diff --git a/lib/tasks/llm/llms/ultravox_s2s.js b/lib/tasks/llm/llms/ultravox_s2s.js
index 1823c2ac..6d0d621d 100644
--- a/lib/tasks/llm/llms/ultravox_s2s.js
+++ b/lib/tasks/llm/llms/ultravox_s2s.js
@@ -32,12 +32,14 @@ class TaskLlmUltravox_S2S extends Task {
     this.auth = this.parent.auth;
     this.connectionOptions = this.parent.connectOptions;
 
-    const {apiKey} = this.auth || {};
+    const {apiKey, agent_id} = this.auth || {};
     if (!apiKey) throw new Error('auth.apiKey is required for Vendor: Ultravox');
     this.apiKey = apiKey;
+    this.agentId = agent_id;
     this.actionHook = this.data.actionHook;
     this.eventHook = this.data.eventHook;
     this.toolHook = this.data.toolHook;
+    this.llmOptions = this.data.llmOptions || {};
 
     this.results = {
       completionReason: 'normal conversation end'
@@ -105,24 +107,29 @@ class TaskLlmUltravox_S2S extends Task {
       }
       );
       // merge with any existing tools
-      this.data.llmOptions.selectedTools = [
+      this.llmOptions.selectedTools = [
         ...convertedTools,
-        ...(this.data.llmOptions.selectedTools || [])
+        ...(this.llmOptions.selectedTools || [])
       ];
     }
 
     const payload = {
-      ...this.data.llmOptions,
-      model: this.model,
+      ...this.llmOptions,
+      ...(!this.agentId && {
+        model: this.model,
+      }),
       medium: {
-        ...(this.data.llmOptions.medium || {}),
+        ...(this.llmOptions.medium || {}),
         serverWebSocket: {
           inputSampleRate: 8000,
           outputSampleRate: 8000,
         }
       }
     };
-    const {statusCode, body} = await request('https://api.ultravox.ai/api/calls', {
+    const baseUrl = 'https://api.ultravox.ai';
+    const url = this.agentId ?
+      `${baseUrl}/api/agents/${this.agentId}/calls` : `${baseUrl}/api/calls`;
+    const {statusCode, body} = await request(url, {
       method: 'POST',
       headers: {
         'Content-Type': 'application/json',