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The PII Redactor That Scrubbed the User's Question and Left the Prompt Cache Untouched

· 11 min read
Tian Pan
Software Engineer

A customer audit finds eleven months of verbatim user PII sitting in a Redis cluster nobody on the residency team knew existed. No system was compromised. No attacker got in. The data was written there on purpose, by a service the inference team built and named "prompt cache," as a performance optimization. The redactor on the analytics path worked perfectly the entire time. The redactor was simply not on this path.

The breach is real anyway. Under GDPR, retention beyond the contracted thirty days is enough; the data does not need to have leaked to trigger Article 33 notification obligations. The residency team's inventory listed every log, every warehouse, every queue — and missed the cache because the cache was on the inference team's side of the org chart. The privacy boundary that everyone trusted ran straight down the analytics pipeline and stopped at the wall where the LLM stack began.