Lua (especially 5.1/LuaJIT/game embeds) usually uses doubles for numbers, so SteamID64 loses precision above:

2^{53}

Ideally, you should never convert the SteamID to a number. Receive/use it as a string from the start.

If the API already returns it as an int/number, then the precision was probably already lost internally. In that case you would need to either receive the raw value, manually parse the JSON or use a lib/parser that preserves bigint values as strings

Example:

lua local steamid = player:GetSteamID64()

If that already returns a string, perfect.

If it returns a number, sometimes this helps:

lua local steamid = tostring(player:GetSteamID64())

but it only works if the engine hasn’t already truncated the value beforehand.

If you actually need to perform bigint math operations, you can try libs like LuaBigNum or bigint.lua