si_send_message

si_send_message is the AdCP task for exchanging messages in an active SI session. Relay user text or action responses to the brand agent and handle active, pending_handoff, or complete session states.

Experimental. Sponsored Intelligence (si_get_offering, si_initiate_session, si_send_message, si_terminate_session) is part of AdCP 3.0 as an experimental surface — it may change between 3.x releases with at least 6 weeks’ notice. Sellers implementing any of these tasks MUST declare sponsored_intelligence.core in experimental_features. See experimental status for the full contract.

Send a message within an active SI session. The host invokes this task to relay user messages and action responses to the brand agent.

Request

Field	Type	Required	Description
session_id	string	Yes	Session ID from si_initiate_session
message	string	No	User’s text message
action_response	object	No	Response to a UI action (button click, form submit)

At least one of message or action_response must be provided.

Action Response Object

When the user interacts with a UI element:

Field	Type	Required	Description
action	string	Yes	The action identifier from the UI element
element_id	string	No	ID of the specific UI element
payload	object	No	Additional data from the interaction

Response

Field	Type	Description
session_id	string	Confirms the active session
response	object	Brand agent’s response
session_status	string	Current session state
handoff	object	Present when session_status is “pending_handoff”

Session Status Values

Status	Description
active	Session continues normally
pending_handoff	Brand agent signals readiness to hand off
complete	Conversation is done

Handoff Object

When session_status is pending_handoff:

Field	Type	Description
type	string	”transaction” or “complete”
intent	object	For transactions: what the user wants to buy
context_for_checkout	object	Summary for ACP handoff

Examples

Simple Message Exchange

Request:

{
  "session_id": "sess_abc123",
  "message": "Do you have any earlier flights?"
}

Response:

{
  "session_id": "sess_abc123",
  "response": {
    "message": "Yes! There's DL628 departing at 5:30 AM. It's a bit earlier but also qualifies for the Premium Economy upgrade.",
    "ui_elements": [
      {
        "type": "carousel",
        "data": {
          "items": [
            {
              "title": "DL628 - 5:30 AM",
              "subtitle": "Arrives 8:57 AM",
              "price": "$199",
              "badge": "Free Upgrade"
            },
            {
              "title": "DL632 - 6:15 AM",
              "subtitle": "Arrives 9:42 AM",
              "price": "$199",
              "badge": "Free Upgrade"
            }
          ]
        }
      }
    ]
  },
  "session_status": "active"
}

Action Response (Button Click)

Request:

{
  "session_id": "sess_abc123",
  "action_response": {
    "action": "select_flight",
    "payload": {
      "flight_number": "DL628",
      "departure_time": "05:30"
    }
  }
}

Response:

{
  "session_id": "sess_abc123",
  "response": {
    "message": "Great choice! DL628 is confirmed with your Premium Economy upgrade. Ready to book?",
    "ui_elements": [
      {
        "type": "product_card",
        "data": {
          "title": "DL628 to Boston",
          "subtitle": "Tue Jan 27, 5:30 AM → 8:57 AM",
          "price": "$199",
          "badge": "Premium Economy",
          "cta": { "label": "Book Now", "action": "checkout" }
        }
      }
    ]
  },
  "session_status": "active"
}

Transaction Handoff

When the user is ready to purchase:

Request:

{
  "session_id": "sess_abc123",
  "action_response": {
    "action": "checkout"
  }
}

Response:

{
  "session_id": "sess_abc123",
  "response": {
    "message": "Perfect! I'll hand you back to complete the booking."
  },
  "session_status": "pending_handoff",
  "handoff": {
    "type": "transaction",
    "intent": {
      "action": "purchase",
      "product": {
        "type": "flight",
        "flight_number": "DL628",
        "departure": "2026-01-27T05:30:00-05:00",
        "arrival": "2026-01-27T08:57:00-05:00",
        "origin": "JFK",
        "destination": "BOS",
        "class": "premium_economy"
      },
      "price": {
        "amount": 199,
        "currency": "USD"
      }
    },
    "context_for_checkout": {
      "conversation_summary": "Jane selected DL628 JFK→BOS on Jan 27 with free Premium Economy upgrade via campaign offer",
      "applied_offers": ["delta_chatgpt_3313"]
    }
  }
}

Handling Handoffs

When you receive session_status: "pending_handoff":

1. For type: "transaction" - Initiate ACP checkout with the provided intent and context
2. For type: "complete" - The conversation is done; return to normal chat

The host should call si_terminate_session after handling the handoff to properly close the session.

Key Points

1. Message or action_response - Each request needs at least one. Users can type messages or interact with UI elements.

2. Session status drives flow - Check session_status on every response to know if the conversation continues or needs handoff.

3. Handoff preserves context - The context_for_checkout object gives ACP everything needed for a seamless purchase experience.

4. UI elements are optional - Brand agent decides when to include cards, carousels, etc. based on the conversation.