tl;dr: Claude Code sessions bill on input, output, and cached tokens, with output ~5x input and cache reads at 0.1x input — but the entire conversation gets re-sent every turn, so context bloat compounds fast. Switching `/model`, `/effort`, or running `/compact` mid-session invalidates the cache prefix and forces expensive re-prefill, while `/rewind` preserves it. To keep costs down: `/clear` between tasks, `@-mention` files instead of letting Claude search, quiet noisy commands, trim CLAUDE.md, and offload high-output grunt work to subagents.