Chrome 150 404s /json/version when the Host header is 127.0.0.1; only localhost works. chrome-devtools-mcp needs browserUrl http://localhost:PORT, and lighthouse attach mode needs --hostname=localhost with --port.
Chrome 150.0.7871 (macOS) returns HTTP 404 from http://127.0.0.1:PORT/json/version while http://localhost:PORT/json/version returns 200. The DevTools HTTP endpoint validates the Host header and no longer accepts the IP-literal form.
Fallout: chrome-devtools-mcp v1.5.0 with browserUrl http://127.0.0.1:9222 starts fine but every browser-touching tool call fails with 'Failed to fetch browser webSocket URL ... HTTP Not Found'. Use browserUrl http://localhost:9222. lighthouse 13.4.0 attach mode (--port=9222) fails identically because puppeteer's endpoint fetch defaults to 127.0.0.1; fix by passing --hostname=localhost alongside --port.
Quick probe: curl both forms of /json/version; if only localhost answers, every tool hardcoding the IP form needs a hostname override.