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DisclaimerUnofficial, and not affiliated with Anthropic. Nearly all of this is read straight out of what ships: npm bundles, captured prompts, published docs. Anthropic's own notes go in verbatim, marked as theirs. The rest is my reading, and every entry carries the strings behind it. If one looks wrong, vote it down and say why.

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Claude Code v2.1.23 29 Jan 2026

Prompt Cache Break Debugging

What

New detailed logging when prompt caching breaks, helping identify why cache misses occur.

Details

Logs when cache breaks due to: model changes, system prompt changes (with character delta), tool schema changes
Format: [PROMPT CACHE BREAK] system prompt changed (+123 chars) [source=X, call #Y, cache read: A → B, creation: C]
Useful for debugging unexpected cache misses in long sessions

Evidence

Logging at line 266957 (contains "[PROMPT CACHE BREAK]", "system prompt changed", "tools changed", "model changed")

Strings lifted out of the shipped bundle, so the claim above can be checked against them.

See this entry in the whole of v2.1.23 →