I need this change to convert a big int array to a string value, working with unix bigstamp, SteamID64, EconItem itemid and etc..
For compatibility with these natives:
https://github.com/alliedmodders/sourcemod/blob/master/plugins/include/sourcemod.inc#L367
https://github.com/alliedmodders/sourcemod/blob/master/plugins/include/clients.inc#L320 by AuthId_SteamID64
https://github.com/komashchenko/PTaH/blob/master/PTaH.inc#L510
https://github.com/alliedmodders/sourcemod/blob/master/plugins/include/protobuf.inc#L56
https://github.com/alliedmodders/sourcemod/blob/master/plugins/include/protobuf.inc#L144
https://github.com/alliedmodders/sourcemod/blob/master/plugins/include/protobuf.inc#L214
https://github.com/ErikMinekus/sm-ripext/blob/main/pawn/scripting/include/ripext/json.inc#L116
https://github.com/ErikMinekus/sm-ripext/blob/main/pawn/scripting/include/ripext/json.inc#L172
...

Nice! This is a good interim solution and something I've been intending to add a format specifier for doing for ages.

Could you add some test cases to the test suite? (Or share a test plugin and test results here.)

BigInt usually stands for some way to store any large number not restricted to 64 bits. Using Int64 might be more descriptive.

I wonder if Int64ToString should be a format specifier <instead / as well>? I imagine most of the use cases are going to be embedding one in a larger string such as a URL or SQL query.

I'm ambivalent. Using int64 ids as anything other than strings is a bad idea unless there's some mathematical calculation going on. As numbers they are not portable to other languages (eg JavaScript/JSON).