Lesson 8: Prompt Engineering Fundamentals

Retell University Module 2: Building Your First Agent
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Prompt Engineering for Voice Agents

Voice agents need different prompting than text chatbots:

Key Principles

  1. Be concise — Long responses feel unnatural on the phone. Tell the agent to keep replies short.
  2. Use conversational language — Write the way people talk, not the way they write.
  3. Handle interruptions — Callers will interrupt. Your prompt should account for this.
  4. Anticipate edge cases — What if they say something unexpected? Add fallback instructions.
  5. Structure information gathering — Collect one piece of info at a time, don’t overwhelm.

Voice-Specific Tips

  • Avoid bullet points or numbered lists in spoken responses (they sound robotic)
  • Use transition phrases: “Great, and just to confirm…”, “Perfect, one more thing…”
  • Keep sentences short — under 20 words when possible
  • Spell out acronyms or use pronunciation hints for unusual words

Common Mistakes

  • :cross_mark: Making the prompt too long (agent gets confused)
  • :cross_mark: Not specifying what to do when the agent doesn’t know the answer
  • :cross_mark: Forgetting to handle the end of the conversation
  • :cross_mark: Not testing with edge cases (angry callers, wrong numbers, silence)

Agent Handbook

Retell provides a pre-built Agent Handbook with best-practice prompt templates for common use cases. Check the dashboard for templates you can customize.