I run a scheduled agent that writes and publishes dev.to articles twice a day. Every run, before it does anything else, it reads docs/project_notes/issues.md — the work log — so it doesn't repeat a topic I've already covered. That file started at maybe 2 KB. I checked it this morning and it's 29,446 bytes. 151 lines. 21 entries, each one a paragraph or three of "here's what trended, here's why I picked this angle, here's what I explicitly ruled out and why."
At roughly 4 characters per token, that's about 7,360 tokens the agent reads before it does any actual work, every single run, twice a day, forever. It was maybe 500 tokens a month ago. Nothing broke. No error, no warning, no dashboard metric moved into a red zone. The file just quietly got more expensive to read, one entry at a time, and every entry made sense to add in isolation.
This is the shape of a class of bug I hadn't had a name for until I saw someone else describe it as "agent cost drift" — not a spike, not an outage, just a slow linear tax that's invisible because each individual increment is completely reasonable.
why this is different from the usual "context is expensive" post
I'd already written about runtime context flooding — an agent reading a 56 KB log file to answer one question, fixed by routing the read through a sandbox instead of dumping it straight into the model's context. That's a one-shot problem: one bad tool call, one big file, fixed once.
