Why does my video bid never win?

Answer

Video adds media constraints on top of the usual auction filters, and any mismatch disqualifies the bid: your creative's MIME type must be in imp.video.mimes, the VAST version you return must map to a value in protocols, duration must sit between minduration and maxduration (or match rqddurs exactly when present), and dimensions should fit w and h. On CTV, buyers and exchanges increasingly key on plcmt; a creative strategy built for plcmt 1 instream will not clear an outstream placement. Compare each constraint in the request against what your ad server actually returned, field by field.

Fields in this answer

FieldTypeSinceStatus in 2.6
imp.video.mimesstring array2.0Current

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All 93 questions are listed in the OpenRTB FAQ.