At the beginning, a custom proxy was used to watch the network packets come and go, while doing things with the client to see what packets they produce.
This approach gets tiring quickly, and if I had done a little more of research earlier on, I would've found that tshock has published the Multiplayer Packet Structure which has been incredibly useful to save a lot of time.
However, another mistake was made, and it was manually writing the hundred of packets that exist.
After a bad automation of the process, a lot of time was spent renaming and reviewing everything by hand, because the generation was only for the skeleton of the packets and very basic.
If I had to start again, I would automatically generating a custom DSL to describe the packets, clean it up, and generate proper code with it.
The original goal was to automate the client for fun (like automatic mining, building or killing bosses, possibly in a realistic way), but the protocol was a lot of work and can probably be used to write a custom Terraria server in Rust too.