${AUCTION_ID}

What it expands to

ID of the bid request, from BidRequest.id.

Expands in win (nurl), billing (burl), and loss (lurl) notice URLs and in adm markup. It carries BidRequest.id, the one identifier every party in the transaction logged.

What to watch for

This is the fastest discrepancy-debugging key there is: when exchange and DSP counts disagree, joining logs on the expanded auction id shows exactly which impressions diverged.

Encoding

When a macro rides inside another URL as a query parameter, the expanded value must survive URL parsing: exchanges support suffixed forms like ${AUCTION_ID:B64} (base64) where documented, and values containing reserved characters must be percent-encoded. An unencoded expansion is the most common reason a notice URL truncates at the first ampersand.

Related

• All substitution macros
• Where nurl, burl, and lurl fire in the delivery flow
• Loss reason codes
• ${AUCTION_BID_ID}
• ${AUCTION_IMP_ID}
• ${AUCTION_SEAT_ID}
• ${AUCTION_AD_ID}
• ${AUCTION_PRICE}