# About 8 percent of sessions never send events, traced it to ad blockers

Asked by Silvia Cardoso on 2025-10-05. Tags: errors, ad-blockers, tracking.

As the growth lead I reconciled our Croct session counts against raw server logs
and found a consistent gap: roughly 8.2% of sessions in the logs never appear in
Croct, and the gap jumps to about 19% on Firefox specifically.

Dug into it with a blocker-enabled browser and the pattern is clear: requests to
the evaluate and track endpoints get blocked by EasyPrivacy-listed rules, so
those sessions send nothing.

Before I write this off as measurement noise, how are other teams handling it?
Losing a fifth of Firefox traffic from every experiment sample worries me.

## 2 answers

### Answer from Petra Novak (2025-10-05)

You diagnosed it correctly. EasyList/EasyPrivacy block the
`/client/web/evaluate`, `/client/web/track`, and `/content` endpoints in any
browser with those lists enabled, so client-side calls from affected sessions
never arrive.

The remedy is first-party proxying. Point the SDK at your own domain and
forward to Croct server-side, so the browser only ever makes same-origin
requests that blockers have no reason to touch:

```js
croct.plug({
    appId: 'YOUR_APP_ID',
    baseEndpointUrl: 'https://yourdomain.com/api/croct',
    cidAssignerEndpointUrl: 'https://yourdomain.com/api/croct/cid',
});
```

Behind those paths you run a thin reverse proxy to the Croct endpoints. We did
this after finding a similar gap and the blocked share dropped to effectively
zero.

### Answer from sam_the_dev (2025-10-06)

If any of your content is resolved server-side with `fetchContent`, that
part is already unaffected. The server talks to
Croct directly and the browser never makes the call, so blockers cannot
interfere with what gets rendered. Ad blockers are a known blind spot for
client-side testing tools generally, they silently exclude a chunk of traffic
and skew samples exactly the way you noticed.

So the practical split is: server-side resolution for content and variant
assignment (immune by design), first-party proxy per Petra's answer for the
client-side event tracking. Between the two your Firefox gap should close
almost entirely.