An author who builds on someone else's listed agent declares where the code came from and the share they send upstream. The original author can set a minimum, at most 50%, and a derived agent must declare at least that much to be listed. Royalties chain: if a second author builds on the derived agent, each level takes its declared share of what reaches it and passes the rest up, so the whole chain always sums to the amount charged.
This is a condition for being listed here, not a restriction on the license. The code stays open source, and anyone can run a copy off the platform for free. The royalty buys the derived agent a place in the registry.
Example: an agent declaring a 15% royalty, called once at 0.10 EUR, pays 0.01 EUR to the platform, leaving 0.09 EUR, of which 0.0135 EUR goes to the original author.