Recurring RP_9 / RP_10 Error Loop on v3 Distributions API (XM Directory)

Hi everyone,

I am a Brand Admin attempting to trigger an Individual Distribution via the /v3/distributions POST endpoint using Google Apps Script. I am stuck in a "Schema Loop" where the API rejects my payload regardless of structure.

The Loop:

1. If I use a nested structure (with recipients, message, and header objects), I get:

   • 400 Bad Request: Expected param not found: sendDate (RP_10)

2. If I then add sendDate at the root level (ISO 8601 format), I get:

   • 400 Bad Request: Unexpected json key provided: sendDate (RP_9)

3. If I remove the action: CreateDistribution key and try a flat transactional structure, I get:

   • 400 Bad Request: Expected param not found: sendDate (RP_10)

My Environment:

• Engine: XM Directory (Transactional)

• Account: Brand Administrator

• Auth: X-API-TOKEN

• Endpoint: https://[DataCenter].qualtrics.com/API/v3/distributions

My Payload (Standard attempt):

{
  "action": "CreateDistribution",
  "surveyId": "SV_XXXXXXXXX",
  "linkType": "Individual",
  "sendDate": "2026-02-20T18:00:00Z",
  "recipients": {
    "contactId": "CID_XXXXXXXXX",
    "mailingListId": "CG_XXXXXXXXX"
  },
  "message": {
    "libraryId": "UR_XXXXXXXXX",
    "messageId": "MS_XXXXXXXXX"
  }
}

Question: Is there a specific brand-level setting or a hidden mandatory parameter (like expirationDate or a specific sendDate format) that causes the API to flip-flop between these two errors? Is my brand forced into a specific Transactional vs. Legacy validation engine?