diff --git a/README.md b/README.md
index b6e9208..6ba2895 100644
--- a/README.md
+++ b/README.md
@@ -33,7 +33,7 @@ To stop the playground, press `Ctrl+C`.
 The `EL` instance is deployed with this deterministic enode address:
 
 ```
-enode://3479db4d9217fb5d7a8ed4d61ac36e120b05d36c2eefb795dc42ff2e971f251a2315f5649ea1833271e020b9adc98d5db9973c7ed92d6b2f1f2223088c3d852f@127.0.0.1:8545
+enode://3479db4d9217fb5d7a8ed4d61ac36e120b05d36c2eefb795dc42ff2e971f251a2315f5649ea1833271e020b9adc98d5db9973c7ed92d6b2f1f2223088c3d852f@127.0.0.1:30303
 ```
 
 Options:
diff --git a/cl-proxy/cl-proxy.go b/cl-proxy/cl-proxy.go
new file mode 100644
index 0000000..df5cb2a
--- /dev/null
+++ b/cl-proxy/cl-proxy.go
@@ -0,0 +1,193 @@
+package clproxy
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"fmt"
+	"io"
+	"net/http"
+	"os"
+	"strings"
+	"time"
+
+	"github.com/flashbots/mev-boost-relay/common"
+	"github.com/sirupsen/logrus"
+)
+
+type Config struct {
+	LogOutput io.Writer
+	Port      uint64
+	Primary   string
+	Secondary string
+}
+
+func DefaultConfig() *Config {
+	return &Config{
+		LogOutput: os.Stdout,
+		Port:      5656,
+	}
+}
+
+type ClProxy struct {
+	config *Config
+	log    *logrus.Entry
+	server *http.Server
+}
+
+func New(config *Config) (*ClProxy, error) {
+	log := common.LogSetup(false, "info")
+	log.Logger.SetOutput(config.LogOutput)
+
+	proxy := &ClProxy{
+		config: config,
+		log:    log,
+	}
+
+	return proxy, nil
+}
+
+// Run starts the HTTP server
+func (s *ClProxy) Run() error {
+	mux := http.NewServeMux()
+	s.server = &http.Server{
+		Addr:         fmt.Sprintf(":%d", s.config.Port),
+		ReadTimeout:  10 * time.Second,
+		WriteTimeout: 10 * time.Second,
+		Handler:      mux,
+	}
+
+	mux.HandleFunc("/", s.handleRequest)
+
+	s.log.Infof("Starting server on port %d", s.config.Port)
+	if err := s.server.ListenAndServe(); err != http.ErrServerClosed {
+		return fmt.Errorf("server error: %v", err)
+	}
+	return nil
+}
+
+// Close gracefully shuts down the server
+func (s *ClProxy) Close() error {
+	s.log.Info("Shutting down server...")
+
+	// Create a context with timeout for shutdown
+	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
+	defer cancel()
+
+	// Attempt graceful shutdown
+	if err := s.server.Shutdown(ctx); err != nil {
+		return fmt.Errorf("server shutdown error: %v", err)
+	}
+
+	return nil
+}
+
+type jsonrpcMessage struct {
+	Version string            `json:"jsonrpc,omitempty"`
+	ID      json.RawMessage   `json:"id,omitempty"`
+	Method  string            `json:"method,omitempty"`
+	Params  []json.RawMessage `json:"params,omitempty"`
+	Result  json.RawMessage   `json:"result,omitempty"`
+}
+
+func (s *ClProxy) handleRequest(w http.ResponseWriter, r *http.Request) {
+	// Only accept POST requests
+	if r.Method != http.MethodPost {
+		http.Error(w, "Method not allowed", http.StatusMethodNotAllowed)
+		return
+	}
+
+	data, err := io.ReadAll(r.Body)
+	if err != nil {
+		http.Error(w, "Bad request", http.StatusBadRequest)
+		return
+	}
+
+	// Multiplex all the request to both primary and secondary but omit the
+	// block building requests (this is, remove 'params' field from FCU and omit get payload).
+	// There are two reasons for this:
+	// - The secondary builder does not use the Engine API to build blocks but the relayer so these requests are not necessary.
+	// - The CL->EL setup is not configured anyway to handle two block builders throught the Engine API.
+	// Note that we still have to relay this request to the primary EL node since we need
+	// to have a fallback node in the CL.
+	var jsonRPCRequest jsonrpcMessage
+	if err := json.Unmarshal(data, &jsonRPCRequest); err != nil {
+		http.Error(w, "Bad request", http.StatusBadRequest)
+		return
+	}
+
+	s.log.Info(fmt.Sprintf("Received request: method=%s", jsonRPCRequest.Method))
+
+	// proxy to primary and consider its response as the final response to send back to the CL
+	resp, err := s.proxy(s.config.Primary, r, data)
+	if err != nil {
+		s.log.Errorf("Error multiplexing to primary: %v", err)
+		http.Error(w, "Internal server error", http.StatusInternalServerError)
+		return
+	}
+	defer resp.Body.Close()
+
+	respData, err := io.ReadAll(resp.Body)
+	if err != nil {
+		s.log.Errorf("Error reading response from primary: %v", err)
+		http.Error(w, "Internal server error", http.StatusInternalServerError)
+		return
+	}
+
+	w.Header().Set("Content-Type", "application/json")
+	w.WriteHeader(resp.StatusCode)
+	w.Write(respData)
+
+	if s.config.Secondary == "" {
+		return
+	}
+
+	if strings.HasPrefix(jsonRPCRequest.Method, "engine_getPayload") {
+		// the only request we do not send since the secondary builder does not have the payload id
+		// and it will always fail
+		return
+	}
+
+	if strings.HasPrefix(jsonRPCRequest.Method, "engine_forkchoiceUpdated") {
+		// set to nil the second parameter of the forkchoiceUpdated call
+		if len(jsonRPCRequest.Params) == 1 {
+			// not expected
+			s.log.Warn("ForkchoiceUpdated call with only one parameter")
+		} else {
+			jsonRPCRequest.Params[1] = nil
+
+			data, err = json.Marshal(jsonRPCRequest)
+			if err != nil {
+				s.log.Errorf("Error marshalling forkchoiceUpdated request: %v", err)
+				return
+			}
+		}
+	}
+
+	// proxy to secondary
+	s.log.Info(fmt.Sprintf("Multiplexing request to secondary: method=%s", jsonRPCRequest.Method))
+	if _, err := s.proxy(s.config.Secondary, r, data); err != nil {
+		s.log.Errorf("Error multiplexing to secondary: %v", err)
+	}
+}
+
+func (s *ClProxy) proxy(dst string, r *http.Request, data []byte) (*http.Response, error) {
+	// Create a new request
+	req, err := http.NewRequest(http.MethodPost, dst, bytes.NewBuffer(data))
+	if err != nil {
+		return nil, err
+	}
+
+	// Copy headers. It is important since we have to copy
+	// the JWT header from the CL
+	req.Header = r.Header
+
+	// Perform the request
+	client := &http.Client{}
+	resp, err := client.Do(req)
+	if err != nil {
+		return nil, err
+	}
+
+	return resp, nil
+}
diff --git a/main.go b/main.go
index 6e41297..aa4cf92 100644
--- a/main.go
+++ b/main.go
@@ -31,6 +31,7 @@ import (
 	ecrypto "github.com/ethereum/go-ethereum/crypto"
 
