If you read the previous article on the sandbox internals , you already know that there is a local playground, what it is made of, and roughly what it can do. What that article did not cover is how to actually play. This one does.

We will build a custom certificate template for a fictional institution called Building Block Academy , simulate issuing certificates in bulk for fictional students on the local devnet, evaluate the on-chain costs, and inspect the final issued certificates in the UI. At the end, we will submit the finished template to the public UVerify UI repository so anyone can see Building Block Academy 's student certificates at app.uverify.io .

You will need:

Docker Desktop 24+

uv : brew install uv on macOS or the PowerShell one-liner on Windows

Deno : for the load test at the end

Claude Code : optional, but having an LLM handle the template styling saves a lot of time

Step 1: Scaffold a Template for Building Block Academy

The sandbox UI compiles templates at startup from a…