It depends on language, but:

• Parsing as a 32-bit or 64-bit int or long will not work. Maybe it used to, but it definitely should not work in this kata.
• Parsing to BigInt should be not allowed and should be blocked in languages which have BigInt types. But some languages have poor tests and if you try hard enough, you might find a way.

General idea of this kata is to perform addition of large numbers "manually", without a support of big integer utilities.

That's what happens when you don't add logs at the beginning, but put them somewhere in the middle. If you put it on first line, you would notice that your code fails for number 0.

They shouldn't. Either C# suddenly has a problem after 7 000 submitted solutions ( unlikely ), or you're reading the logs wrong. I'm going with the latter.

Please mention which language you're using next time, esp. if a kata has multiple.

Closing.