Claude Code now explains why your prompt cache missed

Claude Code 2.1.260 adds a reason field to cache misses. Both /cost and the status line's prompt_cache value now name a likely cause, a changed tool definition, a changed system prompt, or a session that sat idle past the cache's TTL, according to the Claude Code changelog.

That's a debugging convenience. The bigger change in the same release is a fix to how Claude Fable 5.1 handled context attached after tool results. Prompt caching wasn't covering that context, so it got re-sent as uncached input on every tool-call turn. In a long agent session, that's not a one-time miss. It's every tool round-trip paying full input price for context that should have been free.

If you've been running extended Claude Code sessions with heavy tool use on Fable 5.1, this is worth checking against your own bills. The changelog doesn't say how far back the bug went or how many tokens it burned in aggregate, only that it's fixed now.

The release also stops /effort changes mid-session from invalidating the cache on Fable 5.1, another small tax on long sessions that's gone now.

None of this is flashy. It's the kind of fix that shows up as a lower number on next month's usage report and nothing else, which is exactly why it's easy to miss.