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Stop paying to rebuild your Claude Code cache. When your main agent waits on a subagent for more than 5 minutes, its prompt cache silently expires, and the next turn re-encodes your entire conversation at the write rate instead of reading it back cheap. On long sessions with many subagents that's roughly 20% of your bill. claude-thermos keeps the cache warm so you never pay that tax.

Run Claude Code exactly as you normally would, but through claude-thermos with uvx :

Requires Python 3.11+ and the claude CLI on your PATH .

That's it. Warming runs automatically in the background. To disable it for a run without changing the command, set CLAUDE_WARMER_DISABLE=1 .

Claude Code's prompt cache uses a 5-minute TTL . Every turn, your whole conversation history is served from cache at 0.1x the input price instead of being re-sent at full price, as long as the cache stays alive.

The cache expires if more than 5 minutes pass between requests on the same prefix. The dominant trigger for that gap is not you thinking. It's the main agent blocked on a subagent that runs longer than 5 minutes . A subagent has a different system prompt and tool set, so its requests have a different cache prefix and never refresh the main agent's. While the subagent works, the main agent's cached history ages untouched; past 5 minutes it's gone. When the subagent returns, the main agent resumes with a byte-identical, append-only history, and finds its cache missing, forcing a full re-encode at the 1.25x write rate.