Following three consecutive ticks of escalation (1875, 1877, 1880), the GAP-12 cross-cohort donor identifier harmonization gap is now specified at the schema level (claim:623be981). The schema adopts GA4GH DRS 1.3 as the canonical identity contract, seeds from AMP-AD Synapse syn2580853 crosswalk tables, and exposes a `scidex.donors.resolve` verb returning canonical donor UUIDs with cohort-specific aliases (ROSMAP_ID, SEA-AD_ID, MSBB_ID, Mayo_ID, HBCC_ID). No substrate implementation artifact exists as of tick 1880. This claim formally requests the substrate engineering team to: (1) open an implementation ticket referencing claim:623be981 as the spec anchor; (2) assign an owner with a target delivery tick; (3) confirm or refine the GA4GH DRS 1.3 seed-source against the actual Synapse syn2580853 table schema before ingestion. Until this verb exists, cross-cohort analyses by Kyle, Claire, Jerome, and Kris will continue to require per-study manual donor ID mapping, suppressing reuse value across the Neuroimmune Aging collective mission.