How are you actually saving cost on your agent systems?

I've been researching how teams handle cost and FinOps for agent systems in production. Token bills get unpredictable fast, and most tooling stops at per-call or per-agent attribution, which doesn't tell you much about why the bill jumped. a few patterns that keep coming up. Per-call cost is easy. Per-coordination pattern is hard. One team I talked to had a customer workflow burning 10x the others. The bill was correct, but no one could tell which agent loop or handoff pair was driving it. They end up writing custom queries against logs after the fact. Runaway detection is mostly bill-shaped. Someone notices the OpenAI or anthropic bill spiked, then traces back what happened. cursor users have posted forum threads about $1,780 burned overnight from a stuck background agent on a $20/month plan. By the time the bill shows up, the run is already done. caching, model routing, and prompt compression help on the per-call side, but they don't help when an agent loops or fans out into 30 sub-calls because of a logic bug. Curious what people are running. What's the last thing that actually moved the needle on your token bill, model switch, caching, hard caps, something else? If you've had a surprise bill or runaway, how did you find out, and what did the investigation look like after? Where does your current tooling stop short on cost questions you actually need answered?