A tool that quietly launched to address a gap in the measurement ecosystem today surfaced in a LinkedIn discussion that is catching attention from analytics and ad tech professionals. isblocked.fyi, a free domain lookup and monitoring service, aggregates filter lists from the four major adblockers - uBlock Origin, Brave, AdGuard, and AdBlock Plus - into a single searchable index. The announcement comes at a moment when the conflict between tracking infrastructure and browser-level blocking tools has sharpened considerably, with server-side Google Tag Manager subdomains now appearing by name in the same lists that isblocked.fyi indexes.

What isblocked.fyi does

According to isblocked.fyi, the tool is a union index of major adblocker filter lists. It aggregates 22 lists and maps each one to the blockers that ship it by default or as an optional add-on. The service covers lists from uBlock Origin, Brave, AdGuard, and AdBlock Plus - four of the most widely deployed browser-based blocking extensions.

The core function is a free, no-login lookup. A user enters a domain - for example, google-analytics.com or a custom tracking subdomain - and the tool returns every matching entry across its index. Match types distinguish between exact matches, parent domains, subdomains, and keyword substrings. Each row in the results shows the filter list name, the entry category (ads, tracking, malware, and so on), the block type, which blockers use that list, and the date the domain was first seen. Historical data is also shown: if an entry was previously listed and then removed, that removal date appears alongside the original first-seen date.

According to isblocked.fyi, it is the only tool of its kind that combines a searchable union index with built-in domain monitoring in one product. Other resources may cover individual lists or perform one-off lookups, but none offer daily search with alerts when list membership changes.

The block type distinction matters

Not every filter list entry blocks all traffic to a domain. According to isblocked.fyi, many tracking domains are blocked only as third-party resources - meaning when loaded from another site - and not when visited directly. Others block all traffic, scripts only, or XHR calls only.

The distinction is technically significant. A domain flagged as "3rd-party only" still functions when a user navigates to it directly, but any script or pixel from that domain embedded on another page will be blocked. For analytics and conversion tracking infrastructure, which typically operates as embedded resources on a publisher or advertiser's site, even a third-party-only block is effectively a complete block for measurement purposes.

uBlock Origin, the most widely used blocking extension, defaults to EasyList and EasyPrivacy. Brave has its own lists. AdGuard and AdBlock Plus each ship their own defaults. Users can also enable optional lists - Fanboy Annoyances, regional lists, and similar additions. According to isblocked.fyi, a domain blocked only by an optional list affects a smaller share of users than one blocked by a default list. The tool separates "Blocked by default" from "Optional lists" in its results to allow this distinction to be evaluated.

Why this matters now

The timing of the tool's broader visibility is not incidental. Mariusz Brucki, a server-side tracking consultant at TAGGRS, posted about isblocked.fyi on LinkedIn on June 4, 2026, referencing a disclosure by Simo Ahava - a widely cited analytics practitioner and co-founder at Simmer - that his own server-side GTM subdomain, sgtm.simoahava.com, had landed on a blocklist used by uBlock Origin. When a visitor to Ahava's blog browses with uBlock Origin active, the gtm.js request is blocked entirely.

According to Brucki's post, the issue is not limited to Ahava's domain. Brucki noted that during a session at MeasureCamp Zurich, a participant reported that their own subdomain had landed on the same type of list. The data gap that results depends on the audience: according to Brucki, "depending on the audience, that's a 5-40% gap in your data."

The specific phenomenon Brucki described - a custom subdomain used for server-side tracking being individually named in a blocklist - has been documented in detail by PPC Land in coverage published on May 31, 2026. The AdGuard filter file tracking_servers_firstparty.txt contains over 3,400 blocking rules targeting first-party tracking subdomains. Among those are 83 entries matching the gtm. pattern and 42 entries matching sgtm. prefixes - the naming convention adopted widely by organisations deploying server-side GTM containers. The EasyPrivacy file easyprivacy_specific.txt, included by default in uBlock Origin, also contains specific subdomain entries.

