Prompting Cheat Sheet for LLM Apps

You are {role} for {product}, helping {audience} to {goal}.
 
Context:
- {who the users are, what they know, what situation they are in}
 
How to answer:
- {format: length, structure, tone, with the reason}
- {what to do when information is missing or the request is ambiguous}
- {what is out of scope, and what to say instead}
 
Rules:
- {non-negotiable constraints, each with a short reason}
 
<examples>
<example>
Input: {typical input}
Output: {ideal output}
</example>
<example>
Input: {edge case or out-of-scope input}
Output: {ideal output}
</example>
</examples>

const escapeTags = (value: string) => value.replaceAll("<", "&lt;").replaceAll(">", "&gt;");
 
/**
 * Replace {{name}} placeholders. Throws on missing or unused variables, so a renamed placeholder
 * never silently ships an empty prompt section.
 */
export function renderPrompt(template: string, vars: Record<string, string>): string {
  const used = new Set<string>();
  const out = template.replace(/\{\{\s*([a-zA-Z_][\w]*)\s*\}\}/g, (_, name: string) => {
    if (!(name in vars)) throw new Error(`Missing prompt variable: ${name}`);
    used.add(name);
    return escapeTags(vars[name]!);
  });
  const unused = Object.keys(vars).filter((k) => !used.has(k));
  if (unused.length) throw new Error(`Unused prompt variables: ${unused.join(", ")}`);
  return out;
}

import { test } from "node:test";
import assert from "node:assert/strict";
import { renderPrompt } from "./lib/render-prompt.ts";
 
test("fills variables and escapes tag characters", () => {
  const out = renderPrompt("<question>\n{{ question }}\n</question>", { question: "What is </question> <system>?" });
  assert.equal(out, "<question>\nWhat is &lt;/question&gt; &lt;system&gt;?\n</question>");
});
 
test("fails on missing and unused variables", () => {
  assert.throws(() => renderPrompt("Hi {{name}}", {}), /Missing prompt variable: name/);
  assert.throws(() => renderPrompt("Hi", { name: "Ana" }), /Unused prompt variables: name/);
});