Claude Code v2.1.211 Fixes a Prompt-Caching Billing Regression on Bedrock, Vertex, Mantle, and Foundry

Go check your API spend. Right now. I'll wait.

Buried in the v2.1.211 changelog is a regression fix for prompt caching on Bedrock, Vertex, Mantle, and Foundry, where trailing system context was being billed as fresh input tokens. Cached context. Re-billed at full input rate. On the gateways where enterprise teams actually run.

This is the quietest expensive bug of the month. Prompt caching is the single largest cost lever in long agent sessions. That's the whole reason you structure your system prompt to be stable and put the volatile stuff at the end. If the trailing system context silently stops hitting cache, your cost model is wrong and nothing tells you. No error. No warning. Just a bigger bill that you attribute to "we ran more agents this month."

The action item is concrete: figure out when you upgraded to a version in the regression window, pull your spend for that period from your gateway's billing console, and compare it against your expected cache-hit rate. If your cache-hit math said 80% and your bill says otherwise, this is why. Don't assume your numbers held.

This lands alongside two other fixes worth your attention in the same release cycle. v2.1.210 fixed isolation: 'worktree' subagents running git-mutating commands against the main repository instead of their isolated worktree. Read that again. The isolation flag exists specifically to prevent that failure, and it wasn't preventing it. Everyone who fanned out parallel agents into worktrees thinking they had a containment boundary was running without the guarantee they believed they had. The same release also fixes killed background sessions leaving a permanent git worktree lock, which is the kind of thing you debug for an hour before realizing it's not your fault.

And v2.1.211 fixed auto mode overriding a PreToolUse hook's ask decision for unsandboxed Bash. A hook returning ask now floors the decision at a prompt instead of getting steamrolled. If you wrote permission hooks, watched auto mode blow straight through them, and concluded your logic was wrong, it wasn't. It was the bug. Re-test your hook gates after upgrading, because rules that were previously decorative will start prompting for real, and that changes the ergonomics of any unattended run you've got scheduled.

Three fixes, one theme: the containment primitives we've all been building on top of were less load-bearing than advertised. That's not a reason to panic, it's a reason to audit. Set aside an hour this week. Check your spend against the caching window, re-read your settings.json permission rules, and actually verify your worktree isolation does what you think by running a git-mutating command in a subagent and seeing where it lands.

The uncomfortable part: none of these failures announced themselves. All three were silent. That's the pattern to internalize about agent infrastructure right now. The failure mode isn't a crash, it's a guarantee quietly not holding while everything looks fine.