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Abstract

AI agents that pay for resources via the x402 protocol embed payment metadata resource URLs, descriptions, and reason strings in every HTTP payment request. This metadata is transmitted to the payment server and to the centralised facilitator API before any on-chain settlement occurs; neither party is typically bound by a data processing agreement. We present `presidio-hardened-x402` , the first open-source middleware that intercepts x402 payment requests _before_ transmission to detect and redact personally identifiable information (PII), enforce declarative spending policies, and block duplicate replay attempts. To evaluate the PII filter, we construct a labeled synthetic corpus of 2,000 x402 metadata triples spanning seven use-case categories, and run a 42-configuration precision/recall sweep across two detection modes (regex, NLP) and five confidence thresholds. The recommended configuration ( `mode=nlp` , `min_score=0.5` , all entity types) achieves micro-F1 = 0.898 with precision 0.972, at a p99 latency of 5.73 ms well within the 50 ms overhead budget. The middleware, corpus, and all experiment code are publicly available at `https://github.com/presidio-v/presidio-hardened-x402` .