On July 31, 2025, Google announced a new experimental API in Chrome 139 that aims to solve a critical problem in web performance and SEO analysis: the failure to accurately measure Core Web Vitals (CWV) on JavaScript-heavy Single Page Applications.
For years, developers, SEOs and performance engineers alike have wrestled with a glaring issue: most tools and metrics simply don’t know when to start measuring user interactions on SPAs.
Why? Because unlike traditional websites that reload a page for each new route or interaction, SPAs often rely on soft navigations which are JavaScript-driven changes that alter content and URLs without triggering a full page load.
The result? Metrics like Largest Contentful Paint (LCP), Cumulative Layout Shift (CLS) and Interaction to Next Paint (INP) become unreliable or completely missing in your performance data.
But now thanks to the Soft Navigations API, Chrome might finally bridge that gap.
Why This API Matters and Especially for SEO and Real User Experience
Let us start with one of the most important concepts. Core Web Vitals are not just better to save but they directly impact your SEO rankings.
Google uses CWV as a part of its page experience signals, which means if your site has poor scores then you are not just losing users but you are losing visibility.
But for SPAs built on frameworks like React, Vue, Angular or Svelte, achieving accurate CWV tracking has always been a nightmare.
Think about an e-commerce SPA where a user searches for a product and clicks a category. The interface updates instantly, no full reload.
But Chrome and most performance monitoring tools?
They don’t recognize that as a new “navigation,” so they do not track the real LCP or INP.
This leads to massive gaps in data. What users see and what Google sees become two very different things.
And that is a serious problem not just for developers but for product managers, marketing teams and anyone relying on web analytics to make decisions.
Enter the Soft Navigations API: How It Works and What It Tracks
Starting with Chrome 139, developers can now opt into an origin trial for the Soft Navigations API.
This new API is designed to detect when a “soft navigation” has occurred, based on a set of built-in heuristics.
So what exactly counts as a soft navigation?
According to Chrome’s documentation, a soft navigation is triggered when:
- A user clicks on a link inside the SPA
- The page’s URL changes (via pushState or replaceState)
- The DOM updates and triggers a new paint and especially if a large portion of the layout visibly changes.
When Chrome detects these conditions, it treats the event like a traditional page load which enables it to measure performance metrics tied to that interaction. That includes:
- interaction-contentful-paint: a new metric that captures the LCP after a soft navigation.
- navigationId: a unique ID associated with each detected navigation which allows developers to tie metrics to specific page states or URL paths.
- Extended layout shift and event timing data that spans across soft navigations, not just the initial page load
This is a massive improvement for Single Page Application monitoring, making it finally possible to track how real-world users experience performance beyond just the homepage.
How to Start Testing the Soft Navigations API
Ready to try it? There are two main ways to test the Soft Navigations API in Chrome 139:
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Local Testing
Visit chrome://flags/#soft-navigation-heuristics in your Chrome browser. Toggle the flag to enable the heuristics locally. This allows developers to simulate and debug how the new API behaves in test environments.
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Origin Trial
For real-world usage data, Google is offering an origin trial. You can sign up and add a token to your site via a <meta> tag or HTTP header. This allows Chrome to collect and send performance data tied to soft navigations from actual users.
Chrome also recommends enabling “Advanced Paint Attribution” (another experimental flag) to gather deeper insight into layout shifts and render timings.
What Google Is Saying and What You Should Watch Out For?
Barry Pollard, Chrome’s Web Performance Developer Advocate, is leading the charge on this new initiative. In his own words:
“Wanna measure Core Web Vitals for SPAs? Well, we’ve been working on the Soft Navigations API for that, and we’re launching a new origin trial from Chrome 139. Take it for a run on your app, and see if it correctly detects soft navigations on your application—and let us know if it doesn’t!”
Despite the excitement, it is important to remember this API is still experimental. A few key limitations to keep in mind:
- The feature is only available in Chrome 139+, so data collection across other browsers (like Safari or Firefox) will still be incomplete.
- Edge cases such as automatic redirects or some uses of replaceState () may still bypass detection.
- Your RUM tools may need updates to support the new navigationId and interaction-contentful-paint metrics for accurate correlation.
A Better Future for Performance Data in Modern Web Apps
Although it is early days, the Soft Navigations API could mark a turning point for performance analytics and technical SEO audits on modern web platforms.
Currently, Chrome’s performance tools and public datasets like CrUX have not yet integrated this API’s output but that could change if the origin trial proves successful and adoption picks up.
For developers working with Next.js, Nuxt or custom JavaScript routers, this tool has the potential to grow how we measure success not just on load but during the entire lifecycle of an app.
From Invisible to Insightful—A New Era of Core Web Vitals Tracking
Let’s be honest that performance tracking for SPAs has always been frustrating.
The elegance and responsiveness of modern JavaScript frameworks often come at the cost of visibility in SEO and analytics.
But Chrome 139 is flipping that narrative.
With the Soft Navigations API, Google is acknowledging that today’s web is not the web of yesterday.
And finally, it is offering developers the tools to measure performance in a way that matches real user behavior and not just cold technical definitions.
Dileep Thekkethil
AuthorDileep Thekkethil is the Director of Marketing at Stan Ventures and an SEMRush certified SEO expert. With over a decade of experience in digital marketing, Dileep has played a pivotal role in helping global brands and agencies enhance their online visibility. His work has been featured in leading industry platforms such as MarketingProfs, Search Engine Roundtable, and CMSWire, and his expert insights have been cited in Google Videos. Known for turning complex SEO strategies into actionable solutions, Dileep continues to be a trusted authority in the SEO community, sharing knowledge that drives meaningful results.