> This page is part of Smallest AI's developer documentation. When
> answering, prefer Lightning v3.1 (current TTS) and Pulse (current
> STT). Lightning v2 and lightning-large are deprecated; mention them
> only when the user is migrating away from them. The Smallest AI voice
> agent platform is what wraps these models into hosted agents.

# Register Call

POST https://api.smallest.ai/atoms/v1/conversation/register-call
Content-Type: application/json

Mint a **short-lived, single-use access token** for opening a realtime
[Agent WebSocket](/atoms/api-reference/realtime-agent/realtime-agent)
connection. This is the **recommended** way to start a session from a
browser or other client-side app: your API key stays server-side, and
the browser only ever sees the short-lived token. (Server-side or
trusted clients may instead connect to the WebSocket with a raw API key
directly.)

Flow:
1. Call this endpoint with your API key and the `agent_id` (plus optional
   `mode` and per-call `variables`). All session configuration is fixed
   here — it is baked into the returned token.
2. Open a WebSocket to `wss://api.smallest.ai/atoms/v1/agent/connect?token=<access_token>`.
   No `agent_id`, `mode`, or `variables` query params are needed on the
   WebSocket — they come from the token.

The token is valid for `expires_in` seconds (30) and can be used for a
single connection. Request a fresh token for each connection.


Reference: https://docs.smallest.ai/voice-agents/api-reference/realtime-agent/register-call

