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Early Beta — The Web SDK is in early beta. APIs may change between releases.

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Overview

System prompts define the AI’s behavior, personality, and constraints. They’re prepended to every generation request and help ensure consistent, focused responses.

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Basic Usage

import { TextGeneration } from '@runanywhere/web'

const result = await TextGeneration.generate('What should I cook tonight?', {
  systemPrompt: 'You are a helpful chef. Suggest recipes that are quick and easy to make.',
  maxTokens: 200,
})

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Examples

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Coding Assistant

const result = await TextGeneration.generate('How do I sort an array in JavaScript?', {
  systemPrompt: `You are an expert JavaScript developer. Provide concise code examples 
with brief explanations. Use modern ES6+ syntax.`,
  maxTokens: 300,
})

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Concise Responder

const result = await TextGeneration.generate('Explain quantum entanglement', {
  systemPrompt: 'You are a science communicator. Keep all responses under 3 sentences.',
  maxTokens: 100,
  temperature: 0.3,
})

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JSON Output

const result = await TextGeneration.generate('List 3 popular programming languages', {
  systemPrompt: `You are a helpful assistant. Always respond in valid JSON format.
Example: {"items": ["item1", "item2"]}`,
  maxTokens: 200,
  temperature: 0.1,
})

const data = JSON.parse(result.text)

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Persona

const result = await TextGeneration.generate('Tell me about space exploration', {
  systemPrompt: `You are Captain Nova, an enthusiastic space explorer. 
You speak with excitement about space and use space-related analogies.
Keep responses brief and engaging.`,
  maxTokens: 200,
  temperature: 0.8,
})

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System Prompt Tips

TechniqueExampleEffect
Role definition”You are a helpful tutor”Sets personality
Output format”Respond in JSON”Controls format
Length constraint”Keep responses under 2 sentences”Controls length
Tone”Be professional and formal”Sets communication style
Constraints”Never discuss politics”Limits topics

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Structured Output with System Prompts

For type-safe JSON generation, combine system prompts with the StructuredOutput module: 
import { TextGeneration, StructuredOutput } from '@runanywhere/web'

const schema = JSON.stringify({
  type: 'object',
  properties: {
    name: { type: 'string' },
    ingredients: { type: 'array', items: { type: 'string' } },
    prepTime: { type: 'number' },
  },
  required: ['name', 'ingredients', 'prepTime'],
})

const systemPrompt = StructuredOutput.getSystemPrompt(schema)

const result = await TextGeneration.generate('Suggest a quick pasta recipe', {
  systemPrompt,
  maxTokens: 300,
  temperature: 0.3,
})

const extracted = StructuredOutput.extractJson(result.text)
if (extracted) {
  const recipe = JSON.parse(extracted)
  console.log(recipe.name, recipe.ingredients, recipe.prepTime)
}

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Best Practices

Keep system prompts concise. Longer system prompts consume more of the model’s context window, leaving less room for the user’s prompt and the response.
  • Be specific — “You are a Python expert” is better than “You are helpful”
  • Set output format early — Place format instructions at the start of the system prompt
  • Use examples — Show the model what good output looks like
  • Set constraints — Tell the model what NOT to do (e.g., “Do not include disclaimers”)
  • Keep it short — Every token in the system prompt reduces available context

Generate

Full generation with options

Streaming

Real-time token streaming