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Send a message

POST /v1/sessions/{sessionId}/messages
Send a follow-up message to the agent. The agent receives the message in the same sandbox with full conversation context. Returns immediately — the agent works asynchronously and emits events via the SSE stream.
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message
string
required
The user’s message
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user_id
string
required
Must match the session owner

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Example

curl -X POST https://api.opencomputer.dev/v1/sessions/session_abc123/messages \
  -H "Content-Type: application/json" \
  -H "X-API-Key: osb_your_api_key" \
  -d '{
    "message": "Add a dark mode toggle to the header",
    "user_id": "user_123"
  }'

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Response 200

Returns the full session object (same shape as create). 
{
  "id": "session_abc123",
  "status": "running",
  "sandboxId": "sb-31317626",
  "previewUrl": "https://sb-31317626-p3000.preview.opencomputer.dev",
  "project": {
    "title": "Build a todo app",
    "framework": "nextjs",
    "artifacts": []
  },
  "messages": [
    {
      "id": "msg_xxx",
      "role": "user",
      "content": "Build a todo app with Next.js and Tailwind",
      "createdAt": "2026-03-18T17:00:00.000Z"
    },
    {
      "id": "msg_yyy",
      "role": "user",
      "content": "Add a dark mode toggle to the header",
      "createdAt": "2026-03-18T17:05:00.000Z"
    }
  ],
  "events": [],
  "userId": "user_123",
  "createdAt": "2026-03-18T17:00:00.000Z",
  "updatedAt": "2026-03-18T17:05:00.000Z"
}