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You dropped the snippet in <head>. You verified it is there in view source. You opened your test page in a fresh tab. Nothing happens. Nine times out of ten it is one of the five things below.

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1. window.__ABTESTLY_CONFIG__ is undefined

The snippet did not load. Not “did not run”, did not even load.
Check, in order:
  • Is the snippet actually in <head>? Right-click the page → View source. Search for abtestly. It has to be a <script> tag inside <head>.
  • Is it the first script tag? Plugins sometimes inject other tags above ABTestly. Ours has to be first.
  • Is cdn.abtestly.com blocked? Try incognito with ad blockers disabled. Some blockers need it allowlisted.
  • Is the snippet the right one for this domain? Each site has its own snippet, keyed to its own domain. A snippet from site-a.com running on site-b.com does nothing.
If the config still comes back undefined after all of this, the snippet URL itself is probably not reachable, inspect the Network tab for a request to cdn.abtestly.com/s/<key>.js and its status.

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2. Config is loaded but has no experiments

You have no live experiments on this site.
  • Are there any in Live status?
  • If yes, are they targeting a Location that matches this page?
  • Did you save an experiment but never click Start?
Draft experiments do not go in the config. Only Live and Scheduled experiments do.

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3. Config has experiments but none apply

Look at the events buffer for assignment events. If there is none, the experiment did not consider you.
  • Location did not match. The URL rules do not match the current URL. Open the experiment’s Location editor; check operator and case sensitivity.
  • Audience did not match. Your visitor properties (device, country, cookie) did not qualify. The debugger tells you which rule failed.

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4. Assignment happened, variant did not apply

Debugger says the assignment succeeded but the change is not visible.
  • Selector typo in variant JS? document.querySelector('.hero_cta') when the element is .hero-cta. Look for variant_apply_error events.
  • Element was not on the page yet? Use abtestly.waitFor(selector, cb) instead of querySelector.
  • SPA that moved past the target? Turn on SPA support in Settings → SPA. Use onApply and onCleanup in your variant.

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5. Variant applied, no beacon fired

Debugger shows a variant_applied event but no beacon event.
  • Consent gate is still deferred. Check consentMode and consentKey. If consent is deferred and not granted, the runtime waits.
  • Visitor is opted out. Check __abtestly_optout=1 in the cookies for this domain.
  • Preview mode. Preview never fires real beacons, by design.

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The nuclear option

Open preview on the experiment. Preview runs the full flow with the QA overlay on and tells you why each step did or did not fire, you do not have to guess.