2-Way Messaging with RCS

Prelude supports 2-way messaging over RCS, so your application can receive the texts users send to your RCS agent and answer them. Replies leave from the agent that received the message, as plain RCS texts.

2-way messaging works on an RCS agent that is yours, launched for your brand. Messages sent to a shared Prelude agent are not forwarded. The agent must be a transactional, promotional or multi-use agent: an OTP agent cannot send free text, so it takes no part in 2-way messaging. Ask your Customer Success Manager to enable 2-way messaging on your agent and to register the callback URL that receives its messages.

How it works

User sends an RCS message

A user writes to your agent from a conversation your agent opened.

Prelude receives and forwards the message

Prelude stores the message and sends an inbound.message.received webhook event to the callback URL registered for your agent. The event includes the message text, the sender’s phone number, your agent identifier, and a message ID you can use to reply.

You send a reply

Your application calls the Reply to an inbound message endpoint with the to, text, and reply_to fields to send a reply.

User receives a reply

The user sees your reply as an RCS text from your agent.

Reply timing

RCS has no reply window: you can reply to an inbound message at any time, and the reply leaves from the agent that received it. Unlike WhatsApp, a reply sent more than 24 hours after the user’s latest message is not rejected.

Reserved keywords

STOP, START and HELP are handled by Prelude and are not forwarded to your callback URL. A user who sent STOP is unsubscribed from your agent until they send START. A reply to an unsubscribed user is not sent: the message fails with the reason unsubscribed.

Sending a reply

To reply to an inbound message, use the dedicated Reply to an inbound message endpoint:

{
  "to": "+33612345678",
  "text": "Thanks for reaching out! We'll look into your request.",
  "reply_to": "im_01k8aq2zggeyssvt53zgvpx63a"
}

string

required

The recipient’s phone number in E.164 format. Must match the phone number that sent the original inbound message.

string

required

The reply message body, sent as a plain RCS text from the agent that received the message.

string

required

The inbound message ID (prefixed with im_) to reply to. This ID is provided in the inbound.message.received webhook event.

The to field must match the phone number that sent the original inbound message. A phone_number_mismatch error is returned otherwise.

SDK examples

const response = await client.notify.reply({
  to: "+33612345678",
  text: "Thanks for reaching out! We'll look into your request.",
  reply_to: "im_01k8aq2zggeyssvt53zgvpx63a",
});

response = client.notify.reply(
    to="+33612345678",
    text="Thanks for reaching out! We'll look into your request.",
    reply_to="im_01k8aq2zggeyssvt53zgvpx63a",
)

response, err := client.Notify.Reply(context.TODO(), prelude.NotifyReplyParams{
    To:      prelude.F("+33612345678"),
    Text:    prelude.F("Thanks for reaching out! We'll look into your request."),
    ReplyTo: prelude.F("im_01k8aq2zggeyssvt53zgvpx63a"),
})

Receiving inbound messages

When a user sends a text to your agent, Prelude forwards it as an inbound.message.received webhook event to the callback URL registered for that agent.

Webhook event

{
  "id": "evt_01k8aq2zggeyssvt53zgvpx63a",
  "type": "inbound.message.received",
  "payload": {
    "message_id": "im_01k8aq2zggeyssvt53zgvpx63a",
    "channel": "rcs",
    "from": "+33612345678",
    "to": "acme_agent",
    "content": {
      "text": "Hi, I have a question about my order"
    },
    "conversation": {
      "window_expires_at": "2025-10-25T08:59:10.736909995Z"
    }
  },
  "created_at": "2025-10-24T08:59:10.736925502Z"
}

string

The unique inbound message identifier (prefixed with im_). Use this value in reply_to to send a reply.

string

The channel the message was received on: rcs.

string

The sender’s phone number in E.164 format.

string

The identifier of your RCS agent that received the message.

string

The text content of the inbound message.

RFC3339 date string

24 hours after the message. On RCS it is for information only: a reply sent after this time is still delivered.

Error handling

The following errors are specific to 2-way messaging:

Error Code	Description	Action
`invalid_reply_context`	The inbound message ID was not found	Verify the `reply_to` message ID
`reply_not_supported`	The agent that received the message takes no part in 2-way messaging: an OTP agent, or no callback URL is registered	Ask your Customer Success Manager to enable 2-way messaging on a transactional, promotional or multi-use agent
`phone_number_mismatch`	The `to` number does not match the inbound message sender	Use the phone number from the inbound webhook event
`missing_text`	The `text` field is empty or missing	Provide a reply message body

Limitations

Only text messages are forwarded. Other content a user sends is not.
Replies are sent as plain RCS texts, not as rich cards.
2-way messaging is only available on your own RCS agent, not on shared Prelude agents, and not on OTP agents.

Documentation

API Reference

2-Way Messaging with RCS

How it works

Reply timing

Reserved keywords

Sending a reply

SDK examples

Receiving inbound messages

Webhook event

Error handling

Limitations

​How it works

​Reply timing

​Reserved keywords

​Sending a reply

​SDK examples

​Receiving inbound messages

​Webhook event

​Error handling

​Limitations

How it works

Reply timing

Reserved keywords

Sending a reply

SDK examples

Receiving inbound messages

Webhook event

Error handling

Limitations