when your coding agent suddenly feels dumber, check which of its two version numbers moved

noticed a pattern in the recurring "did the model get nerfed" threads: half the time nothing about the model changed. the tool wrapping it auto-updated, or the person switched tools and is comparing across wrappers. the thing that made this click for me: the model never runs anything. It proposes. the program around it assembles what the model sees each turn (system prompt, tool definitions, your rules file, the trimmed history), executes what it proposes, decides what happens on errors, and decides when to stop. A tool update whose changelog says "improved tool descriptions" has quietly rewritten what your model reads every single turn. so an agent is really a pair: weights × loop. two habits this has changed for me, I log both version numbers when something feels off. the tool ships way more often than the model. usual suspect. and i stopped comparing models across different tools. a model that looks smarter in another tool might just be wearing a better wrapper; cross-tool comparisons measure the pair. anyone else tracking harness versions, or am i over-indexing on this?