Coding agents ship UI fast, but they design from vibes. Rubric gives them a source: 554 curated UX rules with severity, evidence and a concrete fix, served over MCP. One corpus, two moments. Before you build, your agent pulls prescriptive guidance, not generic best practices. After you ship, it audits the live site against the same rules and returns findings ranked by severity: issue, impact, actions, references. Claude Code, Cursor, Windsurf, Cline. Browser approval, no API keys.