Message normalization cache aims to keep the prompt cache stable

What

Before each request, Claude Code tidies up the conversation into a standard form, called normalization. The prompt cache lets the service reuse the start of a conversation it has already seen, which only works if that start stays exactly the same. A new cache keeps that start stable:

• Messages already sent are kept as stable objects, so identical copies are reused.
• Checkpoints of the normalization pass are saved and later requests resume from them instead of starting over.
• CLAUDE_CODE_NORMALIZE_PREFIX_CACHE set to false turns it off and set to true turns it on. There is also a kill switch.
• Without the variable, the server value tengu_wise_salamander decides. It is used only if it is a number greater than 0 and at most 1, and the built-in fallback is 0, which leaves the feature off when neither supplies a value. Nothing has been read about this switch.
• With CLAUDE_CODE_NORMALIZE_PREFIX_CACHE_VERIFY set, a resumed pass is compared with a full pass. If they differ, the full pass is sent and every later request in that process is normalized in full. Whether this check runs depends on a self-check share value, not a plain switch.

Why

Where it is active, this could change how often the prompt cache is reused and so how many tokens requests cost. The verify mode falls back to the full pass if the shortcut ever gives a different result.