Cache-break diagnostics now track the system prompt's folded attachment shape too
Claude Code already tracks the shape of messages sent to the model to help diagnose when the prompt cache breaks (a cache break means the model has to reprocess parts of the prompt it would otherwise reuse, which costs time and money). A new function now also computes a fingerprint of the system prompt's "folded attachment" spans, recording the type, length, and hash of each part. When a cache break happens, a diff of this shape (parts added, removed, reordered, or changed, plus length differences) is now included in the tengu_prompt_cache_break telemetry event.
This gives Claude Code more detailed internal diagnostics for figuring out what specifically changed in the system prompt when a cache break occurs, building on the message-shape tracking already used for the same purpose.