Adds a message to a ticket conversation. Requires scope messages:write (tickets:write alone does not grant it), and the endpoint must be enabled for your MSP — otherwise it returns 403 FEATURE_NOT_AVAILABLE.

Content rules. body is capped at 64 KiB of UTF-8 and validated against body_format. Unsafe or unsupported markup is rejected, never silently stripped, so an accepted message is stored as submitted apart from link hardening (rel="noopener noreferrer") and, for text bodies, HTML escaping. The 422 responses distinguish UNSAFE_CONTENT_DETECTED, INVALID_MESSAGE_FORMAT, EMPTY_MESSAGE_BODY and MESSAGE_TOO_LARGE.

Duplicates. Supports X-Idempotency-Key (replays return the original message with X-Idempotent-Replayed: true). Separately, reusing an external_ref returns 409 DUPLICATE_MESSAGE — unlike ticket creation, it does not replay, because a message cannot be read back individually.

Notification of the customer is automatic for public messages and governed by that customer's notification settings; it cannot be requested or suppressed per message.

Note: request-validation failures return 422 VALIDATION_ERROR, the convention across this API.