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templates-and-renderers

id: templates-and-renderers sidebar_position: 1 title: Templates & Renderers​

prompt-fn ships with two rendering paths:

  1. Template functions (default) – pass a function that receives the validated input and returns a string. Great for lightweight prompts and when you want to leverage TypeScript autocomplete.
  2. Eta templates – hand Eta an input object plus a template name and let Eta render loops/partials.

Template functions​

const prompt = definePrompt({
  ...,
  template: ({customer, issue}) => `Customer: ${customer}\nIssue: ${issue}`,
});

Because template is a function, TypeScript enforces the same structure as your inputSchema. No stringly-typed placeholders.

Eta templates​

import {Eta} from 'eta';

const eta = new Eta({ views: path.join(import.meta.dir, 'templates') });

const prompt = definePrompt({
  ...,
  template: 'product-brief',
  eta,
});
  • template becomes the Eta view name.
  • Prompt input is passed as the Eta context (<%= it.body %> etc.).
  • Use Eta when you need conditionals, loops, or shared partials. See examples/templates/product-brief.eta for a full example.

Tips​

  • Keep templates small and push reuse into helpers so TypeDoc docs stay readable.
  • Avoid telling the LLM to “respond as JSON”; the AI SDK handles that instruction under the hood when you supply an outputSchema.
  • When a template contains long text snippets, store them under examples/templates or src/prompts and load them via Eta to keep your TypeScript files tidy.