Understand when third-party cookies are available and make more money.
Since the earliest days of the web, one principle has allowed millions of sites, browsers, devices, and services to work together seamlessly: interoperability.
The internet grew into a universal medium precisely because no single company or platform dictated how information flowed. Shared standards and practices, predictable behavior, and mutual compatibility made the web feel like one coherent place rather than a patchwork of disconnected systems.
But in recent years, this foundation has begun to erode.
Sometimes with good intentions — often with unintended consequences — browsers and platforms have introduced changes that quietly reduce interoperability. These shifts frequently impact developers, analytics teams, publishers, and advertisers who rely on consistent, predictable browser behavior to build reliable digital experiences.
One of the clearest examples of this trend is the handling of third-party cookies.
How the web gained — and is now losing — its memory
In its original form, the web had no memory at all. Each page request was independent, stateless, and unaware of anything that came before. This simplicity made the web easy to scale, but severely limited what developers could build. Shopping baskets, logins, personalised content, and analytics — all the core capabilities we now take for granted — were impossible.
Cookies solved this problem elegantly. By allowing a server to store small pieces of information in the browser, the web suddenly gained the ability to remember.
Are third-party cookies bad for privacy?
Over time, some uses of third-party cookies were associated with invasive tracking practices. This led to a widespread argument that all third-party cookies are inherently “bad for privacy.”
But this view misses an important nuance: cookies themselves are neutral technology. Like JavaScript, HTTPS, HTML forms, or IP addresses, they are simply interoperability mechanisms. They can be used responsibly or irresponsibly — the technology is not the problem; the use of them might be in some situations.
Yet blanket restrictions have been introduced by some web browser vendors. And because browsers implement these restrictions differently, the result is a web where the availability of third-party cookies varies dramatically across platforms, versions, people, and contexts. For organizations trying to operate consistently across devices, this unpredictability is a major issue.
Are third-party cookies good for publishers?
Google's research into the revenue decline to publishers associated with removing third-party cookies in 2019 indicated a 64% reduction.
The UK's Competition and Market's Authority in their 2020 report into digital markets shows the same 70% decline.
Research for Garrett Johnson, Associate Professor at Questrom School of Business, summarizes other studies. The trend is the same, third-party cookies help publishers.
Why 51Degrees built third-party cookie detection
51Degrees really cares about publishers of all sizes.
Our mission has always been to provide accurate, real-time information about the user's environment, enabling organizations to adapt gracefully to the realities of diverse devices and evolving browser behavior. With interoperability decreasing, high-quality detection becomes even more essential.
To address the uncertainty around third-party cookie availability, we’re introducing two new properties:
1. ThirdPartyCookiesEnabled
Indicates whether third-party cookies are enabled. If the supporting JavaScript has not yet executed, this property provides the most likely status, inferred from known browser behaviour, versions, and platform characteristics.
The values returned are;
- NotSupported. The browser is not able to support third-party cookies.
- True. Third-party cookies are enabled and this has been validated by ThirdPartyCookiesEnabledJavaScript.
- False. Third-party cookies are not enabled and this has been validated by ThirdPartyCookiesEnabledJavaScript.
- ProbablyTrue. Third-party cookies are enabled by default for this browser. The current status has not been validated by ThirdPartyCookiesEnabledJavaScript.
- ProbablyFalse. Third-party cookies are not enabled by default for this browser. The current status has not been validated by ThirdPartyCookiesEnabledJavaScript.
- Unknown. The default status of third-party cookies for this browser is not known. The current status has not been validated by ThirdPartyCookiesEnabledJavaScript.
2. ThirdPartyCookiesEnabledJavaScript
Contains JavaScript that can be used to perform a live test in the user’s browser. When executed, its result overrides the most likely value with a true value.
Together, these two properties provide:
- A high-confidence, per-request understanding of third-party cookie availability
- A standardized method of checking support across all major browsers
- A reliable approximation when client-side execution isn’t available
- A unified interface so implementers don’t need to maintain their own detection logic
In essence: we bring clarity to an area where the industry has been forced to rely on assumptions.
Use cases - where this matters most
1. Prebid & Ad Tech supply-chain enhancements
Modern advertising systems increasingly require clear, actionable signals about browser capabilities.By surfacing third-party cookie availability early — even at bid-request time — publishers and SSPs can:
- Tailor demand based on cookie-enabled vs. cookie-restricted environments
- Provide more accurate signals to buyers who have shown a willingness to pay more when third-party cookies are available
- Improve auction efficiency
- Enable strategies for cookieless contexts
This is particularly valuable in Prebid.js integrations, where knowing the cookie landscape upfront can influence bidding logic and adapter behaviour.
2. Analytics and attribution reliability
Analytics platforms frequently rely on cross-site data or need to confirm whether cookie-based methods will function. Detection helps teams:
- Avoid skewed metrics caused by silent cookie blocking
- Switch to first-party or server-side strategies when appropriate
- Improve modelling accuracy by tagging sessions with cookie availability status
3. Feature adaptation and progressive enhancement
Websites can dynamically adjust behaviour — e.g., switching storage mechanisms or adjusting login flows — based on whether third-party cookies will work.
Building a more predictable web
The broader trend in the industry is clear: browsers are diverging, defaults are changing, and assumptions that once held true no longer do. As interoperability decreases, the importance of device and capability intelligence increases.
Our new third-party cookie detection properties give developers, publishers, and platforms the clarity they need to operate confidently in this shifting environment.
Learn more and get started
Please contact us to discuss how our third-party cookie detection capabilities can make your business more money.