Most agent failure modes are not interesting. They are boring. They are the same five problems in different costumes. After eighteen months running agent loops in production, I keep meeting these five and only these five.
I do not build agent frameworks. I build small libraries that fix one failure mode each. You install the one you need. The composition emerges from your code, not a framework's architecture diagram.
Here they are in roughly the order you will hit them.
1. The JSON is not JSON
Your model returns Sure, here you go:\njson\n{...,}\n``. You parse it. You crash.
