Classic vs Soroban USDC routing for x402 facilitators — operational notes from cross-chain delivery

[chopmob-cloud]

We ship Stellar as one of 8 settlement chains in a multi-chain x402 facilitator (AlgoVoi). Two operational notes from running classic and Soroban USDC paths in production that might be useful to other facilitator authors and to the Allbridge / cross-chain integrator community:

Classic vs Soroban issuer collision. Circle USDC on Stellar has two distinct on-chain forms:

• Classic asset: USDC issued by GA5ZSEJYB37JRC5AVCIA5MOP4RHTM335X2KGX3IHOJAPP5RE34K4KZVN, 7 decimal places, settled via the standard Payment op
• Soroban-wrapped: contract CCW67TSZV3SSS2HXMBQ5JFGCKJNXKZM7UQUWUZPUTHXSTZLEO7SJMI75, contract-call settlement

Allbridge /transfer/status lookups for inbound bridges land as the classic asset (verified end-to-end against the issuer's account_credited effect on Horizon). A facilitator that only handles the Soroban-wrapped variant will silently miss the cross-chain inbound. We hit this when first wiring the EVM-to-Stellar xChain path; resolved by matching account_credited effect type at the scanner.

7dp precision in the 402 envelope. USDC on Stellar is 7 decimals, vs the 6dp standard on most other chains. A facilitator that advertises the same amount value across all chains will under-quote by 10x on Stellar. We encode per-leg decimals in the x402 v2 extra object so agents and verifiers see the correct on-chain precision. The decimal-scaling logic runs against a single nominal-price input (6dp microunits) and scales per leg in our _leg() builder, which keeps the cap-guard semantics clean ($1.00 nominal ceiling preserved across all chains).

Trustline as receiver-side prerequisite. Same pattern as Algorand ASA opt-in: before advertising a Stellar leg in a 402 challenge, a facilitator should verify the receiver has a trustline established for the issuer asset. changeTrust op costs 0.5 XLM reserve plus 0.00001 XLM fee. Without trustline, USDC transfers from agents fail silently from the receiver side, producing a "shipped-but-broken" leg.

For anyone curious about the verification mechanics from the agent side, our public bench (https://agent-trust-bench.algovoi.co.uk) advertises a Stellar leg as one of 8, with a profile (/asset-spoof) that exercises issuer-mismatch detection.

Happy to dig into any of these if useful for the Stellar SEP or facilitator-implementor community.