> 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/api-reference/voice-agents/realtime-agent/register-call

## Authentication

- `Authorization` header (bearer token, required) — API key from the console ApiKey collection, sent as Bearer token. Also accepts session cookies for browser-based auth.

## Request

### Body (application/json)

This endpoint expects an object.

- `agent_id` (string, required) — The Atoms agent to connect to.
- `mode` (enum, optional, default: webcall) — Session mode. `webcall` = full voice pipeline (audio in + audio out). `chat` = text-only pipeline. Defaults to `webcall`.
  - Allowed values: `webcall`, `chat`
- `input_audio_format` (enum, optional) — The format you will **send**, as one `<encoding>_<rate>` token naming both the encoding and the rate. Defaults to PCM at the output rate, so a client sending only `sample_rate` needs nothing else. The recogniser is configured at this rate and your audio is not resampled, so the token must match the bytes you send: a mismatch produces wrong transcripts and no error. G.711 exists only at 8000. Opus is raw packets, one per WebSocket message; Ogg-framed Opus is a different format and is refused. A browser client can also declare this later, on the connect URL, once its `AudioContext` reports the rate it actually got.
  - Allowed values: `pcm_8000`, `pcm_16000`, `pcm_22050`, `pcm_24000`, `pcm_44100`, `pcm_48000`, `mulaw_8000`, `alaw_8000`, `opus_8000`, `opus_16000`, `opus_24000`, `opus_48000`
- `output_audio_format` (enum, optional, default: pcm_24000) — The format the agent sends back. Same token shape as `input_audio_format`. Sending this together with `sample_rate` is allowed only if they agree; disagreeing values are refused rather than resolved by a precedence rule.
  - Allowed values: `pcm_8000`, `pcm_16000`, `pcm_24000`, `pcm_44100`
- `variables` (map from string to ConversationRegisterCallPostRequestBodyContentApplicationJsonSchemaVariables, optional) — 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.
- `sample_rate` (enum, optional, default: 24000, deprecated) — **Deprecated. Use `output_audio_format` instead.** Still accepted, and will keep being accepted - existing integrations do not need to change. A bare rate meaning PCM at that rate, so `sample_rate: 24000` and `output_audio_format: "pcm_24000"` are the same request. It predates the one-token-per-direction contract and is the reason two fields exist for one thing. `44100` is only rendered by the `lightning-v3.1` and `lightning-v3.1-pro` voices; asking for a rate the agent's voice cannot render is refused here rather than failing mid-call. This does **not** set the rate of the audio you send - use `input_audio_format`.
  - Allowed values: `8000`, `16000`, `24000`, `44100`

## Response

### 201

Access token created.

- `status` (boolean, optional)
- `data` (ConversationRegisterCallPostResponsesContentApplicationJsonSchemaData, optional)

## Errors

### 400 Bad Request Error

Validation failed (e.g. missing `agent_id`), or the organization has no remaining credits.

- `status` (boolean, optional)
- `errors` (list of string, optional)

### 401 Unauthorized Error

Unauthorized access

- `status` (boolean, optional)
- `errors` (list of string, optional)

### 404 Not Found Error

Resource not found. The referenced ID does not exist or does not belong to the caller's organization.

- `status` (boolean, optional)
- `errors` (list of string, optional)

### 500 Internal Server Error

Internal server error

- `status` (boolean, optional)
- `errors` (list of string, optional)

## Types

### ConversationRegisterCallPostRequestBodyContentApplicationJsonSchemaVariables

### ConversationRegisterCallPostResponsesContentApplicationJsonSchemaData

- `access_token` (string, optional) — Short-lived, single-use token (prefixed `wct_`). Pass it as the `token` query param when opening the Agent WebSocket.
- `expires_in` (integer, optional) — Token lifetime in seconds.
- `sample_rate` (integer, optional) — Negotiated audio sample rate (Hz) for the session. Echoed back in the WebSocket `session.created` event.
- `input_audio_format` (string, optional) — The input format resolved for this session. Read it back to confirm the server understood what you meant before you send any audio.
- `output_audio_format` (string, optional) — The output format resolved for this session.

## 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,
    "input_audio_format": "pcm_24000",
    "output_audio_format": "pcm_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()
```