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+## 1.58.1
+**`(fix):`** Preserve required-nullable fields that arrive as `null` when a decoded model is
+marshaled again. Generated `UnmarshalJSON` now records which required-nullable
+keys were present in the incoming JSON (via a per-type wire-name-to-field-bit
+table) so that `MarshalJSON` re-emits them as `null` instead of dropping them.
+Fields absent from the input stay absent, and freshly constructed values
+(e.g. request bodies) serialize exactly as before.
+
+