Append a message to a session

`POST /sessions/{id}/messages`

Append a message to a session. On the first message the session is lazily created and the initiator is recorded as a participant. A human agent who is not yet a participant may append to an unclaimed (owner-less) cortex warm-start session on a ticket they can access — this claims it (they become its first participant), the discover-and-continue path. Posting to a session already claimed by another human returns 403. Auth rules: - Cookie/JWT (human agent): sender = {id, name, email}, direction = 'in' - Internal token (cortex/NEO): sender = null, direction = 'out'; costUsd, usageJson, model accepted only from the internal caller. The to field is stamped by Mind from the current participant set — never read from the request body.

Path parameters

idstringpathrequired

Session ID (client-minted UUIDv4)

Request body

idstringbodyrequired

Client-minted message UUIDv4

typeenumbodyrequired

Options: text, tool_use, tool_result, result, thinking

contentstringbodyrequired

Raw payload (opaque string or JSON-encoded object)

ticketIdintegerbody

Associated ticket ID (used on first message to set session.ticketId)

attachmentsobject[]body

No description.

createdAtstringbody

Caller-supplied event time (defaults to now if absent)

Request Example

{
  "id": "string",
  "type": "text",
  "content": "string",
  "ticketId": 1,
  "attachments": [
    null
  ],
  "createdAt": "2026-01-01T12:00:00Z"
}

Responses

201 — Message created

Response Example

{
  "success": true,
  "id": "string",
  "sessionId": "string",
  "direction": "in",
  "to": [
    {
      "id": 1
    }
  ]
}

400 — Missing or invalid fields

401 — Unauthorized