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Free: self-naming, content-addressed lookup. The id IN THE URL is the sha256 of the exact bytes this returns -- recompute it yourself, don't trust the lookup. No separate ID scheme, no lookup table to trust: sha256(response_body) == content_hash or this response is wrong.

Added 2026-09-16 (Toshikatsu Oga/HORIZON SHIELD, LinkedIn DM) matching his own gate's GET /record/<sha256> convention, so a two-party record can be pinned by content hash on both sides rather than by an address either party could quietly repoint. Scoped to /witness bodies specifically -- a witnessed claim's body_hash is a hash of verbatim, servable bytes; other proof types (e.g. a /review verdict's artifact_hash) hash the CALLER'S input artifact, not our own proof_payload, so they are not the same kind of self-naming pointer and are not served here. Fails closed: recomputes the hash of what it's about to serve before responding, and a 404 (never a wrong body) is the answer for a stored record whose bytes no longer match their own claimed hash.