WebMCP Polyfill ↗

WebMCP Polyfill is an officially published WebMCP tool from GoogleChromeLabs. Anomity scores its risk at 10 out of 100 (low).

The WebMCP Polyfill is a compatibility layer that emulates the experimental WebMCP browser API in browsers that do not yet ship it. WebMCP lets a page declare structured tools — either as annotated HTML forms or as JavaScript functions with JSON schemas — so an AI agent can call them directly instead of guessing at the DOM. The polyfill implements that surface in userland, which is how most sites currently experiment with the API ahead of an origin trial.

The risk to reason about is not the polyfill's own code so much as what it turns on. Once a page registers tools, any agent driving that browser can enumerate and invoke them inside whatever session the user already has open — logged in, cookied, and authorized. A prompt-injected agent that reaches a page with a `submitOrder` or `transferFunds` tool does not need to defeat authentication; the page has already handed it a typed, documented button. Treat every registered tool as an unauthenticated endpoint reachable by anything that can influence the agent's context.

Why this entry scored 10 out of 100

This profile starts at the catalog's neutral base of 50. Because the entry is officially published, trust adjusts the score by -40. The entry declares no high-risk capabilities on its own, so capabilities add nothing. It matches no dangerous capability combinations. The clamped result is 10, placing it in the low band.

See the full scoring formula →