← Blogby Marcell Almeida

How to test LLM prompt templates with variables

Write a template with {{placeholders}} — fill variables and preview the final prompt.

Prompt engineering uses templates with slots for user data. The Prompt Variable Tester detects {{variables}}, lets you fill them, and shows the rendered prompt before you call an API.

Try it — template and variable values

Open full tool →

How it works

  1. Template — prompt with {{variable}} placeholders.
  2. Variables — auto-detected fields to fill.
  3. Preview — final prompt ready to copy.

Mustache-style syntax

Placeholders use double curly braces: {{name}}, {{context}}. Missing variables stay visible so you catch gaps.

Token budgeting

Estimate cost with the AI Token Calculator or Embedding Cost Calculator.

The How to test LLM prompt templates with variables is free, runs locally, and never sends your data to a server. More developer tools at maratool.com.