How to Build a WebMCP Scoring Tool in Plain HTML With No Dependencies

A developer tutorial published on DEV Community walks through building a WebMCP tool from scratch using only a plain HTML file served via Python's built-in HTTP server. The guide demonstrates how to register a tool called 'score_facts' that evaluates a YAML document against five required fields — name, goal, who, what, and why — and returns a 0–100 score. Chrome supports the document.modelContext API natively behind an experimental flag, while other browsers can use a polyfill as a fallback. The tutorial emphasises defensive coding practices, including input validation and try/catch error handling, so that tool failures return structured error objects rather than raw stack traces. A security section also introduces URL allowlisting to prevent remote-fetch tools from being exploited as open proxies.