What makes this structurally different from earlier blocking approaches is the method. Traditional blocklist rules targeted known third-party origins: scripts loading from googletagmanager.com or google-analytics.com were caught because those domains were publicly associated with tracking infrastructure. Server-side GTM was designed partly to sidestep that mechanism - events would originate from the advertiser's own subdomain, appearing as first-party requests that generic rules would not match.

That assumption has been narrowing. Filter list maintainers have shifted toward cataloguing specific subdomains by name, whether through DNS inspection, public records, or community reporting. Once a specific subdomain is named in a list that ships by default in uBlock Origin or AdGuard, the first-party origin no longer provides protection.

The monitoring layer

The lookup function on isblocked.fyi requires no account. Monitoring is different: it requires sign-in, and once logged in, a user can add a domain or regex pattern to a dashboard. According to isblocked.fyi, all indexed lists are checked daily. When a monitored domain is added to or removed from any list, the system dispatches an email alert or a webhook POST, depending on the user's setup.

The history tracking feature is particularly relevant for retroactive diagnosis. According to isblocked.fyi, users can see when a domain first appeared on a list and when it was removed, which allows changes in blocklist membership to be correlated against drops in analytics data. This closes a diagnostic gap that has been a recurring problem in measurement: a data volume drop appears in a dashboard, but without knowing when a blocklist change occurred, isolating the cause is slow and often inconclusive.

Brucki described the tool in his LinkedIn post as the only aggregator on the market with built-in monitoring and notifications. The tool's About page positions it for three categories of professional: analytics engineers validating data pipeline coverage, MarTech teams managing third-party tag inventories, and anyone running third-party tracking, analytics, or attribution infrastructure.

The broader measurement context

The question of which tracking domains are blocked - and by which blockers - sits at the intersection of several trends that have been reshaping measurement infrastructure since 2020. Browser-level privacy features, beginning with Apple's Intelligent Tracking Prevention and its application to iOS browsers, progressively restricted the lifespan and scope of cookies set by JavaScript. Server-side approaches were developed in part to work around those restrictions, because cookies set by a server running on the operator's domain are treated differently from those set by third-party scripts.

Google has pursued a parallel track through its tag gateway programme, which routes Google tags through content delivery network infrastructure integrated with an advertiser's own domain. The tag gateway reached general availability in May 2025 with Cloudflare as the initial partner, reporting an 11% signal uplift. Subsequent CDN integrations with Google Cloud Platform, Akamai, and Fastly followed in sequence. A Fastly integration announced in April 2026 cited a 14% uplift figure. A May 2026 update to the tag gateway introduced the option to randomise the serving path for GTM container IDs, addressing a further vector by which blockers could identify GTM implementations even when first-party routing was in use.

Chrome's 2025 transition to Manifest V3 altered the technical surface available to browser extensions, requiring extensions to use the declarativeNetRequest API rather than the webRequest API that uBlock Origin had relied on for dynamic filtering. uBlock Origin moved to a Lite version for Manifest V3 environments, with some dynamic filtering capabilities reduced. The effect on blocking rates in Chrome - and the degree to which blocklist coverage of server-side infrastructure compensates for reduced dynamic capabilities - remains an open empirical question.

First-party mode for GTM, the forerunner of the tag gateway product, was described by PPC Land in July 2024, at which point industry observers including Ahava were already raising questions about whether the architecture's primary effect was measurement improvement or ad blocker circumvention. That debate has continued through each subsequent iteration of the product. isblocked.fyi's positioning as infrastructure for "the tracking and analytics side of the market" reflects how that debate has evolved: the problem is no longer whether blocking happens, but whether practitioners know about it before it appears as unexplained gaps in their dashboards.

What the tool does not do

A distinction that isblocked.fyi is careful to draw on its About page: it is not an adblocker. According to the site, it does not block anything. It indexes what blockers block. This distinction matters in practice because the tool's utility flows from that indexing function - it answers the question of whether a domain is already being blocked, not whether it should be.

