diff --git a/op-node/rollup/derive/attributes.go b/op-node/rollup/derive/attributes.go
index c65198bcec7..92ccd32cf5f 100644
--- a/op-node/rollup/derive/attributes.go
+++ b/op-node/rollup/derive/attributes.go
@@ -121,9 +121,21 @@ func (ba *FetchingAttributesBuilder) PreparePayloadAttributes(ctx context.Contex
 		return nil, NewCriticalError(fmt.Errorf("failed to create l1InfoTx: %w", err))
 	}
 
-	txs := make([]hexutil.Bytes, 0, 1+len(depositTxs)+len(upgradeTxs))
+	var afterForceIncludeTxs []hexutil.Bytes
+	// TODO I'm usure if this needs to be only done AFTER interop is enabled, but not on the same block.
+	// This is similar to what Proto mentioned about blocking the ExecutingMessages on the same block as the interop activation.
+	if ba.rollupCfg.IsInterop(nextL2Time) {
+		depositsCompleteTx, err := DepositsCompleteBytes(seqNumber, l1Info)
+		if err != nil {
+			return nil, NewCriticalError(fmt.Errorf("failed to create depositsCompleteTx: %w", err))
+		}
+		afterForceIncludeTxs = append(afterForceIncludeTxs, depositsCompleteTx)
+	}
+
+	txs := make([]hexutil.Bytes, 0, 1+len(depositTxs)+len(afterForceIncludeTxs)+len(upgradeTxs))
 	txs = append(txs, l1InfoTx)
 	txs = append(txs, depositTxs...)
+	txs = append(txs, afterForceIncludeTxs...)
 	txs = append(txs, upgradeTxs...)
 
 	var withdrawals *types.Withdrawals
diff --git a/op-node/rollup/derive/attributes_test.go b/op-node/rollup/derive/attributes_test.go
index 68c7c71aa1e..e428d952e5b 100644
--- a/op-node/rollup/derive/attributes_test.go
+++ b/op-node/rollup/derive/attributes_test.go
@@ -195,6 +195,54 @@ func TestPreparePayloadAttributes(t *testing.T) {
 		require.Equal(t, l1InfoTx, []byte(attrs.Transactions[0]))
 		require.True(t, attrs.NoTxPool)
 	})
+	t.Run("same origin with deposits on post-Isthmus", func(t *testing.T) {
+		rng := rand.New(rand.NewSource(1234))
+		l1Fetcher := &testutils.MockL1Source{}
+		defer l1Fetcher.AssertExpectations(t)
+		l2Parent := testutils.RandomL2BlockRef(rng)
+		l1CfgFetcher := &testutils.MockL2Client{}
+		l1CfgFetcher.ExpectSystemConfigByL2Hash(l2Parent.Hash, testSysCfg, nil)
+		defer l1CfgFetcher.AssertExpectations(t)
+		l1Info := testutils.RandomBlockInfo(rng)
+		l1Info.InfoParentHash = l2Parent.L1Origin.Hash
+		l1Info.InfoNum = l2Parent.L1Origin.Number + 1
+
+		receipts, depositTxs, err := makeReceipts(rng, l1Info.InfoHash, cfg.DepositContractAddress, []receiptData{
+			{goodReceipt: true, DepositLogs: []bool{true, false}},
+			{goodReceipt: true, DepositLogs: []bool{true}},
+			{goodReceipt: false, DepositLogs: []bool{true}},
+			{goodReceipt: false, DepositLogs: []bool{false}},
+		})
+		require.NoError(t, err)
+		usedDepositTxs, err := encodeDeposits(depositTxs)
+		require.NoError(t, err)
+
+		// sets config to post-interop
+		cfg.AfterHardfork(rollup.Interop, 2)
+
+		epoch := l1Info.ID()
+		l1InfoTx, err := L1InfoDepositBytes(cfg, testSysCfg, 0, l1Info, 0)
+		require.NoError(t, err)
+
+		l2Txs := append(append(make([]eth.Data, 0), l1InfoTx), usedDepositTxs...)
+
+		l1Fetcher.ExpectFetchReceipts(epoch.Hash, l1Info, receipts, nil)
+		attrBuilder := NewFetchingAttributesBuilder(cfg, l1Fetcher, l1CfgFetcher)
+		attrs, err := attrBuilder.PreparePayloadAttributes(context.Background(), l2Parent, epoch)
+		require.NoError(t, err)
+		require.NotNil(t, attrs)
+		require.Equal(t, l2Parent.Time+cfg.BlockTime, uint64(attrs.Timestamp))
+		require.Equal(t, eth.Bytes32(l1Info.InfoMixDigest), attrs.PrevRandao)
+		require.Equal(t, predeploys.SequencerFeeVaultAddr, attrs.SuggestedFeeRecipient)
+		require.Equal(t, len(l2Txs), len(attrs.Transactions), "Expected txs to equal l1 info tx + user deposit txs + DepositsComplete")
+		require.Equal(t, l2Txs, attrs.Transactions)
+		require.Equal(t, DepositsCompleteBytes4, attrs.Transactions[1+len(depositTxs)][:4])
+		require.True(t, attrs.NoTxPool)
+		depositsCompleteTx, err := DepositsCompleteBytes(l2Parent.SequenceNumber, l1Info)
+		require.NoError(t, err)
+		require.Equal(t, l2Txs, append(attrs.Transactions, depositsCompleteTx))
+	})
+
 	// Test that the payload attributes builder changes the deposit format based on L2-time-based regolith activation
 	t.Run("regolith", func(t *testing.T) {
 		testCases := []struct {
diff --git a/op-node/rollup/derive/deposit_source.go b/op-node/rollup/derive/deposit_source.go
index f7a9730ad02..0e663be95ab 100644
--- a/op-node/rollup/derive/deposit_source.go
+++ b/op-node/rollup/derive/deposit_source.go
@@ -13,9 +13,10 @@ type UserDepositSource struct {
 }
 
