Sanity Library Reference Docs
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    InitialFieldValue: {
        [K in FieldKind]: {
            name: string;
            type: K;
            value: NonNullable<FieldValueMap[K]>;
        }
    }[FieldKind]

    Initial value for an input-sourced entry — what a caller supplies at startInstance (or setStage). Same discriminator shape as ResolvedFieldEntry minus _key and with null values disallowed (omission signals "no initial value").