Promptyard
Prompt

Prompt Template Builder

Compose a prompt with named variables, see the rendered output side-by-side.

Referenced: role, language, goal, code
Rendered prompt
You are a senior backend engineer reviewing TypeScript code.

Goal: flag bugs and unclear naming

Code to review:
function fetch(u) { return f(u).then(r => r.json()) }

Respond with concrete suggestions referencing line numbers.
228 chars
Variables

Why templating matters

Most production LLM features assemble prompts at runtime: system instructions + user query + RAG context + few-shot examples. Hand-concatenating strings drifts — a stray period, a missed variable, an extra newline — and the model\'s behaviour shifts unpredictably.

Centralising the template + variable substitution means: changes to the template are tracked in one place, missing variables are caught before runtime, and you can swap variable values to A/B test without rewriting code.

FAQ

Is this a Jinja / Mustache compatible engine?
Subset. We support {{ name }} substitution only — no loops, conditionals, filters, or whitespace control. For full templating use a real engine (Jinja2 in Python, Handlebars/Nunjucks in JS).
Why does the rendered prompt show <code>{{foo}}</code> if I forgot to declare <code>foo</code>?
On purpose — passing an un-substituted placeholder to the model is almost always a bug, and seeing the literal in the rendered output makes it impossible to miss. The "Missing variables" hint surfaces it explicitly too.
Should I use markdown headers or XML tags inside the template?
XML for Claude (use Anthropic XML Prompt Builder); markdown for GPT and Gemini. For cross-provider prompts, plain markdown is safest.

Common pitfalls

  • Substituting unsanitised user input into a system-level template — opens prompt injection.
  • Trailing whitespace in variable values that disrupts formatting.
  • Hard-coding the prompt as a string literal in code, then editing in 4 different places when the requirement changes.

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