openrtb.bid.mtype_missing
What it means
A bid carries creative markup in adm but does not declare mtype (1 banner, 2 video, 3 audio, 4 native), on an OpenRTB version that defines the field. The exchange is left to sniff what kind of markup arrived.
Why it matters
Markup sniffing is guesswork, and misclassified creatives fail at render time, after the auction cleared and the win was billed. Exchanges increasingly reject unclassifiable bids up front instead of risking the discrepancy.
How to fix it
Set mtype on every bid that carries adm, matching the creative you actually returned. It is one integer, known at bid time, and removes an entire failure class.
Check your payload
Paste a bid request or response into the tester to see whether this code fires against it, or gate on the id in CI with the CLI: the id is stable, wording is not.
Back to the full diagnostic code reference.