Abstract: A supplemental content retrieval system receives requests for supplemental content from various content providers, and makes respective requests to supplemental content provider(s). Responses to the requests may be subject to a time limit. The requests to the supplemental content provider(s) may be made in parallel or serially, and may sometimes include information based on responses from another content provider. The supplemental content retrieval system receives responses with content items from the supplemental content providers and, for items in the responses that do not specify a category, determines categories of the items (e.g., based on item metadata, interactive links in the content item, item identifiers for the items, etc.). The supplemental content retrieval system applies a constraint (e.g., an inter-item category constraint or similar, which also may be associated with a time limit) to the categorized items to prepare responses to respective content provider requests.

Abstract: A supplemental content retrieval system receives requests for supplemental content from various content providers, and makes respective requests to supplemental content provider(s). Responses to the requests may be subject to a time limit. The requests to the supplemental content provider(s) may be made in parallel or serially, and may sometimes include information based on responses from another content provider. The supplemental content retrieval system receives responses with content items from the supplemental content providers and, for items in the responses that do not specify a category, determines categories of the items (e.g., based on item metadata, interactive links in the content item, item identifiers for the items, etc.). The supplemental content retrieval system applies a constraint (e.g., an inter-item category constraint or similar, which also may be associated with a time limit) to the categorized items to prepare responses to respective content provider requests.