It's a struct which contains an int field, which iirc has more encoding, transfer, and decoding costs than an int (nobody cares when it's one, you might care wh

It's a struct which contains an int field, which iirc has more encoding, transfer, and decoding costs than an int (nobody cares when it's one, you might care when it's many thousands), and implies nullability because all message types are nullable. And unless you're using a Google-blessed code generator in a Google-blessed language, it's also noticeably more verbose to use[1]. But yes, you do indeed have user-defined structure (message) types in protobuf, and you can bend them quite far. There are also annotations of a sort, though support there seems worse, personally I've only very rarely been able to use them cross-language at all. [1] yeah, there are alternate code generators, but their support and quality is "mixed" to put it mildly. They do exist though, and many have a plugin system of some kind for making your own less-painful custom types.