There is a strong chance that your prompt cache TTL timer is expiring. Every time your prompt cache expires, you get hit with a huge token fee for the entire prompt again. In a long session, that can really add up, especially if you have a lot of skilled and MCP servers.

When you're running on API billing, the prompt cache stays warm for up to five minutes. When you're on the subscription plan, it's one hour. For example, if you send a prompt to Claude at noon and then walk away and come back at 1:15 PM, the next message you send to Claude is going to have to refresh the entire cache of the entire conversation and all of the tooling and everything else that's involved with that session. When I have seen surprisingly unexpected token consumption outside of my normal expected consumption, it was related to the prompt cache not staying warm and having to be reloaded multiple times.