FpJSON limits the key name to 255 bytes (due to the use of ShortString strings in Contnrs.TFPHashObjectList)

FPC [git main]

---

program app;
{$mode objfpc}{$h+}

uses
  SysUtils, fpjson;

var
  jso: TJSONObject;
  s: string;

begin
  s := StringOfChar('A', 256);

  jso := TJSONObject.Create;
  jso.Add(s + '1', 777);

  WriteLn(jso.Find(s + '2').AsInteger);
  WriteLn(jso.Find(s + '3').AsInteger);
  WriteLn(jso.Find(s + '4').AsInteger);
  if Assigned(jso.Find('AAA4')) then
    WriteLn(jso.Find('AAA4').AsInteger)
  else
    WriteLn('kek!');

  ReadLn;
end.

Output:

777
777
777
kek!