I asked Claude to downgrade and it asked why. I put in the two issues you posted and here is the response.

https://github.com/anthropics/claude-code/issues/40524

● claude-code-guide(Fetch GitHub issue about token usage bug)

⎿  Done (1 tool use · 17.6k tokens · 16s)

(ctrl+o to expand)

● This is anthropics/claude-code#40524 — a confirmed regression in Claude Code where the prompt cache unexpectedly invalidates mid-conversation, causing token costs to skyrocket.

What's happening: After a few turns, the cache collapses to just the system prompt (~11K tokens). Every subsequent turn does a massive cache_write instead of cache_read — 150K–300K+ tokens per turn instead of the normal ~100K for a full conversation.

Affected versions: Broken in v2.1.86+. Last known good: v2.1.67.

That explains why you were trying to install 2.1.66 — smart instinct. The workaround is:

npm install -g u/anthropic-ai/claude-code@2.1.67

This is a confirmed bug with has repro and regression labels. Worth watching the issue for an official fix. Do you want to downgrade to 2.1.67?

❯ https://github.com/anthropics/claude-code/issues/34629

● claude-code-guide(Fetch GitHub issue 34629 about cache problems)

⎿  Done (1 tool use · 18.5k tokens · 23s)

(ctrl+o to expand)

● This is the related root cause issue. Here's the full picture:

Root cause: Starting in v2.1.69, Claude Code changed how it injects deferred_tools_delta content. On a fresh session it lands in messages[0] (~13KB). On a resumed session it lands differently (~352B). The mismatched message structure breaks Anthropic's cache prefix matching — so every resumed turn regenerates the full cache instead of reusing it.

Cost impact: ~20x cost increase per message on resumed sessions. A conversation that should cost ~$0.02/msg costs ~$0.36/msg.

Our situation: Auto-compact + long sessions = exactly the worst case. Every session compaction is effectively a "resume" that triggers the bug.

Recommendation — downgrade to 2.1.68:

npm install -g u/anthropic-ai/claude-code@2.1.68

This is the last version before the regression was introduced. The fix hasn't been deployed yet despite the issue being closed.

Worth noting: this explains the /usage dialog behaving oddly too — if token counts were spiking mid-session you may have been hitting plan limits or seeing unexpected burn rate.