Self-hosting an x402 facilitator: the EIP-3009 gotchas that almost cost us money

Short summary

A founder shares four critical engineering gotchas from self-hosting an x402 payment facilitator that lets AI agents pay per action in USDC via EIP-3009 signatures. The most dangerous trap: trusting payer-controlled nonce state instead of verifying actual Transfer events on-chain, which adversarial agents could exploit to get tool outputs for free. Additional lessons cover idempotency via a unique-nonce claim table, multi-chain complexity (especially Solana's lack of EIP-3009), and measuring real demand by splitting the funnel from manifest fetches to settled transfers.

• •Verify on-chain Transfer events, not nonce state — payers can burn their own nonce to fake settlement
• •Use a UNIQUE-constrained claim table keyed by EIP-3009 nonce to prevent double-execution of side-effecting tools
• •Start with Base only; multi-chain USDC introduces contract-address, bridged-vs-native, and Solana incompatibility headaches

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