Prompt Variable Filler
Fill a prompt using {{name}} placeholders and JSON string values, and find missing variables or unused keys.
Input
Editable English syntax example.
Result
Run immediately with the editable example.
Supported syntax and processing
Substitute your own strings once into {{name}} placeholders in a reusable prompt and check missing values or unused keys. Nothing is sent to an AI model.
Variable names have no spaces, start with a letter or underscore and contain at most 64 letters, digits or underscores. Values are JSON strings keyed by the same case-sensitive names. Repeated names use the same value. Inserted {{other}}, $& and HTML are left literal rather than interpreted again.
Input example
The example maps tone to short and topic to a daily walking plan, producing Write a short summary about a daily walking plan. Removing topic leaves {{topic}} in the preview and lists it as missing. The result is complete only when all values are supplied.
Supported range and boundaries
Limits are 20,000 template characters and 500 occurrences; 50,000 JSON characters, 100 keys and 20,000 characters per value; and 100,000 output characters. Duplicate and reserved prototype keys are unsupported. Conditions, loops, functions, nested paths, spaced placeholders and double-brace escaping are unsupported. No code is evaluated and no AI request is sent.
Inputs and results stay in your browser. They are not saved or put in URLs or analytics. Leaving or refreshing the page restores the public example.
Frequently Asked Questions
Can JSON values be numbers or objects?
Only strings are supported. Put numbers in double quotes if they should be text. Objects and arrays are not automatically converted, avoiding unintended substituted values.
What happens to missing values or unused keys?
Missing values leave the original {{name}} token and mark the preview incomplete. Unused keys are listed but not inserted. The original template and JSON are never automatically modified.
Is this the same syntax as other template engines or AI services?
Only the documented one-pass {{name}} substitution rule applies. This is not a full Jinja or Mustache engine or an implementation of any AI service variable rules. Check the destination service rules separately.
Calculation criteria and reference materials
- IETF RFC 8259 — JSON Data Format
Reference for JSON strings, objects and escapes. Variable names and one-pass substitution are separately defined by this tool.
Verification of calculation criteria: · Calculation·verification principles · Report errors