 	"github.com/ferranbt/builder-playground/artifacts"
+	clproxy "github.com/ferranbt/builder-playground/cl-proxy"
 	mevboostrelay "github.com/ferranbt/builder-playground/mev-boost-relay"
 
 	"github.com/hashicorp/go-uuid"
@@ -61,6 +62,7 @@ var genesisDelayFlag uint64
 var watchPayloadsFlag bool
 var latestForkFlag bool
 var useRethForValidation bool
+var secondaryBuilderPort uint64
 
 var rootCmd = &cobra.Command{
 	Use:   "playground",
@@ -168,6 +170,7 @@ func main() {
 	rootCmd.Flags().BoolVar(&watchPayloadsFlag, "watch-payloads", false, "")
 	rootCmd.Flags().BoolVar(&latestForkFlag, "electra", false, "")
 	rootCmd.Flags().BoolVar(&useRethForValidation, "use-reth-for-validation", false, "enable flashbots_validateBuilderSubmissionV* on reth and use them for validation")
+	rootCmd.Flags().Uint64Var(&secondaryBuilderPort, "secondary", 1234, "port to use for the secondary builder")
 
 	downloadArtifactsCmd.Flags().BoolVar(&validateFlag, "validate", false, "")
 	validateCmd.Flags().Uint64Var(&numBlocksValidate, "num-blocks", 5, "")
@@ -369,6 +372,31 @@ func setupServices(svcManager *serviceManager, out *output) error {
 		return err
 	}
 
+	// Start the cl proxy
+	{
+		cfg := clproxy.DefaultConfig()
+		cfg.Primary = "http://localhost:8551"
+
+		if secondaryBuilderPort != 0 {
+			cfg.Secondary = fmt.Sprintf("http://localhost:%d", secondaryBuilderPort)
+		}
+
+		var err error
+		if cfg.LogOutput, err = out.LogOutput("cl-proxy"); err != nil {
+			return err
+		}
+		clproxy, err := clproxy.New(cfg)
+		if err != nil {
+			return fmt.Errorf("failed to create cl proxy: %w", err)
+		}
+
+		go func() {
+			if err := clproxy.Run(); err != nil {
+				svcManager.emitError()
+			}
+		}()
+	}
+
 	rethVersion := func() string {
 		cmd := exec.Command(rethBin, "--version")
 		out, err := cmd.Output()
@@ -404,13 +432,14 @@ func setupServices(svcManager *serviceManager, out *output) error {
 			"--p2p-secret-key", defaultRethDiscoveryPrivKeyLoc,
 			"--addr", "127.0.0.1",
 			"--port", "30303",
-			"--disable-discovery",
+			// "--disable-discovery",
 			// http config
 			"--http",
 			"--http.api", "admin,eth,net,web3",
 			"--http.port", "8545",
 			"--authrpc.port", "8551",
 			"--authrpc.jwtsecret", "{{.Dir}}/jwtsecret",
+			"-vvvv",
 		).
 		If(useRethForValidation, func(s *service) *service {
 			return s.WithReplacementArgs("--http.api", "admin,eth,web3,net,rpc,flashbots")
@@ -470,7 +499,7 @@ func setupServices(svcManager *serviceManager, out *output) error {
 			"--http-port", "3500",
 			"--disable-packet-filter",
 			"--target-peers", "0",
-			"--execution-endpoint", "http://localhost:8551",
+			"--execution-endpoint", "http://localhost:5656",
 			"--execution-jwt", "{{.Dir}}/jwtsecret",
 			"--builder", "http://localhost:5555",
 			"--builder-fallback-epochs-since-finalization", "0",
@@ -538,6 +567,11 @@ func setupServices(svcManager *serviceManager, out *output) error {
 		ports: []*port{
 			{name: "http", port: 5555},
 		},
+	}, &service{
+		name: "cl-proxy",
+		ports: []*port{
+			{name: "jsonrpc", port: 5656},
+		},
 	})
 
 	// print services info