This is Prompt Canvas - a simple, open-source web app that lets you visually build LLM prompt templates as YAML schemas and then generate complete prompts by populating values in the templates. It’s based on a single HTML file with no real privacy concerns and everything is portable thanks to YAML exports.

Check out the example in the dropdown and read the guide to see how it works: https://promptcanvas.ml4den.com/

LLMs like structure and I found that generating prompts like this is an easy way of giving it to them. It can be useful if you’re doing a degree of prompt engineering and you want to test small variations in your prompts; or if you have a use case where you submit the same promps many times but with some input variations. I found that browsing to a YAML file and tweaking one parameter for a particular job is much cleaner than a web of Notion pages which is what I had before.

Some thought and iteration has gone into the templating engine but everything is still early stage! Some of it is opinionated, and some of it is meant to be quite extensible to different use cases. Let me know if it makes sense.

I built this mostly with Gemini 2.5 Pro out of my own necessity. Would love to know if it's useful for you! Feedback welcome; as are bugs and things on GitHub: https://github.com/ml4den/PromptCanvas