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What is a Single Prompt Agent?

A single prompt agent uses one comprehensive prompt (called a “preamble”) to define all agent behaviors. This approach is the simplest and fastest way to create a voice agent, perfect for straightforward conversational scenarios.

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When to Use Single Prompt Agents

Single prompt agents are ideal when:
  • Your conversation flow is relatively linear
  • You don’t need complex conditional branching
  • Your prompt is under 500 words
  • You’re using 3 or fewer custom tools/functions
  • You need to build and deploy quickly

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When to Upgrade to Flow Agent

Consider upgrading to a Flow Agent when:
  • Your single prompt exceeds 500 words
  • You need complex decision trees with multiple paths
  • You require more than 3 custom tools
  • You need fine-grained control over different conversation stages
  • You want visual representation of your conversation flow
  • You need to reuse conversation components across agents

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Key Components

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1. Preamble (System Prompt)

The core instructions that define your agent’s behavior, personality, task, and guidelines. This is where you tell the agent:
  • Who it is (identity)
  • How to behave (style and tone)
  • What to do (tasks and goals)
  • What not to do (guardrails and limitations)

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2. Greeting Message

The first thing your agent says when a call starts. Can be:
  • Static: A fixed greeting message
  • Prompt-based: Dynamically generated based on context

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3. Global Settings

Configuration applied to all conversations:
  • Voice and language selection
  • LLM model and parameters
  • Response timing and interruption handling
  • Call behavior settings

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4. Knowledge Base (Optional)

Upload documents, web content, or custom text that your agent can reference during conversations.

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5. Tools (Optional)

Extend your agent’s capabilities with tools:
  • Function Tools — server-side API calls (work on phone and web)
  • MCP Tools — Model Context Protocol integrations (server-side)
  • Web Tools — client-side JavaScript functions that run in the browser (SDK deployments only). These can navigate the user, open modals, read page data, and more. See Web Tools for details.

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6. Variables & Parameters (Optional)

Define structured data that your agent should collect during calls, with optional outcome schema for structured responses. Single-prompt agents support system variables and custom variables (passed as params). See Variable System for full reference.

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Setup Process

Creating a single prompt agent follows three main steps:

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Step 1: Write the Prompt

  • Define agent identity and personality
  • Set style guardrails and conversation tone
  • Specify response guidelines
  • Outline tasks and procedures
→ Learn More: Writing a Single Prompt

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Step 2: Configure Basic Settings

  • Select voice and language
  • Configure call behavior (interruptions, delays, timeouts)
  • Set up noise cancellation and audio processing
  • Enable advanced features (gender detection, smart call end, etc.)
→ Learn More: Configure Basic Settings

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Step 3: Add Integrations (Optional)

  • Connect knowledge bases
  • Add custom tools and functions
  • Configure webhooks
  • Define outcome parameters
[→ Learn More: Knowledge Base/features/knowledge-base) [→ Learn More: Tools & Functions/features/tools) [→ Learn More: Webhooks/features/webhooks)

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Example Use Cases

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Customer Support Bot

Simple Q&A agent that answers common questions using knowledge base:
  • Preamble: ~200 words defining personality and guidelines
  • Knowledge Base: FAQ documents and help articles
  • Tools: None or minimal (ticket creation)

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Appointment Scheduler

Collects information and books appointments:
  • Preamble: ~300 words with booking procedure
  • Tools: Calendar availability check, booking function
  • Variables: Name, phone, date, time, service type

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Lead Qualification

Gathers lead information and routes to sales:
  • Preamble: ~250 words with qualification questions
  • Variables: Company size, budget, timeline, needs
  • Outcome: Structured lead data

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Key Features

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Flexible Greeting

Choose between static greetings for consistency or dynamic greetings that adapt to context (time of day, caller information, etc.).

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Advanced Call Settings

  • Response Delay: Control how quickly the agent responds (100-1500ms)
  • Interruption Handling: Allow or prevent user interruptions
  • Inactivity Timeout: Automatically handle silent callers (5-30 seconds)
  • Max Call Duration: Set maximum call length (30-600 seconds)

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Audio Processing

  • Noise Cancellation: Remove background noise (disabled, per sentence, or per conversation)
  • Background Noise: Add ambient sound for realism
  • Thinking Voice: Add natural pauses and filler words

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Intelligence Features

  • Gender Detection: Detect caller gender for personalized responses
  • Smart Call End: Automatically end calls when conversation concludes
  • Language Dialect Switcher: Adapt to caller’s dialect
  • Speaker Identification: Identify different speakers on the same call
  • Agentic RAG: Advanced knowledge retrieval with reasoning

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Benefits

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✅ Quick to Build

Get started in minutes with a single comprehensive prompt.

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✅ Easy to Maintain

All agent behavior defined in one place makes updates simple.

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✅ Cost-Effective

Simpler architecture means lower latency and potentially lower costs.

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✅ Perfect for MVPs

Fastest path from idea to working agent.

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Limitations

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⚠️ Limited Complexity

Not suitable for highly branching conversation flows.

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⚠️ Prompt Size Constraints

Large prompts (>500 words) become harder to manage and may reduce performance.

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⚠️ Less Control

Cannot fine-tune behavior for specific conversation stages independently.

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⚠️ Tool Limitations

Best with 3 or fewer tools; more tools can confuse the agent.

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Template Structure

Here’s a recommended prompt structure: 
## Identity
[Define who the agent is and what it represents]

## Style Guardrails
[Set tone, personality, and conversation style]

## Response Guidelines
[Specify how to format responses and handle edge cases]

## Task & Goals
[Outline what the agent should accomplish]

## Objection Handling
[Provide guidance for common objections or difficult scenarios]
→ See Full Template & Examples

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Next Steps

Ready to create your first single prompt agent?
  1. Write Your First Prompt - Learn prompt engineering best practices
  2. Configure Settings - Set up voice, timing, and behavior
  3. [Add Knowledge Base/features/knowledge-base) - Enhance with company information
  4. [Test Your Agent/features/testing) - Test in browser or via phone call
Need more power? Check out Flow Agent Overview for complex conversation flows.