## Problem

Problem

The REST-SSZ Engine API introduced by #21729 does not yet implement two Amsterdam requirements from execution-apis#793:

• POST /engine/v1/blobs/v4 is not implemented or advertised. The current draft requires an Amsterdam EL to serve v4, which adds cell-range selection.
• POST /engine/v1/forkchoice decodes and validates custody_columns but does not apply it. The current draft requires an accepted custody set to remain active until another forkchoice update replaces it; omitting the field must leave the set unchanged.

Consequently, Erigon can advertise Amsterdam among its supported fork schemas without providing the complete Amsterdam behavior.

Required work

• Add the /blobs/v4 request and response SSZ types and connect them to a cell-range backend.
• Advertise blob revision v4 only when the endpoint is available.
• Apply custody_columns in external-CL mode and preserve the set until it is replaced.
• Keep custody processing independent from forkchoice payload status, as required by the draft.
• Cover v4 selection, partial availability, 204 No Content, capability advertisement, custody replacement, omitted custody, persistence, and custody-error behavior in tests.

Specification

• REST transport and endpoint semantics
• SSZ container catalogue

Follow-up to #21729.

Originally posted by @yperbasis in https://github.com/erigontech/erigon/issues/23045