openrtb.bid.impid_unknown
What it means
The bid's impid does not match the id of any Imp in the paired bid request. Every bid must reference exactly which impression it is buying.
Why it matters
A bid on a nonexistent impression can never win; it is discarded on arrival. This is nearly always an id mapping bug in the bidder: echoing an internal placement id, an off-by-one in multi-imp requests, or answering a cached copy of a different request.
How to fix it
Echo the Imp.id from the request you are answering, exactly. Multi-impression requests make this bug visible fast; test with a request carrying several imps with distinct ids.
Example that trips it
{ "seatbid": [{ "bid": [{ "id": "1", "impid": "placement-7", "price": 1.1 }] }] }The request's imps were "1" and "2"; "placement-7" is the bidder's internal id leaking out.
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.