable keys cannot reach this API. A key may carry an expiry, and an expired key is refused exactly like an unknown one, with a 401 that names no reason; check the key's expires_at in the dashboard rather than inferring it from a response. When a merchant rolls a key's secret they choose a grace window of up to 3 days, and for its duration BOTH the new secret and the one it replaced authenticate, so an integration moves over on its own deploy schedule instead of at the instant the button is pressed. Move before the window closes: after it, the old secret is refused. Nothing else about this contract moves with a roll. The key keeps its id and its scopes, so the only thing an integration updates is the credential itself.
Headers
A unique key per logical write. Replaying a request with the same key returns the first response byte for byte instead of applying the write twice.
Path Parameters
Body
Arms may only be changed while the campaign's content is mutable (draft or scheduled); anything else is 409 invalid_transition. Adding one mid-send would reassign the recipients not yet reached and split one campaign's audience across two experiments.
The arm's stable label ('a', 'b', 'control'), at most 32 bytes and unique per campaign. It is what the stats readout groups by and what each message carries, so it is the join key between an arm and its results.
A RELATIVE SHARE, not a percentage: an arm's odds are its weight over the sum of every arm's, so two arms of 1 and 3 split 25/75. Omit the key for an even split. An explicit 0 is 400: the server reads a zero as "no preference" and stores 1, so sending it to disable an arm does the opposite of what you asked.
Response
Created
