Based on tournament data from the last ~3 months (60 decks), the defining cards for Esper Affinity are pretty clear.
The non-negotiable core, appearing in 95-100% of all registered Esper Affinity lists, is: Refurbished Familiar, Myr Enforcer, Thoughtcast, Seat of the Synod, Razortide Bridge, Mistvault Bridge, Ancient Den, Goldmire Bridge, and Vault of Whispers. These are essentially the cards that define the archetype's identity: artifact lands across White, Blue, and Black, the affinity payoffs (Myr Enforcer, Thoughtcast), and the recursive engine (Refurbished Familiar).
Beyond that core, the deck runs a suite of flex slots. Utrom Monitor (78%), Blood Fountain (75%), Glint Hawk (72%), Cryogen Relic and Dispatch (both 68%), and Reckoner's Bargain (67%) are very common but not universal. Cards like Baleful Strix, Kor Skyfisher, Moon-Circuit Hacker, and Springleaf Drum show up in roughly 25-50% of lists, representing build variation. In short: if a deck plays artifact lands in W/U/B with Myr Enforcer and Thoughtcast as the primary payoffs, it classifies as Esper Affinity.
Sources: Data from 37 tournaments (Jul-Sep 2026): 62% Melee, 27% MTGO, 11% Cardsrealm.