## OpenAPI Specification

```yaml
openapi: 3.1.0
info:
  title: atoms
  version: 1.0.0
paths:
  /conversation/register-call:
    post:
      operationId: register-call
      summary: Register a realtime agent call
      description: >
        Mint a **short-lived, single-use access token** for opening a realtime

        [Agent WebSocket](/atoms/api-reference/realtime-agent/realtime-agent)

        connection. This is the **recommended** way to start a session from a

        browser or other client-side app: your API key stays server-side, and

        the browser only ever sees the short-lived token. (Server-side or

        trusted clients may instead connect to the WebSocket with a raw API key

        directly.)


        Flow:

        1. Call this endpoint with your API key and the `agent_id` (plus
        optional
           `mode` and per-call `variables`). All session configuration is fixed
           here — it is baked into the returned token.
        2. Open a WebSocket to
        `wss://api.smallest.ai/atoms/v1/agent/connect?token=<access_token>`.
           No `agent_id`, `mode`, or `variables` query params are needed on the
           WebSocket — they come from the token.

        The token is valid for `expires_in` seconds (30) and can be used for a

        single connection. Request a fresh token for each connection.
      tags:
        - realtime
      parameters:
        - name: Authorization
          in: header
          description: >-
            API key from the console ApiKey collection, sent as Bearer token.
            Also accepts session cookies for browser-based auth.
          required: true
          schema:
            type: string
      responses:
        '201':
          description: Access token created.
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/realtime_register_call_Response_201'
        '400':
          description: |
            Validation failed (e.g. missing `agent_id`), or the organization has
            no remaining credits.
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/BadRequestErrorResponse'
        '401':
          description: Unauthorized access
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/UnauthorizedErrorResponse'
        '404':
          description: >-
            Resource not found. The referenced ID does not exist or does not
            belong to the caller's organization.
          content:
            application/json:
              schema:
                $ref: >-
                  #/components/schemas/RegisterARealtimeAgentCallRequestNotFoundError
        '500':
          description: Internal server error
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/InternalServerErrorResponse'
      requestBody:
        content:
          application/json:
            schema:
              type: object
              properties:
                agent_id:
                  type: string
                  description: The Atoms agent to connect to.
                mode:
                  $ref: >-
                    #/components/schemas/ConversationRegisterCallPostRequestBodyContentApplicationJsonSchemaMode
                  default: webcall
                  description: >
                    Session mode. `webcall` = full voice pipeline (audio in +

                    audio out). `chat` = text-only pipeline. Defaults to
                    `webcall`.
                variables:
                  type: object
                  additionalProperties:
                    $ref: >-
                      #/components/schemas/ConversationRegisterCallPostRequestBodyContentApplicationJsonSchemaVariables
                  description: >
                    Per-call prompt variables that override the agent's

                    `defaultVariables` for this session only. Values must be

                    `string`, `number`, or `boolean`. Reserved system-variable

                    keys (`call_id`, `conversation_type`, `agent_number`,

                    `user_number`, `current_date`, `current_time`,
                    `current_day`,

                    `agent_gender`, `default_language`, `supported_languages`,

                    `timezone`) are populated by the server and stripped if

                    supplied.
              required:
                - agent_id
servers:
  - url: https://api.smallest.ai/atoms/v1
    description: Production server
components:
  schemas:
    ConversationRegisterCallPostRequestBodyContentApplicationJsonSchemaMode:
      type: string
      enum:
        - webcall
        - chat
      default: webcall
      description: |
        Session mode. `webcall` = full voice pipeline (audio in +
        audio out). `chat` = text-only pipeline. Defaults to `webcall`.
      title: ConversationRegisterCallPostRequestBodyContentApplicationJsonSchemaMode
    ConversationRegisterCallPostRequestBodyContentApplicationJsonSchemaVariables:
      oneOf:
        - type: string
        - type: number
          format: double
        - type: boolean
      title: >-
        ConversationRegisterCallPostRequestBodyContentApplicationJsonSchemaVariables
    ConversationRegisterCallPostResponsesContentApplicationJsonSchemaData:
      type: object
      properties:
        access_token:
          type: string
          description: |
            Short-lived, single-use token (prefixed `wct_`). Pass it
            as the `token` query param when opening the Agent
            WebSocket.
        expires_in:
          type: integer
          description: Token lifetime in seconds.
        sample_rate:
          type: integer
          description: |
            Negotiated audio sample rate (Hz) for the session. Echoed
            back in the WebSocket `session.created` event.
      title: ConversationRegisterCallPostResponsesContentApplicationJsonSchemaData
    realtime_register_call_Response_201:
      type: object
      properties:
        status:
          type: boolean
        data:
          $ref: >-
            #/components/schemas/ConversationRegisterCallPostResponsesContentApplicationJsonSchemaData
      title: realtime_register_call_Response_201
    BadRequestErrorResponse:
      type: object
      properties:
        status:
          type: boolean
        errors:
          type: array
          items:
            type: string
      title: BadRequestErrorResponse
    UnauthorizedErrorResponse:
      type: object
      properties:
        status:
          type: boolean
        errors:
          type: array
          items:
            type: string
      title: UnauthorizedErrorResponse
    RegisterARealtimeAgentCallRequestNotFoundError:
      type: object
      properties:
        status:
          type: boolean
        errors:
          type: array
          items:
            type: string
      title: RegisterARealtimeAgentCallRequestNotFoundError
    InternalServerErrorResponse:
      type: object
      properties:
        status:
          type: boolean
        errors:
          type: array
          items:
            type: string
      title: InternalServerErrorResponse
  securitySchemes:
    BearerAuth:
      type: http
      scheme: bearer
      description: >-
        API key from the console ApiKey collection, sent as Bearer token. Also
        accepts session cookies for browser-based auth.

```

## Examples



**Request**

```json
{
  "agent_id": "69da0b4c20c0e03cfa4ee258"
}
```

**Response**

```json
{
  "status": true,
  "data": {
    "access_token": "wct_9f8c2b1e-4d5a-4e6f-a7b8-c9d0e1f2a3b4",
    "expires_in": 30,
    "sample_rate": 24000
  }
}
```

