Bid caching window

The bid caching window is the period during which a bid can still be used after its auction, set by the bid's time-to-live (TTL) and any safety buffer the wrapper subtracts.

In Prebid.js every bid adapter must return a ttl, the number of seconds Prebid may use the bid. Prebid core subtracts a buffer of one second, so a bid with a 30-second TTL counts as expired after 29 seconds. Publishers can change the buffer with ttlBuffer (Prebid.org, setConfig, accessed 9 November 2026). Expired bids cannot win, and minBidCacheTTL decides how long they stay in memory.

OpenRTB has the same idea at exchange level. The impression's exp field advises how many seconds may pass between auction and impression, and the bid's exp field says how long the bidder is willing to wait (IAB Tech Lab, OpenRTB 2.6). The implementation notes give the reason: an app may prefetch a video during a game level and show it minutes later. Exchanges may drop bids whose expiry they expect to miss. Whether a late billing notice is still billable is a contract question, not a protocol rule.

Example: An SSP returns a display bid with ttl: 300. With the default buffer, Prebid treats it as usable for 299 seconds. On a page with bid caching on, the bid can fill a refreshed slot four minutes later, but not six.

Related terms: bid caching, Prebid.js, tmax, burl, timeout.

Authoritative source: Prebid.org, "pbjs.setConfig", section "Set TTL Buffer" (accessed 9 November 2026).

Read the full guide: Prebid.js guide: events, bidder adapters, releases and debugging.