VAST error code
900 — Undefined error
What the spec says
A catch-all the player uses when a failure does not map to any defined code.
What actually causes it
- A player-specific failure with no corresponding VAST code.
- An SDK bug or an unhandled state.
- An integration that reports 900 for everything rather than classifying failures, which some do.
How to fix it
The VAST code alone will not diagnose this. Go to the player or SDK logs, which usually carry a more specific internal error. If one integration reports 900 for a large share of its failures, treat the code as uninformative for that partner rather than as a real signal.
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.