VAST error code

303 — No ads returned after one or more wrappers

What the spec says

The chain resolved successfully but the final response contained no ad. The wrappers worked; there was simply nothing at the end.

What actually causes it

Genuine no-fill — the most common cause by a wide margin, and not an error in any meaningful sense.
Targeting at the final hop excluding the request.
Floor price above what the demand at the end of the chain would pay.
A frequency cap already reached for that user.

How to fix it

Treat a moderate rate of 303 as normal rather than as a fault; it is what no-fill looks like from the player's side. Investigate only when the rate climbs sharply or reaches 100%, which points at targeting or floor configuration at the last hop rather than anything in the tag.

To see whether a tag would hit this before it runs, paste it into the VAST validator. It reads the XML and reports the conditions that map to this code.

Related codes

Code definitions from the IAB Tech Lab VAST 4.3 specification. Causes and fixes are practitioner guidance, last verified July 2026.