If the JSON decoder/encoder you use is outdated, it may strip a crucial comma or bracket from the code. Corrupted saves usually result in a "Failed to load data" error.
On Android, your save data is typically located in: Android/data/com.rovio.gold/files/ angry birds epic save editor
An Angry Birds Epic Save Editor is technically feasible, useful for players who wish to bypass repetitive grinding, and serves as a solid learning project in reverse engineering, file format analysis, and GUI programming. However, developers must respect the original game’s design intent and clearly communicate that the tool is unofficial, for offline use only, and not for disrupting any remaining online features. If the JSON decoder/encoder you use is outdated,