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Using the right system prompt is especially important when building Voice AI Agents. LLMs are primarily trained on written text, so they tend to produce output that is more formal and structured than natural speech. By carefully crafting your prompt, you can guide the model to generate responses that sound more conversational and human-like.

Base System Prompt for Voice AI

Minimal base prompt for Voice AI

Pronunciation of numbers, dates & times

Pronunciation of numbers, dates, times, and special characters is also crucial for voice applications. TTS (text-to-speech) providers handle pronunciations in different ways. A good base prompt that guides the LLM to use words to spell out numbers, dates, addresses etc will work for common cases.
Numbers & data rules

Keep long paragraphs sounding natural

Most text-to-speech systems will change prosody if they receive each sentence individually. If your voice agent needs to speak a large amount of text (e.g. a long legal disclosures or policy statements), follow this guidance to keep paragraphs sounding natural:
  • Send multiple sentences together. When you already have the full copy (for example, a static disclosure), pass the entire paragraph in a single stream.tts message so the speech engine can maintain the correct intonation.
  • Wait for the model to finish generating. Fast models such as Gemini 2.5 Flash Lite can produce a paragraph quickly. Instead of streaming each partial sentence as soon as it appears, collect the complete paragraph and then forward the whole string to the TTS provider.
This approach avoids sentence-by-sentence delivery that can make disclosures sound choppy.