> 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=`. 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 `_` 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 ", "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 ', '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 ") 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 ' 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 response = Unirest.post("https://api.smallest.ai/atoms/v1/conversation/register-call") .header("Authorization", "Bearer ") .header("Content-Type", "application/json") .body("{\n \"agent_id\": \"69da0b4c20c0e03cfa4ee258\"\n}") .asString(); ``` ```php request('POST', 'https://api.smallest.ai/atoms/v1/conversation/register-call', [ 'body' => '{ "agent_id": "69da0b4c20c0e03cfa4ee258" }', 'headers' => [ 'Authorization' => 'Bearer ', '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 "); 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 ", "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() ```