Fix critical MCP integration bugs from code review

Address 6 issues identified in PR #13 review:
- Honor network flag in server/client (was always Base)
- Only treat 402 errors as payment failures after retry
- Include all payment sentinels in IsPaymentError
- Use settle timeout for HTTP client (was too short)
- Copy requirements slice to prevent concurrent mutation

Author: ezynda3

examples/mcp/main.go

@@ -55,13 +55,26 @@ func runServer() {
 	}
 
 	// Determine network and payment requirements
-	var requirement func(string, string, string) x402.PaymentRequirement
-	networkName := *network
-	if *testnet {
+	var (
+		requirement func(string, string, string) x402.PaymentRequirement
+		networkName string
+	)
+
+	switch {
+	case *testnet:
 		requirement = server.RequireUSDCBaseSepolia
 		networkName = "base-sepolia"
-	} else {
+	case *network == "base":
 		requirement = server.RequireUSDCBase
+		networkName = "base"
+	case *network == "polygon":
+		requirement = server.RequireUSDCPolygon
+		networkName = "polygon"
+	case *network == "solana":
+		requirement = server.RequireUSDCSolana
+		networkName = "solana"
+	default:
+		log.Fatalf("unsupported network: %s", *network)
 	}
 
 	// Create x402 MCP server
@@ -158,10 +171,17 @@ func runClient() {
 
 	// Determine chain configuration
 	var chain x402.ChainConfig
-	if *testnet {
+	switch {
+	case *testnet:
 		chain = x402.BaseSepolia
-	} else {
+	case *network == "base":
 		chain = x402.BaseMainnet
+	case *network == "polygon":
+		chain = x402.PolygonMainnet
+	case *network == "solana":
+		chain = x402.SolanaMainnet
+	default:
+		log.Fatalf("unsupported network: %s", *network)
 	}
 
 	// Create EVM signer with network and USDC token

mcp/client/transport.go

@@ -215,7 +215,7 @@ func (t *Transport) retryWithPayment(ctx context.Context, req transport.JSONRPCR
 
 	// Check if payment succeeded
 	if resp.Error != nil {
-		if t.config.OnPaymentFailure != nil {
+		if resp.Error.Code == 402 && t.config.OnPaymentFailure != nil {
 			t.config.OnPaymentFailure(PaymentEvent{
 				Type:    PaymentFailure,
 				Error:   fmt.Errorf("payment rejected: %s", resp.Error.Message),

mcp/errors.go

@@ -99,8 +99,14 @@ func IsPaymentError(err error) bool {
 	return errors.As(err, &paymentErr) ||
 		errors.Is(err, ErrPaymentRequired) ||
 		errors.Is(err, ErrNoMatchingSigner) ||
+		errors.Is(err, ErrInsufficientBalance) ||
 		errors.Is(err, ErrPaymentRejected) ||
 		errors.Is(err, ErrSettlementFailed) ||
+		errors.Is(err, ErrInvalidPaymentPayload) ||
+		errors.Is(err, ErrNoPaymentRequirements) ||
+		errors.Is(err, ErrFacilitatorUnavailable) ||
+		errors.Is(err, ErrVerificationTimeout) ||
+		errors.Is(err, ErrSettlementTimeout) ||
 		errors.Is(err, x402.ErrNoValidSigner) ||
 		errors.Is(err, x402.ErrSigningFailed) ||
 		errors.Is(err, x402.ErrVerificationFailed)

mcp/server/facilitator.go

@@ -28,7 +28,7 @@ type HTTPFacilitator struct {
 func NewHTTPFacilitator(facilitatorURL string) *HTTPFacilitator {
 	client := &http.FacilitatorClient{
 		BaseURL:       facilitatorURL,
-		Client:        &nethttp.Client{Timeout: mcp.PaymentVerifyTimeout},
+		Client:        &nethttp.Client{Timeout: mcp.PaymentSettleTimeout},
 		VerifyTimeout: mcp.PaymentVerifyTimeout,
 		SettleTimeout: mcp.PaymentSettleTimeout,
 		MaxRetries:    2,

mcp/server/handler.go

@@ -188,14 +188,17 @@ func (h *X402Handler) checkPaymentRequired(toolName string) ([]x402.PaymentRequi
 		return nil, false
 	}
 
-	// Set resource field on requirements
-	for i := range requirements {
-		if requirements[i].Resource == "" {
-			requirements[i].Resource = fmt.Sprintf("mcp://tools/%s", toolName)
+	// Work on a copy to avoid mutating shared config
+	reqCopy := make([]x402.PaymentRequirement, len(requirements))
+	copy(reqCopy, requirements)
+
+	for i := range reqCopy {
+		if reqCopy[i].Resource == "" {
+			reqCopy[i].Resource = fmt.Sprintf("mcp://tools/%s", toolName)
 		}
 	}
 
-	return requirements, true
+	return reqCopy, true
 }
 
 // extractPayment extracts payment from params._meta["x402/payment"]