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Working with Passthru Applet for AgentStream (Beta)

Beta Feature

The Passthru Applet for AgentStream is currently in Beta. Features and behavior may change.

Overview​

The Passthru Applet sends call and streaming metadata from Voice Applet flows to your server, especially useful when executing:

  • Voicebot Applet flows
  • Stream Applet flows

It enables your backend to receive metadata about the current call and streaming session, allowing you to coordinate your streaming server with the call flow.

How It Works​

When a call reaches the Passthru Applet in your call flow, Exotel makes an HTTP POST request to the server URL you configure. The POST body contains a JSON payload with identifiers and context for the current call and stream. Your server responds with a 200 OK, and the call flow continues to the next applet — typically the Stream or Voicebot Applet.

This gives your server a window of time to prepare before the WebSocket audio connection opens. Common uses include:

  • Allocating a session or loading a language model in advance
  • Storing the StreamSid so you can correlate WebSocket events to call records
  • Triggering a downstream service (CRM lookup, agent assignment) before audio begins

Metadata Payload Fields​

Exotel sends the following fields in the HTTP POST body:

FieldTypeDescription
CallSidstringUnique identifier for the call leg
StreamSidstringUnique identifier for the streaming session
StreamUrlstringThe WebSocket URL Exotel will connect to for streaming
AccountSidstringYour Exotel Account SID
EventTypestringAlways stream-start for the Passthru Applet trigger
FromstringThe caller's phone number (E.164 format)
TostringThe Exotel virtual number that received the call
Directionstringinbound or outbound

Sample JSON Payload​

{
  "CallSid": "a1b2c3d4e5f6a1b2c3d4e5f6a1b2c3d4",
  "StreamSid": "MZ1234567890abcdef1234567890abcdef",
  "StreamUrl": "wss://yourserver.example.com/stream",
  "AccountSid": "your_account_sid",
  "EventType": "stream-start",
  "From": "+919876543210",
  "To": "+918041234567",
  "Direction": "inbound"
}

Your server must respond with HTTP 200 OK. Any non-200 response or a timeout will cause the call flow to halt at the Passthru Applet.

When to Use the Passthru Applet​

Place the Passthru Applet before a Stream or Voicebot Applet in your call flow when:

  • Your WebSocket server needs time to initialize a session before audio arrives (for example, to warm up an AI model or create a database record)
  • You need the StreamSid available in your backend before the WebSocket connection is established
  • You want to perform a conditional action (such as routing to a different applet) based on the response from your server

If your WebSocket server can handle the connection immediately without any advance preparation, the Passthru Applet is optional.

Setup Steps​

  1. Open your call flow in the Exotel dashboard. Go to my.exotel.com → ExoPhone → App Bazaar and open the call flow where you are using a Stream or Voicebot Applet.

  2. Add the Passthru Applet to the flow. From the applet panel, drag the Passthru Applet onto the canvas and position it before the Stream or Voicebot Applet.

  3. Configure the callback URL. Click the Passthru Applet to open its settings. Enter the HTTPS URL of the endpoint on your server that should receive the metadata POST (for example, https://yourserver.example.com/call-init).

  4. Set the response timeout. Configure an appropriate timeout (typically 3–5 seconds). If your server does not respond within this window, the call flow will not advance.

  5. Connect the applet to the stream applet. Draw a connection from the Passthru Applet's success output to the Stream or Voicebot Applet.

  6. Save and test the flow. Place a test call to verify that your server receives the POST, logs the StreamSid, and responds with 200 OK before the WebSocket audio connection opens.

Configuration Notes​

  • HTTPS required: The callback URL must use HTTPS. Plain HTTP endpoints are not accepted.
  • Response body: The Passthru Applet ignores the body of your server's response — only the HTTP status code matters. Return 200 OK with an empty body if no action is needed.
  • Idempotency: Design your handler to be idempotent. In rare cases, Exotel may retry the POST if the initial request times out.