 const (
-	UserDepositSourceDomain    = 0
-	L1InfoDepositSourceDomain  = 1
-	UpgradeDepositSourceDomain = 2
+	UserDepositSourceDomain       = 0
+	L1InfoDepositSourceDomain     = 1
+	UpgradeDepositSourceDomain    = 2
+	AfterForceIncludeSourceDomain = 3
 )
 
 func (dep *UserDepositSource) SourceHash() common.Hash {
@@ -63,3 +64,19 @@ func (dep *UpgradeDepositSource) SourceHash() common.Hash {
 	copy(domainInput[32:], intentHash[:])
 	return crypto.Keccak256Hash(domainInput[:])
 }
+
+// Used for DepositsComplete/ResetDeposits post-deposits transactions.
+type AfterForceIncludeSource struct {
+	L1BlockHash common.Hash
+}
+
+func (dep *AfterForceIncludeSource) SourceHash() common.Hash {
+	var input [32 * 2]byte
+	copy(input[:32], dep.L1BlockHash[:])
+	depositIDHash := crypto.Keccak256Hash(input[:])
+
+	var domainInput [32 * 2]byte
+	binary.BigEndian.PutUint64(domainInput[32-8:32], AfterForceIncludeSourceDomain)
+	copy(domainInput[32:], depositIDHash[:])
+	return crypto.Keccak256Hash(domainInput[:])
+}
diff --git a/op-node/rollup/derive/deposit_source_test.go b/op-node/rollup/derive/deposit_source_test.go
index 10fb7048a2a..d851325e370 100644
--- a/op-node/rollup/derive/deposit_source_test.go
+++ b/op-node/rollup/derive/deposit_source_test.go
@@ -3,6 +3,7 @@ package derive
 import (
 	"testing"
 
+	"github.com/ethereum/go-ethereum/common"
 	"github.com/stretchr/testify/assert"
 )
 
@@ -34,3 +35,17 @@ func TestEcotone4788ContractSourceHash(t *testing.T) {
 
 	assert.Equal(t, expected, actual.Hex())
 }
+
+// TestAfterForceIncludeSourceHash
+// cast keccak $(cast concat-hex 0x0000000000000000000000000000000000000000000000000000000000000003 $(cast keccak 0x01))
+// # 0x8afb1c4a581d0e71ab65334e3365ba5511fb15c13fa212776f9d4dafc6287845
+func TestAfterForceIncludeSource(t *testing.T) {
+	source := AfterForceIncludeSource{
+		L1BlockHash: common.Hash{0x01},
+	}
+
+	actual := source.SourceHash()
+	expected := "0x8afb1c4a581d0e71ab65334e3365ba5511fb15c13fa212776f9d4dafc6287845"
+
+	assert.Equal(t, expected, actual.Hex())
+}
diff --git a/op-node/rollup/derive/fuzz_parsers_test.go b/op-node/rollup/derive/fuzz_parsers_test.go
index 95ce94bc7cc..4f76c4ac742 100644
--- a/op-node/rollup/derive/fuzz_parsers_test.go
+++ b/op-node/rollup/derive/fuzz_parsers_test.go
@@ -83,15 +83,26 @@ func FuzzL1InfoEcotoneRoundTrip(f *testing.F) {
 		}
 		enc, err := in.marshalBinaryEcotone()
 		if err != nil {
-			t.Fatalf("Failed to marshal binary: %v", err)
+			t.Fatalf("Failed to marshal Ecotone binary: %v", err)
 		}
 		var out L1BlockInfo
 		err = out.unmarshalBinaryEcotone(enc)
 		if err != nil {
-			t.Fatalf("Failed to unmarshal binary: %v", err)
+			t.Fatalf("Failed to unmarshal Ecotone binary: %v", err)
 		}
 		if !cmp.Equal(in, out, cmp.Comparer(testutils.BigEqual)) {
-			t.Fatalf("The data did not round trip correctly. in: %v. out: %v", in, out)
+			t.Fatalf("The Ecotone data did not round trip correctly. in: %v. out: %v", in, out)
+		}
+		enc, err = in.marshalBinaryIsthmus()
+		if err != nil {
+			t.Fatalf("Failed to marshal Isthmus binary: %v", err)
+		}
+		err = out.unmarshalBinaryIsthmus(enc)
+		if err != nil {
+			t.Fatalf("Failed to unmarshal Isthmus binary: %v", err)
+		}
+		if !cmp.Equal(in, out, cmp.Comparer(testutils.BigEqual)) {
+			t.Fatalf("The Isthmus data did not round trip correctly. in: %v. out: %v", in, out)
 		}
 
