Bug: Bankr eth_signTypedData_v4 signatures fail fxUSD EIP-3009 (Base)

[huwangtao123]

Summary

bankr sign --type eth_signTypedData_v4 produces signatures that recover to the expected address locally, but fail on-chain validation for fxUSD EIP-3009 transferWithAuthorization on Base (FiatTokenV2: invalid signature).

This blocks x402 premium payments at $0.01 fxUSD when Bankr is used as the EIP-712 signer.

Impact

• x402 flow cannot settle payments when signature is produced via Bankr signer.
• Integrations that rely on Bankr signing for EIP-3009 fail with simulation_failed / invalid signature.

Environment

• Bankr CLI: latest from npm i -g @bankr/cli (as of 2026-02-26)
• Network: Base (chainId=8453)
• Token: fxUSD 0x55380fe7A1910dFf29A47B622057ab4139DA42C5
• Flow: x402 premium payment to https://smartclaw.xyz/api/premium

Reproduction

1. Get x402 402 payload from GET https://smartclaw.xyz/api/premium.
2. Build EIP-3009 typed data from payment requirements:
   • domain: name=FxUSD, version=2, chainId=8453, verifyingContract=0x55380fe7A1910dFf29A47B622057ab4139DA42C5
   • primaryType: TransferWithAuthorization
   • message: from,to,value,validAfter,validBefore,nonce
3. Sign with:
   • bankr sign --type eth_signTypedData_v4 --typed-data '<json>'
4. Submit via x402 payment header (PAYMENT-SIGNATURE) and retry premium endpoint.

Actual Result

• Server returns 402 with settlement error:
  • {"error":"Settlement failed","details":"simulation_failed"...}
• Local simulation against token contract reverts with:
  • FiatTokenV2: invalid signature

Expected Result

• Signature should be accepted by fxUSD transferWithAuthorization.
• x402 settlement should complete and premium API should return business data.

Diagnostics Performed

• Signature is parsed correctly from Bankr CLI output.
• recoverTypedDataAddress matches signer address (local cryptographic recovery succeeds).
• Signature is low-s canonical (not a high-s rejection case).
• Domain parameters are confirmed correct against on-chain DOMAIN_SEPARATOR:
  • Match found for name=FxUSD, version=2, chainId=8453.
• Despite above, contract verification fails (invalid signature).

Likely Area to Check

Potential mismatch in how Bankr constructs/serializes EIP-712 typed data for eth_signTypedData_v4 (field encoding/order/type normalization) versus what FiatTokenV2 expects for EIP-3009.

Request

Please prioritize a fix or provide a compatibility guideline/workaround for EIP-3009 on Base fxUSD.

If this is the wrong repository, please transfer this issue to the correct Bankr CLI/signing backend repo.