Seznam is changing how campaigns are measured. Seznam Event Measurement (SEM) is the new unified standard that replaces the previous separate retargeting and conversion codes. Here's what it means and how to run SEM server-side.
What SEM is and why it exists
Previously you placed a separate retargeting code (rc.js) and separate conversion codes — a snippet per conversion. SEM unifies this into a single sul.js script that handles both retargeting and conversions and supports multiple event types with detailed reporting. SEM activation rolls out during 2026.
What changes vs the old codes
- One
sul.jsscript instead of separate retargeting and conversions - Multiple event types + automatic retargeting straight from events (no manual lists)
- Consent via IAB TCF or Google Consent Mode
- New: server-to-server (S2S) measurement via
sem.seznam.cz
How SEM works server-to-server
S2S is the key addition for anyone losing client-side events to ad-blockers and Safari ITP (typically 20–35%). How it works:
sul.json the site generates the first-party cookiessidandudid— Seznam uses them to match identity.- Your server (i.e. server-side GTM) reads those cookies, hashes personal data with SHA-256 and sends the event with your SEM ID to
sem.seznam.cz/rtgconv. - The call runs asynchronously in the background (3–5 s timeout), so it never blocks order processing.
Note: S2S is not a standalone solution — sul.js must run in the browser (without it there are no sid/udid cookies and Seznam can't match identity). Server-to-server is a complement to client measurement: the server call itself isn't blocked by ad-blockers, so the client + server combination recovers the data that client-only sul.js loses.
Why do it server-side
Sklik Smart bidding and retargeting are only as good as the data. If you lose a fifth to a third of conversions client-side, optimisation runs on incomplete numbers and CPA rises. SEM S2S closes that gap, and Sklik and GA4 reports finally agree (same transaction_id, same value).
SEM with DataNostro
DataNostro deploys SEM in server-side mode out of the box: sul.js on the site, reading sid/udid on the server, SHA-256 hashing and sending to sem.seznam.cz/rtgconv with your SEM ID. The same "purchase" event goes to GA4, Meta CAPI, Google Ads and SEM at once — one source, attribution everywhere. Remove the old rc.js and conversion codes once SEM measures correctly.