GA4 hostname filters gain allowlists; source grouping explained

What changed

Google’s 21 September 2026 release notes add Include filters for hostnames to Google Analytics. A team can define the domains approved to send event data to a property, instead of maintaining only an exclusion list for unwanted hostnames. This extends the hostname exclusion capability documented on 11 June; it does not mean hostname filtering first appeared in September.

Two exceptions determine how the new control should be assessed. Google says hostname Include filters do not apply to Measurement Protocol events. It also says events with an empty hostname are automatically blocked by an Include filter. An approved-domain list therefore does not validate every event reaching the property, and an empty hostname needs investigation before the team assumes that all of the affected traffic is unwanted.

For advertisers, the useful preparation is an inventory of the domains that legitimately contribute to the same measurement journey. Check the main site, relevant subdomains and any other intended web destinations against the proposed list. Keep web-tag traffic and Measurement Protocol submissions distinct when checking coverage. Compare expected events with what the property records after an authorised change; the existence of a filter is not evidence that a campaign’s conversions are complete.

The June announcement also introduced Source Group, a separate reporting dimension that consolidates inconsistent source values into a common label. Google describes Source Platform as an existing field whose classifications were being updated alongside it. The earlier PMC summary blurred these roles. Google’s example pairs Instagram as the Source Group with Meta Ads as the Source Platform, rather than treating Source Platform as the new grouping dimension.

Google says Source Group is populated retrospectively. That statement concerns reporting classification and should not be read as a promise that hostname filters clean historical data. Use the source fields to compare channel reporting, and assess hostname filters as collection controls. They address different questions, even though both appeared in the June release notes.

Why it matters for advertisers

Cleaner source labels can make cross-channel comparisons easier, while a hostname allowlist can reduce unwanted web-event input. Neither establishes that attribution is correct or that every legitimate event is retained. The original June newsletter coverage and its credits to Nichelle Halstrom, Himanshu Sharma and Michele Pisani remain attached to this page; the September update is supported by Google’s release notes.