diff --git a/eth/catalyst/api_testing_test.go b/eth/catalyst/api_testing_test.go
new file mode 100644
index 0000000000..95d390b92a
--- /dev/null
+++ b/eth/catalyst/api_testing_test.go
@@ -0,0 +1,105 @@
+package catalyst
+
+import (
+	"context"
+	"math/big"
+	"testing"
+
+	"github.com/ethereum/go-ethereum/beacon/engine"
+	"github.com/ethereum/go-ethereum/common"
+	"github.com/ethereum/go-ethereum/common/hexutil"
+	"github.com/ethereum/go-ethereum/core/types"
+	"github.com/ethereum/go-ethereum/crypto"
+	"github.com/ethereum/go-ethereum/params"
+)
+
+func TestBuildBlockV1(t *testing.T) {
+	genesis, blocks := generateMergeChain(5, true)
+	n, ethservice := startEthService(t, genesis, blocks)
+	defer n.Close()
+
+	parent := ethservice.BlockChain().CurrentBlock()
+	attrs := engine.PayloadAttributes{
+		Timestamp:             parent.Time + 1,
+		Random:                crypto.Keccak256Hash([]byte("test")),
+		SuggestedFeeRecipient: parent.Coinbase,
+		Withdrawals:           nil,
+		BeaconRoot:            nil,
+	}
+
+	currentNonce, _ := ethservice.APIBackend.GetPoolNonce(context.Background(), testAddr)
+	tx, _ := types.SignTx(types.NewTransaction(currentNonce, testAddr, big.NewInt(1), params.TxGas, big.NewInt(params.InitialBaseFee*2), nil), types.LatestSigner(ethservice.BlockChain().Config()), testKey)
+
+	api := NewTestingAPI(ethservice)
+
+	t.Run("buildOnCurrentHead", func(t *testing.T) {
+		envelope, err := api.BuildBlockV1(parent.Hash(), attrs, nil, nil)
+		if err != nil {
+			t.Fatalf("BuildBlockV1 failed: %v", err)
+		}
+		if envelope == nil || envelope.ExecutionPayload == nil {
+			t.Fatal("expected non-nil envelope and payload")
+		}
+		payload := envelope.ExecutionPayload
+		if payload.ParentHash != parent.Hash() {
+			t.Errorf("parent hash mismatch: got %x want %x", payload.ParentHash, parent.Hash())
+		}
+		if payload.Number != parent.Number.Uint64()+1 {
+			t.Errorf("block number mismatch: got %d want %d", payload.Number, parent.Number.Uint64()+1)
+		}
+		if payload.Timestamp != attrs.Timestamp {
+			t.Errorf("timestamp mismatch: got %d want %d", payload.Timestamp, attrs.Timestamp)
+		}
+		if payload.FeeRecipient != attrs.SuggestedFeeRecipient {
+			t.Errorf("fee recipient mismatch: got %x want %x", payload.FeeRecipient, attrs.SuggestedFeeRecipient)
+		}
+	})
+
+	t.Run("wrongParentHash", func(t *testing.T) {
+		wrongParent := common.Hash{0x01}
+		_, err := api.BuildBlockV1(wrongParent, attrs, nil, nil)
+		if err == nil {
+			t.Fatal("expected error when parentHash is not current head")
+		}
+		if err.Error() != "parentHash is not current head" {
+			t.Errorf("unexpected error: %v", err)
+		}
+	})
+
+	t.Run("buildEmptyBlock", func(t *testing.T) {
+		emptyTxs := []hexutil.Bytes{}
+		envelope, err := api.BuildBlockV1(parent.Hash(), attrs, &emptyTxs, nil)
+		if err != nil {
+			t.Fatalf("BuildBlockV1 with empty txs failed: %v", err)
+		}
+		if envelope == nil || envelope.ExecutionPayload == nil {
+			t.Fatal("expected non-nil envelope and payload")
+		}
+		if len(envelope.ExecutionPayload.Transactions) != 0 {
+			t.Errorf("expected empty block, got %d transactions", len(envelope.ExecutionPayload.Transactions))
+		}
+	})
+
+	t.Run("buildBlockWithTransactions", func(t *testing.T) {
+		enc, _ := tx.MarshalBinary()
+		txs := []hexutil.Bytes{enc}
+		envelope, err := api.BuildBlockV1(parent.Hash(), attrs, &txs, nil)
+		if err != nil {
+			t.Fatalf("BuildBlockV1 with transaction failed: %v", err)
+		}
+		if len(envelope.ExecutionPayload.Transactions) != 1 {
+			t.Errorf("expected 1 transaction, got %d", len(envelope.ExecutionPayload.Transactions))
+		}
+	})
+
+	t.Run("buildBlockWithTransactionsFromTxPool", func(t *testing.T) {
+		ethservice.TxPool().Add([]*types.Transaction{tx}, true)
+		envelope, err := api.BuildBlockV1(parent.Hash(), attrs, nil, nil)
+		if err != nil {
+			t.Fatalf("BuildBlockV1 with transaction failed: %v", err)
+		}
+		if len(envelope.ExecutionPayload.Transactions) != 1 {
+			t.Errorf("expected 1 transaction, got %d", len(envelope.ExecutionPayload.Transactions))
+		}
+	})
+}