Ad tag tester: test any ad tag with the right tool
"Ad tag" covers several different formats, and each one needs a different test. Match what you were handed to the row below, then open the tool built for it. Everything here is free and runs without signup.
Identify your ad tag
| What it looks like | Tag type | Test it with |
|---|---|---|
| URL that returns XML, or a document starting with <VAST | VAST video ad tag | VAST tag tester |
| Raw VAST XML you already resolved | VAST document | VAST tag validator |
| <script> snippet or HTML from an ad server | Display / JavaScript tag | Sandbox page + devtools (guide below) |
| 1x1 <img> or bare tracking URL | Tracking pixel | pixellint |
| JSON bid request or response | OpenRTB payload | rtblint |
Video ad tags (VAST)
Most "test my ad tag" requests in video land here. A VAST tag is either a URL that returns XML or the XML document itself. The VAST tag tester fetches the URL, previews the creative, and shows tracking and click URLs in one pass. If you already have resolved XML and only need spec compliance, the VAST tag validator runs a rule-by-rule check against IAB VAST 2.0–4.4. For wrapper chains that break somewhere between hops, the VAST inspector traces each redirect.
Display and JavaScript ad tags
Display tags are executable code, so the honest way to test them is to run them in an environment you control and watch what they do:
- Drop the tag into a bare HTML page (or your ad server's preview sandbox), never a production page.
- Open the browser network panel. Every request the tag makes should be HTTPS; one impression should fire, not zero and not two.
- Confirm the creative renders inside its declared width and height and does not use
document.writeafter page load. - Click through and verify the landing page and any click trackers in the chain.
If the "tag" you are testing is actually a 1x1 image or a bare tracking URL, it is a pixel, not a display creative: pixellint validates those.
Free tools by tag type
VAST Tag Tester
Test a live VAST tag URL: fetch, preview, tracking, and click URLs.
VAST Tag Validator
Validate resolved VAST XML rule by rule against IAB VAST 2.0–4.4.
VAST Inspector
Trace wrapper chains hop by hop when the tag dies between redirects.
Common VAST errors
The failures that show up most in real tags, with fixes.
Frequently asked questions
What is an ad tag tester?
An ad tag tester is a tool that takes the tag an ad server or partner hands you and checks that it loads, renders, and tracks correctly before launch. Because "ad tag" covers several different formats (VAST XML for video, script tags for display, image pixels for tracking), the right tester depends on the tag type.
How do I test a video (VAST) ad tag?
Paste the VAST tag URL or raw XML into a VAST tag tester. It fetches the tag, previews the creative, shows tracking and click URLs, and validates the XML against the IAB spec. The vastlint tester at vastlint.org/tester does this in the browser with nothing stored.
How do I test a display or JavaScript ad tag?
Load the tag in an isolated HTML page or a publisher sandbox and watch the network panel in browser devtools. Check that the script loads over HTTPS, renders inside its declared dimensions, fires exactly one impression, and does not document.write into the parent page. Display tags are executable code, so test them in an environment you control, never directly on a production page.
How do I test a third-party tracking pixel?
Request the pixel URL directly and confirm it returns a 200 with an image content type over HTTPS, then confirm it fires exactly once per impression in the page or player. For structural checks on pixel tags, pixellint covers tracking pixel validation the same way vastlint covers VAST.
Can I preview an ad tag before launch?
Yes. For VAST tags, a tester with creative preview shows the video, companion banners, and click-through behavior before any live traffic hits the tag. For display tags, preview in a sandbox page. Previewing before launch catches broken creatives, mixed-content blocks, and wrong click URLs while they are still cheap to fix.
What is the difference between a tag tester and a tag validator?
A tester exercises the tag end to end: fetch, render, track, click. A validator checks the tag document itself against a specification, rule by rule. For video, run both: validate the VAST XML for spec compliance, then test the live URL for delivery behavior.