Errors

Every error uses one envelope, with a matching HTTP status:

{
  "error": {
    "code": "VALIDATION_FAILED",
    "message": "`to` must be a phone number in international format.",
    "details": null
  }
}

Status codes

HTTP	`code`	When you'll see it
400	`VALIDATION_FAILED`	Missing/invalid fields (bad phone, empty body, …).
401	`NOT_AUTHENTICATED`	Missing, invalid, revoked, or wrong-scope API key.
404	`NOT_FOUND`	The template (or other resource) doesn't exist in this workspace.
422	`TEMPLATE_NOT_APPROVED`	The template isn't approved yet.
422	`WINDOW_CLOSED`	Free-form text outside the 24-hour window — send a template instead.
422	`META_ERROR`	WhatsApp rejected the send; `message` forwards Meta's reason.
429	`RATE_LIMITED`	A rate limit was exceeded (e.g. OTP sends per recipient).

Treat any 2xx as success and read the JSON body for ids.
On 401, stop and check the key + its scopes — retrying won't help.
On 429, back off and retry later.
WINDOW_CLOSED is expected for proactive outreach — fall back to a template automatically.