I was three weeks into building an Agent for my work (a productivity helper for data analysts) when I noticed certain flows were costing noticeably more than others. I assumed it was response length — longer answers, more output tokens, higher bill. So I added a system prompt instruction to be concise, watched the costs barely move, and moved on. Two weeks later I finally token-counted the inputs. The problem wasn't the output. The problem was me passing raw JSON data as context on every single request. The same information serialized as plain prose used 60% fewer tokens. I had been paying a 2.5x markup on every API call that touched the data — for weeks — because I never checked what I was actually sending. That sent me back to the transformer paper. Not to feel bad about the cost, but to understand why this happens at an architectural level. What I found turned several things I treated as configuration choices into things I now understand as architectural requirements.…