When are discriminated unions useful in a FastAPI request model?

Before adding a variant, I decide whether it is additive for existing consumers and whether generated clients can represent it safely. Producers and consumers may deploy independently, so unknown-variant behavior matters: strict writers, tolerant storage, and readers do not automatically share one policy. I avoid callable discriminators unless the wire format truly lacks a stable tag, because they must handle both raw dictionaries and model instances during validation and serialization. Contract tests cover every variant, unknown tags, and ambiguous legacy payloads.