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A modular inventory source is an operator-managed source made of smaller modules: an inventory feed, a booking ledger, and optional execution modules and human work queues. Use one when your inventory and execution do not come from a single upstream sales agent. The modules and lifecycle stages are explained in Modular inventory source lifecycle. V3 gives a modular source three Pages. Each has one job. You open them with open_page, and a person completes the work in the Page. Only a Seller Account can open them. A Buyer Account is refused with a wrong-account error. sourceId comes from search({ "kind": "inventory_source" }).

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Availability

Custom modular sources are part of a Premium or Enterprise Merchandising plan. Without them the Pages are refused with ACCESS_DENIED and the message “Custom modular inventory sources require a Premium or Enterprise Merchandising plan.” The one exception is a synthetic source attached by the Demo Storefront. With its sourceId, the modular_inventory_source Page opens it read-only so you can inspect it.

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What happens in each Page

Add a modular source creates a feed-backed source. It does not ask how you book, traffic, clear creative or report. Those answers come from the modules attached to the source and appear in the workspace. A new source starts with an avails-feed module and Apostra’s booking ledger. Modular inventory source is the page you return to. Its summary shows readiness above the fold. Below it are the lifecycle stages, the attached modules and the source-side work queue. The Page launches the add-source and avails-feed Pages when you need them. Commit avails feed sends the file straight from the Page to private storage, so the model never receives the feed’s bytes. The Page previews the rows before you confirm. Creating a source and committing a feed are explicit, confirmed writes. Nothing is offered to buyers until avails rows are committed. A source with no committed rows is registered but has no sellable capacity.

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What you can do from V3 without a Page

The reads and the typed saves below are V3 tools. They work on the same sources the Pages open. A modular source’s products come from the committed feed. You cannot change them with save_wholesale_product. Upload a new feed instead. See Create and manage seller products. A follow-up is addressed by both sourceId and id, and a completion that names no source is refused. Saving the same completed result again returns unchanged, and a conflicting correction is refused rather than overwriting the recorded result. See Operate work items and approvals.

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What V3 does not cover

V3 has no tool that ingests a feed, lists a modular source’s products, or searches ad-server selectors. Feed ingest goes through the modular_inventory_feed Page. Credential entry stays in the Page and the operator UI, and no credential passes through a model.