Google Chrome is rolling out an experimental feature designed to solve one of the trickiest problems in web performance: accurately measuring Core Web Vitals in Single Page Applications (SPAs).
From Chrome 139 onward, developers can join an origin trial for the Soft Navigations API — a tool that makes it possible to track key metrics like LCP (Largest Contentful Paint), CLS (Cumulative Layout Shift), and INP (Interaction to Next Paint) even when pages update without a full reload.
Why SPAs Have Been Hard to Measure
SPAs are beloved for their speed and app-like experience, but they’ve long posed a challenge for performance analytics and SEO audits.
Unlike traditional sites, SPAs often swap out content using JavaScript rather than triggering a full page load. That means standard measurement systems — whether it’s Lighthouse, Chrome UX Report (CrUX), or most Real User Monitoring (RUM) setups — simply don’t “see” those updates, leaving big gaps in the data Google uses for rankings and site evaluations.
The Soft Navigations API closes this gap, allowing developers to measure user experience during in-app transitions as accurately as they can for traditional page loads.
How the Soft Navigations API Works
Chrome uses built-in heuristics to detect when a “soft navigation” occurs, such as:
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A user clicks a link
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The page URL changes
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The DOM updates in a visible way and triggers a new paint
When these conditions line up, Chrome treats it as a navigation event for performance tracking — even though the page never fully reloads.
The API also adds new measurement capabilities:
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interaction-contentful-paint: Captures LCP specifically for soft navigations -
navigationId: Tags performance entries so you can link metrics to specific navigations, even if the URL changes mid-session -
Enhanced CLS, INP, and event timing support across soft navigations
How to Try It
You can start testing now in Chrome 139 via:
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Local testing – Enable the flag at:
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Origin trial – Add the trial token to your site via a meta tag or HTTP header to gather real-world RUM data
For best results, Chrome also suggests turning on the Advanced Paint Attribution flag.
Early Caveats
Barry Pollard from the Chrome team, who is leading the project, stresses that the API is still experimental:
“We’ve been working on the Soft Navigations API for measuring Core Web Vitals in SPAs. Give it a try, and let us know if it misses any soft navigations in your application.”
A few things to keep in mind:
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Not all browsers (or older Chrome versions) will support it
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RUM providers may need to add support for
navigationIdandinteraction-contentful-paint -
Edge cases like automatic redirects or
replaceState()may not register as navigations
What’s Next
The Soft Navigations API isn’t yet integrated into public Chrome datasets like CrUX — but that could change if testing goes well.
If you’re building with React, Vue, Angular, or any JavaScript-heavy SPA framework, now is a good time to try the trial and see how it improves your Core Web Vitals visibility.
With accurate SPA tracking, site owners could finally get a full picture of user experience — and possibly a performance edge in search rankings.