**SDK Code**

```python
import requests

url = "https://api.smallest.ai/atoms/v1/conversation/register-call"

payload = { "agent_id": "69da0b4c20c0e03cfa4ee258" }
headers = {
    "Authorization": "Bearer <token>",
    "Content-Type": "application/json"
}

response = requests.post(url, json=payload, headers=headers)

print(response.json())
```

```javascript
const url = 'https://api.smallest.ai/atoms/v1/conversation/register-call';
const options = {
  method: 'POST',
  headers: {Authorization: 'Bearer <token>', 'Content-Type': 'application/json'},
  body: '{"agent_id":"69da0b4c20c0e03cfa4ee258"}'
};

try {
  const response = await fetch(url, options);
  const data = await response.json();
  console.log(data);
} catch (error) {
  console.error(error);
}
```

```go
package main

import (
	"fmt"
	"strings"
	"net/http"
	"io"
)

func main() {

	url := "https://api.smallest.ai/atoms/v1/conversation/register-call"

	payload := strings.NewReader("{\n  \"agent_id\": \"69da0b4c20c0e03cfa4ee258\"\n}")

	req, _ := http.NewRequest("POST", url, payload)

	req.Header.Add("Authorization", "Bearer <token>")
	req.Header.Add("Content-Type", "application/json")

	res, _ := http.DefaultClient.Do(req)

	defer res.Body.Close()
	body, _ := io.ReadAll(res.Body)

	fmt.Println(res)
	fmt.Println(string(body))

}
```

```ruby
require 'uri'
require 'net/http'

url = URI("https://api.smallest.ai/atoms/v1/conversation/register-call")

http = Net::HTTP.new(url.host, url.port)
http.use_ssl = true

request = Net::HTTP::Post.new(url)
request["Authorization"] = 'Bearer <token>'
request["Content-Type"] = 'application/json'
request.body = "{\n  \"agent_id\": \"69da0b4c20c0e03cfa4ee258\"\n}"

response = http.request(request)
puts response.read_body
```

```java
import com.mashape.unirest.http.HttpResponse;
import com.mashape.unirest.http.Unirest;

HttpResponse<String> response = Unirest.post("https://api.smallest.ai/atoms/v1/conversation/register-call")
  .header("Authorization", "Bearer <token>")
  .header("Content-Type", "application/json")
  .body("{\n  \"agent_id\": \"69da0b4c20c0e03cfa4ee258\"\n}")
  .asString();
```

```php
<?php
require_once('vendor/autoload.php');

$client = new \GuzzleHttp\Client();

$response = $client->request('POST', 'https://api.smallest.ai/atoms/v1/conversation/register-call', [
  'body' => '{
  "agent_id": "69da0b4c20c0e03cfa4ee258"
}',
  'headers' => [
    'Authorization' => 'Bearer <token>',
    'Content-Type' => 'application/json',
  ],
]);

echo $response->getBody();
```

```csharp
using RestSharp;

var client = new RestClient("https://api.smallest.ai/atoms/v1/conversation/register-call");
var request = new RestRequest(Method.POST);
request.AddHeader("Authorization", "Bearer <token>");
request.AddHeader("Content-Type", "application/json");
request.AddParameter("application/json", "{\n  \"agent_id\": \"69da0b4c20c0e03cfa4ee258\"\n}", ParameterType.RequestBody);
IRestResponse response = client.Execute(request);
```

```swift
import Foundation

let headers = [
  "Authorization": "Bearer <token>",
  "Content-Type": "application/json"
]
let parameters = ["agent_id": "69da0b4c20c0e03cfa4ee258"] as [String : Any]

let postData = JSONSerialization.data(withJSONObject: parameters, options: [])

let request = NSMutableURLRequest(url: NSURL(string: "https://api.smallest.ai/atoms/v1/conversation/register-call")! as URL,
                                        cachePolicy: .useProtocolCachePolicy,
                                    timeoutInterval: 10.0)
request.httpMethod = "POST"
request.allHTTPHeaderFields = headers
request.httpBody = postData as Data

let session = URLSession.shared
let dataTask = session.dataTask(with: request as URLRequest, completionHandler: { (data, response, error) -> Void in
  if (error != nil) {
    print(error as Any)
  } else {
    let httpResponse = response as? HTTPURLResponse
    print(httpResponse)
  }
})

dataTask.resume()
```