fix(evm): Base Sepolia USDC EIP-712 domain name is "USDC", not "USD Coin"

pkg/evm/chains.go

@@ -61,17 +61,34 @@ func KnownChainIDs() []uint64 {
 	}
 }
 
+// usdcDomainName returns the EIP-712 domain name of the USDC contract
+// deployed on chainID. The mainnet FiatToken deployments use "USD Coin",
+// but Circle's Base Sepolia testnet deployment uses "USDC" — signing with
+// the wrong name produces a different domain separator, so the contract
+// (and any x402 facilitator) rejects the signature. Values verified
+// against each contract's on-chain name() and DOMAIN_SEPARATOR(); they
+// also match the per-network EIP-712 metadata in x402's asset registry
+// (go/mechanisms/evm/constants.go).
+//
+// Unknown chains fall back to "USD Coin", preserving the previous
+// behavior for custom deployments passed via token override.
+func usdcDomainName(chainID uint64) string {
+	if chainID == ChainBaseSepolia {
+		return "USDC"
+	}
+	return "USD Coin"
+}
+
 // USDCDomain returns the EIP-712 Domain for USDC on chainID, ready to
-// pass to EIP3009Digest. USDC's EIP-712 domain uses name="USD Coin"
-// and version="2" on every chain (verified against the deployed FiatToken
-// contracts).
+// pass to EIP3009Digest. version="2" on every supported chain; the
+// domain name is per-chain (see usdcDomainName).
 func USDCDomain(chainID uint64) (Domain, error) {
 	addr, err := USDCAddress(chainID)
 	if err != nil {
 		return Domain{}, err
 	}
 	return Domain{
-		Name:              "USD Coin",
+		Name:              usdcDomainName(chainID),
 		Version:           "2",
 		ChainID:           chainID,
 		VerifyingContract: addr,

pkg/evm/evm_test.go

@@ -115,13 +115,19 @@ func TestTransferWithAuthorizationTypeHash(t *testing.T) {
 }
 
 func TestUSDCDomainKnownChains(t *testing.T) {
-	for _, chainID := range []uint64{ChainEthereumMainnet, ChainBaseMainnet, ChainBaseSepolia} {
+	wantName := map[uint64]string{
+		ChainEthereumMainnet: "USD Coin",
+		ChainBaseMainnet:     "USD Coin",
+		ChainPolygonMainnet:  "USD Coin",
+		ChainBaseSepolia:     "USDC", // Circle's testnet deployment differs
+	}
+	for chainID, name := range wantName {
 		d, err := USDCDomain(chainID)
 		if err != nil {
 			t.Fatalf("USDCDomain(%d): %v", chainID, err)
 		}
-		if d.Name != "USD Coin" || d.Version != "2" || d.ChainID != chainID {
-			t.Errorf("domain for chain %d wrong: %+v", chainID, d)
+		if d.Name != name || d.Version != "2" || d.ChainID != chainID {
+			t.Errorf("domain for chain %d wrong: %+v (want name %q)", chainID, d, name)
 		}
 		// Separator must be 32 bytes.
 		sep := d.Separator()
@@ -131,6 +137,30 @@ func TestUSDCDomainKnownChains(t *testing.T) {
 	}
 }
 
+// TestUSDCDomainSeparatorsMatchOnChain pins Domain.Separator() for every
+// supported chain to the value the deployed USDC contract returns from
+// DOMAIN_SEPARATOR() (fetched via eth_call, 2026-07-29). If any of these
+// fail, signatures produced for that chain are unverifiable on-chain —
+// this is the regression test for the Base Sepolia "USD Coin"/"USDC"
+// domain-name mismatch.
+func TestUSDCDomainSeparatorsMatchOnChain(t *testing.T) {
+	onChain := map[uint64]string{
+		ChainEthereumMainnet: "06c37168a7db5138defc7866392bb87a741f9b3d104deb5094588ce041cae335",
+		ChainBaseMainnet:     "02fa7265e7c5d81118673727957699e4d68f74cd74b7db77da710fe8a2c7834f",
+		ChainPolygonMainnet:  "caa2ce1a5703ccbe253a34eb3166df60a705c561b44b192061e28f2a985be2ca",
+		ChainBaseSepolia:     "71f17a3b2ff373b803d70a5a07c046c1a2bc8e89c09ef722fcb047abe94c9818",
+	}
+	for chainID, want := range onChain {
+		d, err := USDCDomain(chainID)
+		if err != nil {
+			t.Fatalf("USDCDomain(%d): %v", chainID, err)
+		}
+		if got := hex.EncodeToString(d.Separator()); got != want {
+			t.Errorf("chain %d separator %s, on-chain DOMAIN_SEPARATOR() is %s", chainID, got, want)
+		}
+	}
+}
+
 func TestUSDCDomainRejectsUnknownChain(t *testing.T) {
 	_, err := USDCDomain(999999)
 	if err == nil {

pkg/evm/method.go

@@ -128,7 +128,7 @@ func (m *EVMMethod) Satisfy(_ context.Context, c payment.Contract) (payment.Rece
 		Nonce:       nonce,
 	}
 	domain := Domain{
-		Name:              "USD Coin",
+		Name:              usdcDomainName(m.chainID),
 		Version:           "2",
 		ChainID:           m.chainID,
 		VerifyingContract: m.token,
@@ -225,7 +225,7 @@ func (m *EVMMethod) Verify(_ context.Context, c payment.Contract, r payment.Rece
 		Nonce: nonce,
 	}
 	domain := Domain{
-		Name:              "USD Coin",
+		Name:              usdcDomainName(p.ChainID),
 		Version:           "2",
 		ChainID:           p.ChainID,
 		VerifyingContract: p.Token,