A Claude Code cache-resume bug re-bills a finished sub-agent's entire context on follow-up.

An r/ClaudeAI user traced part of this week's quota burn to a specific shape: when a sub-agent finishes and you send it a follow-up, Claude Code rebuilds the conversation in a form the prompt cache doesn't recognize, so the next request re-sends and re-pays for nearly everything that agent read. It happens inside the 1-hour cache window; messages sent while the agent is still working are cheap. The same user separately found sub-agent prompt cache lifetime defaults to 5 minutes against the main conversation's 1 hour, and fixed that with a setting. Their weekly usage went 49% to 60% in one session while deliberately holding back. Filed with Anthropic, unconfirmed. The workaround is concrete either way: message sub-agents while they're running, not after.