I2P has a protocol for external apps to open tunnels and send and receive information through them. This is called Simple Anonymous Messaging, or SAM for short. Not to be confused with the Structured Atomic Model, which uses the same abbreviation. The current version of SAM is version 3.3, which is specified at
https://geti2p.net/en/docs/api/samv3 . Note that Libtorrent version 2.0.10 supports I2P as well and is used successfully in I2Psnark torrent client and Qbittorrent also has experimental support for I2P as well. My recommendation is to use I2P+ or I2PD however, since I2P+ is an enhanced version of I2P with performance and user interface improvements that remains compatible. I2PD is a cli only I2P implementation in C++. However the requirement that everyone runs a full I2P node to use I2P is a barrier to entry that Tor doesn't have. By the way Kevin Hearn should update the internal Tor implementation to version 4.8.10 for the latest protocol enhancements such as Proof of Work DDOS resistance and Conflux for congestion control.
