What is prompt inventory?

How it differs from page inventory

A publisher's page inventory is knowable in advance. The pages exist, their number is roughly predictable, and each can carry several placements.

Prompt inventory is created by users in real time and is unknowable until it arrives. It cannot be increased by producing more of it — the publisher does not write the prompts. And most of it is not sellable at all, because most conversations have no commercial dimension or are ones where advertising would be inappropriate.

The practical consequence is that inventory forecasting is a demand-side estimation problem rather than a supply-side inventory count, and the numbers are far smaller than page-based intuitions suggest.

How a publisher segments it

• By topic, which is what advertisers actually buy against.
• By [intent density](/resources/glossary/intent-density), which is what sets the price.
• By decision stage — exploring, comparing, deciding — because the same topic behaves very differently at each.
• By exclusion. The categories where no advertising is permitted regardless of what it would pay: health and legal concerns, distress, minors, anything sensitive.
• By format eligibility, since not every eligible prompt suits every placement type.

Exclusion is the first decision

On a web page the default is that a slot is sellable and exceptions are carved out. On an AI surface the workable default is the opposite: a conversation is not monetisable unless it clearly qualifies.

That inversion is what protects the product. A user who encounters advertising while asking about a medical symptom or a financial difficulty has learned something about the assistant that no amount of relevance elsewhere repairs. Publishers who have made this work run conservative eligibility rules and treat them as a product constraint rather than a revenue ceiling to be optimised against.

Measuring it

The natural unit is per query rather than per thousand impressions, because queries are what the publisher produces and only a fraction are monetisable. Revenue per query is the metric most operators track, with eligible-prompt rate — the share of conversations that qualify at all — as the supply-side number underneath it.