Digital Engagement API 400 Bad Request on Message Send Payload

Can’t get this config to load properly… The Genesys Cloud Digital Engagement API (/api/v2/interactions/message) is rejecting our outbound message payload with a 400 Bad Request error. The validation fails on the to address format despite matching the documentation for SMS gateways.

{
 "to": [{"address": "+15550199834", "type": "sms"}],
 "from": [{"address": "+15550199833", "type": "sms"}],
 "content": [{"text": "Test message from AppFoundry widget"}]
}