 	})
diff --git a/op-node/rollup/derive/l1_block_info.go b/op-node/rollup/derive/l1_block_info.go
index 26f3f6711f5..7631494e42c 100644
--- a/op-node/rollup/derive/l1_block_info.go
+++ b/op-node/rollup/derive/l1_block_info.go
@@ -20,14 +20,24 @@ import (
 const (
 	L1InfoFuncBedrockSignature = "setL1BlockValues(uint64,uint64,uint256,bytes32,uint64,bytes32,uint256,uint256)"
 	L1InfoFuncEcotoneSignature = "setL1BlockValuesEcotone()"
+	L1InfoFuncIsthmusSignature = "setL1BlockValuesIsthmus()"
+	DepositsCompleteSignature  = "depositsComplete()"
 	L1InfoArguments            = 8
 	L1InfoBedrockLen           = 4 + 32*L1InfoArguments
 	L1InfoEcotoneLen           = 4 + 32*5 // after Ecotone upgrade, args are packed into 5 32-byte slots
+	DepositsCompleteLen        = 4        // only the selector
+	// We set the gas limit to 15k to ensure that the DepositsComplete Transaction does not run out of gas.
+	// GasBenchMark_L1BlockIsthmus_DepositsComplete:test_depositsComplete_benchmark() (gas: 7768)
+	// GasBenchMark_L1BlockIsthmus_DepositsComplete_Warm:test_depositsComplete_benchmark() (gas: 5768)
+	// see `test_depositsComplete_benchmark` at: `/packages/contracts-bedrock/test/BenchmarkTest.t.sol`
+	DepositsCompleteGas = uint64(15_000)
 )
 
 var (
 	L1InfoFuncBedrockBytes4 = crypto.Keccak256([]byte(L1InfoFuncBedrockSignature))[:4]
 	L1InfoFuncEcotoneBytes4 = crypto.Keccak256([]byte(L1InfoFuncEcotoneSignature))[:4]
+	L1InfoFuncIsthmusBytes4 = crypto.Keccak256([]byte(L1InfoFuncIsthmusSignature))[:4]
+	DepositsCompleteBytes4  = crypto.Keccak256([]byte(DepositsCompleteSignature))[:4]
 	L1InfoDepositerAddress  = common.HexToAddress("0xdeaddeaddeaddeaddeaddeaddeaddeaddead0001")
 	L1BlockAddress          = predeploys.L1BlockAddr
 	ErrInvalidFormat        = errors.New("invalid ecotone l1 block info format")
@@ -144,7 +154,7 @@ func (info *L1BlockInfo) unmarshalBinaryBedrock(data []byte) error {
 	return nil
 }
 
