fastmon Beacon Inspector

DevTools panel that live-captures a page's fastmon collector beacons, grouped by pageview, and shows how the data evolves.

Inspect the fastmon Real User Monitoring (RUM) beacons your site sends, right in the browser. For every pageview it shows exactly what data fastmon captured and how it evolves across the beacon lifecycle: init, loaded, soft navigation, updates, and the final hidden / frozen / terminated beacon. What it does: • Live-captures the collector beacons of the current page and groups them by pageview (pvid). • Full DevTools panel (the "fastmon" tab): Core Web Vitals (LCP, INP, CLS, FCP, TTFB) rated good / needs-improvement / poor, the merged current state, an optional per-beacon-version diff, navigation-waterfall and vitals-subpartcharts, third-party breakdown, JavaScript errors with clickable stack frames, and a Markdown export (raw or anonymised). • Reads the terminal beacons the DevTools Network tab cannot show (sendBeacon request bodies), so a pageview finalises correctly instead of looking stuck. • The toolbar icon shows at a glance whether the current page uses fastmon. • Click the icon for a quick overview: key Core Web Vitals, CDN and origin cache status (from Server-Timing), and a JavaScript-error hint, plus a link to the fastmon dashboard. • Optional on-page overlay (bottom-right) with TTFB, LCP, CLS and cache status, live and always visible. • Works even on pages without fastmon: measures basic RUM locally from the browser's Performance APIs (Core Web Vitals, navigation timings, Server-Timing) and shows it in the panel, popup and overlay, clearly marked as "basic RUM". Real fastmon beacons always take over when present. • English and German interface, chosen automatically from the browser language and switchable in the panel. Privacy: everything runs locally in your browser. The extension reads the page's fastmon collector beacons and, on pages without a fastmon snippet, measures basic RUM via the browser's Performance APIs; with an optional setting it also reads a few cache headers of the main document. It never reads page content or request/response bodies and sends nothing to any server. See the privacy policy for details. What is fastmon? fastmon is a Real User Monitoring service: it measures the performance your real visitors actually experience (Core Web Vitals such as LCP, INP and CLS, load and navigation timings, cache and Server-Timing, JavaScript errors, third-party impact) and reports it in a dashboard at https://app.fastmon.eu. This extension is a companion developer tool for inspecting the raw fastmon beacons of a page - or, on pages without fastmon, its own basic RUM measurement; it does not collect, store or transmit any monitoring data itself.