Send a message on a specific channel

The channel reference — the same three forms from accepts: a phone number (URL-encode + as %2B), a channel alias, or the exact channel id (ch_...).

Optional key for safe retries. Replaying the same key with an identical body returns the original result; a different body returns 409. Use a unique value (such as a UUID) per logical request.

Plain text message. May be combined with attachments (sent as media with a caption) — except a lone audio file, which is sent as a standalone voice memo and cannot carry a caption. Set format: "markdown" to interpret Markdown styling in this field (and in parts[].text).

How to interpret text (and each parts[].text). Defaults to plain, which sends the string exactly as given. With markdown, four constructs are parsed and delivered as real iMessage rich text — the recipient sees styled text, not delimiters: | Construct | Syntax | | --- | --- | | Bold | **bold** or __bold__ | | Italic | *italic* or _italic_ | | Underline | ++underline++ | | Strikethrough | ~~strike~~ | They nest freely (**bold and _italic_**). Everything else Markdown can express — headings, lists, links, code spans, blockquotes, images — is NOT styling iMessage can carry, so it is passed through as literal characters: [Blooio](https://blooio.com) is delivered with its brackets and URL intact, and # Heading keeps its #. Escape a delimiter with a backslash (\*not italic\*) to send it literally. The styling travels in the message's attributed body, so the stored text and the text returned on reads and webhooks is always the plain string the recipient sees, with the delimiters removed. The Markdown itself comes back as formatted_text, re-serialized into a normalized spelling rather than echoed verbatim (__bold__ returns as **bold**). Only valid on Blooio iMessage channels — 422 format_unsupported_for_channel_type on any other channel type, since no other channel type has a rich-text equivalent and would otherwise deliver your delimiters as literal text. Rich text also requires the message to be delivered over iMessage: a Blooio send that falls back to SMS arrives as unstyled plain text (the text string), because SMS cannot carry styling. Rejected with 422 invalid_content when set without text or parts, when the Markdown source exceeds 20000 characters, or when it compiles to more than 256 distinct formatting ranges.

Public https media URLs. May be combined with text. On Blooio iMessage, two or more image/video URLs are grouped into one photo/video carousel/collage bubble by default; set carousel: false to send them as separate bubbles. **Voice memos:** a single audio file (.mp3, .m4a, .wav, .aac, .opus, .ogg) is automatically sent as a voice memo (the native waveform/scrubber bubble), not a plain audio-file attachment — no extra field is needed. A voice memo is a standalone bubble, so it cannot be combined with text or any other attachment; do that and the send is rejected with 422 invalid_content. To send audio with a message, send the voice memo and the text as two separate requests.

Message id to reply to. Applies to the whole outgoing message.

iMessage screen/bubble effect (Blooio, text only).

Optional attribution badge shown under the bubble: sent_with_siri renders "Sent with Siri" and sent_with_facetime renders "Sent with FaceTime", the way a message dictated to Siri or sent during a FaceTime call appears. Omit for no badge. Any other value is rejected with 422 invalid_content. Applies to text, attachments (media), parts (multipart), rich_link, and app_clip sends on Blooio iMessage channels only; rejected with 422 invalid_content on other content types. No effect on non-Blooio channels.

Group image/video attachments into ONE message that Messages renders as a swipeable photo/video carousel/collage, instead of separate bubbles (Blooio p2p iMessage only). DEFAULTS ON: a Blooio send whose attachments (via attachments or parts) are two or more image/video URLs is grouped automatically — set carousel: false to send them as independent bubbles instead. Set carousel: true explicitly to also enforce the visual-only rule (a known non-visual extension like .pdf, a text part, or fewer than two attachments then returns 422 invalid_carousel); left unset, a non-qualifying send simply isn't grouped. Image extensions: jpg/jpeg/png/gif/heic/heif/webp/bmp/tiff/avif; video: mov/mp4/m4v/qt/3gp/avi/mkv/webm; a URL with no extension is allowed (its real type resolves on the device). An optional text is carried as the carousel's caption. Only valid on Blooio iMessage channels (422 carousel_unsupported_for_channel_type if set on another channel type).

Validate and route only; do not send.

Transport state or resolved wire service; never null. pending = accepted and dispatched, wire service not resolved yet; imessage / rcs / sms = the device-confirmed transport (iMessage blue bubble, RCS, or SMS/MMS green bubble); whatsapp = sent over P2P WhatsApp; whatsapp_business = sent over the WhatsApp Business Platform (WABA); amb = sent over Apple Messages for Business; unknown = accepted but the transport could not be resolved before the tracking window closed (see error).

The resolved sender as a human value: the sending channel's phone number (numbered channels) or alias (non-numbered channels). This is the primary sender reference; channel_id remains available for exact follow-up and audit.

Initial lifecycle state for the send: normally queued (accepted for delivery), or failed if rejected before dispatch. Later transitions (sent → delivered / read, or failed) are reported via the message status endpoint and message.* webhooks.

Existing group id for a group-targeted send.

Hybrid routing phase and optional transition placement.

Recipient identifier. Present only on per-recipient items inside a fan-out (FanOutResult) response.