GBA cheat codes are a complete pain because there’s so many formats. After some painstaking testing I have established that generally, the following 3 formats will work in the mGBA core:
- encrypted GameShark Advance codes
- decrypted Codebreaker codes
- decrypted Action Replay MAX codes
You can click ‘info’ on any code on gamehacking.org and it will show you the code in all formats, if at all possible. Choose one of the supported formats above and it should work - so long as the original code is correct and listed in the correct original format (there are a lot of broken codes out there).
Lastly, you sometimes encounter RAW codes - I’ve not found any way to convert these so they are usable in Retroarch. However, you can use a program called GBACCC to patch RAW codes directly into the ROM, so that’s one solution.
Oh, and there is occasional additional weirdness - for example I was trying to apply a bunch of codes to Aria of Sorrow, and was finding that only one of them worked. The solution, very strangely, was to apply them one at a time (as in apply one, go back to the game, apply the next, back to the game, etc…) Very odd.