Prompt Template Tester (Fill {{variables}} & Preview)
Write a prompt with {{variable}} placeholders, fill in values, and preview the final prompt. Detects every variable and flags any you left unfilled. 100% local.
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CommentsWrite a prompt with {{variable}} placeholders, fill in the values on the right, and the final prompt renders live below — with any placeholders you forgot to fill listed so nothing slips through. Runs locally.
You are a professional assistant. Translate the following text into French, keeping the tone formal: Hello, how are you?
Everything runs in your browser. Unfilled placeholders are left as-is and listed above so you never ship a prompt with a stray {{variable}} in it.
What is Prompt Template Tester?
Most production prompts are templates: a fixed instruction with a few slots filled in per request ({{user_input}}, {{language}}, {{tone}}). When you're iterating on a template it's easy to leave a placeholder unfilled or mistype a variable name and only notice after a bad generation. This tester extracts every placeholder from your template, gives you an input for each, substitutes the values, and clearly flags anything left blank — so the prompt you copy out is exactly what the model will see.
How to test a prompt template
- 1Write or paste your template using {{variable}} placeholders.
- 2Fill in a value for each detected variable.
- 3Copy the rendered final prompt — unfilled variables are listed as a warning.
Key Concepts
- Template variable
- A {{placeholder}} substituted with per-request values before sending.
- Token cost
- Longer filled values mean more tokens — check the breakdown after filling.
Frequently Asked Questions
What placeholder syntax does it support?
Double-brace {{variable}} (the most common convention, used by LangChain-style templates) and single-brace {variable} when no double-brace variables are present.
What happens to variables I don't fill in?
They're left in the output as-is and listed as 'unfilled' above the preview, so you never accidentally ship a prompt with a stray {{variable}}.
Is my template uploaded?
No. Substitution runs locally in your browser.
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