A screenshot of the above-the-fold, a handful of findings, coordinates. The kind of note I'd give a client on a Tuesday call. Vision call with a cost cap and rate limit, roughly $0.005 per analysis; your URL is hashed before anything leaves the page.
Works best on marketing pages. Try https://stripe.com, https://linear.app, https://framer.com.
Same pattern I'd build inside an internal review tool — yours, or your client's if you're a studio: ingest a screenshot, return specific findings with coordinates, skip the generic platitudes. The flow above is the public-facing version — two runs per IP per day so it stays cheap. Architectures, prompts, and tool schemas are documented per engagement.