The practical consequence of appearing on a default list such as EasyPrivacy - which ships enabled in uBlock Origin's standard mode - is that scripts and pixels from the listed domain will not load for a meaningful share of users without any user action required. According to isblocked.fyi, no pixel fires, no session replay runs, and no attribution data is collected from those users. Revenue reporting drops. Campaign attribution breaks. Product analytics go dark - and the team finds out weeks later, when someone notices a gap.

Brucki's comment thread on LinkedIn illustrates what that looks like in practice. In a reply, Brucki confirmed that there are already top-level domains that are fully blocked by default, pointing to webwise.com as an example on Dan Pollock's Hosts File list. A visitor to that domain with uBlock Origin configured to use that optional list would see a blocked warning rather than the intended page.

Who built it and how it fits the market

According to isblocked.fyi's About page, the tool's stated target is professionals who need to know when their domains get flagged - described in the framing as "infrastructure for the tracking and analytics side of the market." The positioning is deliberately narrow. It does not offer tag management, consent management, or implementation services. It offers visibility.

The free tier covers the search function without requiring registration. Monitoring - the daily check of all indexed lists and the delivery of alerts via email or webhook when membership changes - requires sign-in. The tool's mention of webhook POST delivery as a notification option indicates that it is designed to fit into existing monitoring workflows, not only to serve as a manual lookup destination.

The 22 lists currently aggregated span the defaults and optional lists across the four major blockers. Each list is mapped to the blockers that ship it, which means a result flagged on EasyPrivacy carries a different practical significance than one flagged only on a regional optional list that fewer users activate

Timeline

Summary

Who: Mariusz Brucki, a server-side tracking consultant at TAGGRS, surfaced isblocked.fyi in a LinkedIn post on June 4, 2026. The tool itself is a domain lookup and monitoring service built to serve analytics engineers, MarTech teams, and professionals managing tracking and attribution infrastructure. The wider industry context involves Simo Ahava, co-founder at Simmer, whose own server-side GTM subdomain appeared in adblocker filter lists, and Denis Golubovskyi, founder and CEO at Stape, whose infrastructure has also attracted blocklist entries.

What: isblocked.fyi is a free, publicly accessible tool that aggregates 22 major adblocker filter lists - including those from uBlock Origin, Brave, AdGuard, and AdBlock Plus - into a single searchable index. It allows any domain to be checked against all indexed lists simultaneously. The monitoring feature, which requires sign-in, checks all indexed lists daily and delivers email alerts or webhook notifications when a domain's blocklist status changes. The tool also tracks historical list membership, showing when a domain first appeared and when it was removed.

When: The tool was publicly highlighted on June 4, 2026, in the context of ongoing industry discussion about server-side tracking subdomains appearing in major adblocker filter lists. The specific blocklist entries for sgtm. subdomains were reported by PPC Land on May 31, 2026. The AdGuard tracking_servers_firstparty.txt file, which contains over 3,400 first-party tracking subdomain rules, and the EasyPrivacy easyprivacy_specific.txt file, which is included by default in uBlock Origin, both contain specific named subdomain entries.

Where: isblocked.fyi is a web-based tool accessible at isblocked.fyi. The filter lists it indexes are maintained in publicly accessible GitHub repositories by the EasyList and AdGuard projects. The discussion that brought the tool to wider attention originated on LinkedIn, with references to a session at MeasureCamp Zurich.

Why: Blocklist exposure is a measurement failure mode that typically surfaces weeks after it begins - when someone notices a gap in data volume - rather than at the moment a domain is added to a list. According to isblocked.fyi, when a tracking domain lands on a default filter list such as EasyPrivacy, scripts and pixels from that domain stop loading for users with blockers active, with no visible error. Attribution data goes missing, campaign signals degrade, and product analytics lose coverage. The 5 to 40% data gap estimate cited by Brucki reflects real variability depending on the audience composition of a given site. The tool addresses this by making blocklist status visible before it reaches dashboards.