What is the difference between proto2 and proto3?

Proto3 is the current recommended version. Key differences from proto2: (1) Field presence — proto2 supports optional, required, and repeated; proto3 makes all singular fields optional by default (no required), removing required field validation; (2) Default values — proto3 uses zero values as defaults (0 for numbers, "" for strings, false for bools) and doesn't serialize them, reducing message size; proto2 allows explicit defaults; (3) Unknown fields — proto3 originally discarded unknown fields (fixed in 3.5+ to preserve them); proto2 preserves them; (4) Maps — proto3 has native map fields (map<string, Value>); proto2 uses repeated message entries; (5) JSON mapping — proto3 defines a standard JSON encoding; proto2 doesn't. Proto3 is simpler and preferred for new projects. Proto2 is still used in legacy Google codebases.