ReplySparrow Unibox
Send a unibox LinkedIn message
Sends a LinkedIn message in a unibox thread, from the seat the thread is on. Provide chat_id and message in the body. chat_id is the unibox thread id returned as id by GET /api/v1/unibox/threads; it is not the LinkedIn conversation id. Needs the inbox:send scope.
Send multipart/form-data to attach a file or a voice message (one of them, up to 15 MB), or application/json { "chat_id", "message" } for text only. message may be left out only when a file or voice message is sent.
Files and voice messages need a seat connected through Unipile.
The message is sent right away (it is not queued) and stored in the thread; the thread's AI draft reply, if any, is discarded.
https://app.replysparrow.com/api/v1/unibox/messages/send-messageAuthorizations
Body
application/jsonResponse
application/jsonMessage sent successfully.
Message id
Thread the message belongs to
LinkedIn seat (account) that received or sent the message
True when your seat sent it
Message text (empty for an attachment-only message)
Files, images and voice notes on the message
When the message was sent (same as delivered_at)
Who sent it
Time LinkedIn gave the message when it was delivered
curl --request POST \ --url https://app.replysparrow.com/api/v1/unibox/messages/send-message \ --header 'Authorization: Bearer <token>' \ --header 'Content-Type: application/json' \ --data '{ "chat_id": "c1d2e3f4-0000-4a51-9d0e-2b6f8f6d9c11", "message": "Great, Tuesday 10am works. Here is the invite link."}'{ "id": "m_1f2e3d", "thread_id": "c1d2e3f4-0000-4a51-9d0e-2b6f8f6d9c11", "seat_id": "acc_42", "sender_full_name": "You", "sender_is_self": true, "content": "Great, Tuesday 10am works.", "attachments": [], "delivered_at": "2026-10-23T15:04:05.000Z", "created_at": "2026-10-23T15:04:05.000Z"}