Document-existence checks have a deliberately small contract. The Elasticsearch 8 API reference and the OpenSearch Get Document API reference both describe HEAD /<index>/_doc/<id> as HTTP 200 when the document exists and HTTP 404 when it does not. A storage outage is neither result.

The gateway’s HEAD path previously caught every Qdrant error and returned 404. A connection failure, missing collection, or Qdrant 5xx therefore looked like a trustworthy “document absent” answer. That can make health checks, conditional writes, and reconciliation jobs take destructive action on false information.

HEAD now uses the same narrow missing-point classification as GET without retrieving the document source. The known Qdrant missing-point response remains a 404, while transport failures and upstream errors return 502. All gateway HEAD routes also attach X-Elastic-Product: Elasticsearch, and the response body remains empty. The focused regression injects a Qdrant 503 and verifies an empty 502 response in both embedded-source and document-projection modes:

cargo test head_document_does_not_hide_qdrant_failures_as_not_found

This validation is request-level and uses a mocked Qdrant response; it is not a live Elasticsearch, OpenSearch, or Qdrant benchmark. The native status contract comes from the public engine documentation above. Query parameters such as routing, refresh, realtime, and explicit version checks remain outside the gateway’s current HEAD compatibility surface.