Retell AI Core Workflow A

Overview

Build and configure voice agents with custom prompts, function calling, and call flow logic.

Prerequisites

Completed retellai-hello-world

Instructions

Step 1: Agent with Function Calling

const llm = await retell.llm.create({
  model: 'gpt-4o',
  general_prompt: `You are a booking assistant for Dr. Smith's office.
    - Help callers book, reschedule, or cancel appointments
    - Collect: name, phone, preferred date/time
    - Confirm all details before booking`,
  functions: [
    {
      name: 'book_appointment',
      description: 'Book a new appointment',
      parameters: {
        type: 'object',
        properties: {
          patient_name: { type: 'string' },
          phone: { type: 'string' },
          date: { type: 'string', description: 'YYYY-MM-DD format' },
          time: { type: 'string', description: 'HH:MM format' },
        },
        required: ['patient_name', 'phone', 'date', 'time'],
      },
      url: 'https://your-api.com/appointments',
      speak_during_execution: true,
      speak_after_execution: true,
    },
  ],
});

Step 2: Configure Voice and Behavior

const agent = await retell.agent.create({
  response_engine: { type: 'retell-llm', llm_id: llm.llm_id },
  voice_id: '11labs-Rachel',
  agent_name: 'Dr. Smith Booking Agent',
  language: 'en-US',
  opt_out_sensitive_data_storage: false,
  end_call_after_silence_ms: 10000,  // End call after 10s silence
  max_call_duration_ms: 300000,       // 5-minute max
  enable_backchannel: true,           // "mhm", "yeah" responses
  boosted_keywords: ['appointment', 'schedule', 'Dr. Smith'],
});

Step 3: Update Agent Configuration

await retell.agent.update(agent.agent_id, {
  voice_id: '11labs-Dorothy',  // Change voice
  end_call_after_silence_ms: 15000,
});

Output

Agent with custom LLM prompt and function calling
Voice and behavior configuration
Real-time function execution during calls

Error Handling

Error	Cause	Solution
Function not triggering	Prompt doesn't guide to function	Include function use in prompt
Voice quality issues	Wrong voice selection	Test different voices
Call ending too early	Short silence timeout	Increase `end_call_after_silence_ms`

Resources

Next Steps

Phone call management: retellai-core-workflow-b