-// Ecotone Binary Format
+// Isthmus & Ecotone Binary Format
 // +---------+--------------------------+
 // | Bytes   | Field                    |
 // +---------+--------------------------+
@@ -159,10 +169,24 @@ func (info *L1BlockInfo) unmarshalBinaryBedrock(data []byte) error {
 // | 32      | BlockHash                |
 // | 32      | BatcherHash              |
 // +---------+--------------------------+
-
+// Marshal Ecotone and Isthmus
 func (info *L1BlockInfo) marshalBinaryEcotone() ([]byte, error) {
-	w := bytes.NewBuffer(make([]byte, 0, L1InfoEcotoneLen))
-	if err := solabi.WriteSignature(w, L1InfoFuncEcotoneBytes4); err != nil {
+	out, err := marshalBinaryWithSignature(info, L1InfoFuncEcotoneBytes4)
+	if err != nil {
+		return nil, fmt.Errorf("failed to marshal Ecotone l1 block info: %w", err)
+	}
+	return out, nil
+}
+func (info *L1BlockInfo) marshalBinaryIsthmus() ([]byte, error) {
+	out, err := marshalBinaryWithSignature(info, L1InfoFuncIsthmusBytes4)
+	if err != nil {
+		return nil, fmt.Errorf("failed to marshal Isthmus l1 block info: %w", err)
+	}
+	return out, nil
+}
+func marshalBinaryWithSignature(info *L1BlockInfo, signature []byte) ([]byte, error) {
+	w := bytes.NewBuffer(make([]byte, 0, L1InfoEcotoneLen)) // Ecotone and Isthmus have the same length
+	if err := solabi.WriteSignature(w, signature); err != nil {
 		return nil, err
 	}
 	if err := binary.Write(w, binary.BigEndian, info.BaseFeeScalar); err != nil {
@@ -200,14 +224,21 @@ func (info *L1BlockInfo) marshalBinaryEcotone() ([]byte, error) {
 	return w.Bytes(), nil
 }
 
+// Unmarshal Ecotone and Isthmus
 func (info *L1BlockInfo) unmarshalBinaryEcotone(data []byte) error {
+	return unmarshalBinaryWithSignatureAndData(info, L1InfoFuncEcotoneBytes4, data)
+}
+func (info *L1BlockInfo) unmarshalBinaryIsthmus(data []byte) error {
+	return unmarshalBinaryWithSignatureAndData(info, L1InfoFuncIsthmusBytes4, data)
+}
+func unmarshalBinaryWithSignatureAndData(info *L1BlockInfo, signature []byte, data []byte) error {
 	if len(data) != L1InfoEcotoneLen {
 		return fmt.Errorf("data is unexpected length: %d", len(data))
 	}
 	r := bytes.NewReader(data)
 
 	var err error
-	if _, err := solabi.ReadAndValidateSignature(r, L1InfoFuncEcotoneBytes4); err != nil {
+	if _, err := solabi.ReadAndValidateSignature(r, signature); err != nil {
 		return err
 	}
 	if err := binary.Read(r, binary.BigEndian, &info.BaseFeeScalar); err != nil {
@@ -250,9 +281,24 @@ func isEcotoneButNotFirstBlock(rollupCfg *rollup.Config, l2BlockTime uint64) boo
 	return rollupCfg.IsEcotone(l2BlockTime) && !rollupCfg.IsEcotoneActivationBlock(l2BlockTime)
 }
 
+// isInteropButNotFirstBlock returns whether the specified block is subject to the Isthmus upgrade,
+// but is not the actiation block itself.
+func isInteropButNotFirstBlock(rollupCfg *rollup.Config, l2BlockTime uint64) bool {
+	// note from Proto:
+	// Since we use the pre-interop L1 tx one last time during the upgrade block,
+	// we must disallow the deposit-txs from using the CrossL2Inbox during this block.
+	// If the CrossL2Inbox does not exist yet, then it is safe,
+	// but we have to ensure that the spec and code puts any Interop upgrade-txs after the user deposits.
+	return rollupCfg.IsInterop(l2BlockTime) && !rollupCfg.IsInteropActivationBlock(l2BlockTime)
+}
+
 // L1BlockInfoFromBytes is the inverse of L1InfoDeposit, to see where the L2 chain is derived from
 func L1BlockInfoFromBytes(rollupCfg *rollup.Config, l2BlockTime uint64, data []byte) (*L1BlockInfo, error) {
 	var info L1BlockInfo
+	// Important, this should be ordered from most recent to oldest
+	if isInteropButNotFirstBlock(rollupCfg, l2BlockTime) {
+		return &info, info.unmarshalBinaryIsthmus(data)
+	}
 	if isEcotoneButNotFirstBlock(rollupCfg, l2BlockTime) {
 		return &info, info.unmarshalBinaryEcotone(data)
 	}
@@ -271,6 +317,7 @@ func L1InfoDeposit(rollupCfg *rollup.Config, sysCfg eth.SystemConfig, seqNumber
 		BatcherAddr:    sysCfg.BatcherAddr,
 	}
 	var data []byte
+
 	if isEcotoneButNotFirstBlock(rollupCfg, l2BlockTime) {
 		l1BlockInfo.BlobBaseFee = block.BlobBaseFee()
 		if l1BlockInfo.BlobBaseFee == nil {
@@ -283,11 +330,19 @@ func L1InfoDeposit(rollupCfg *rollup.Config, sysCfg eth.SystemConfig, seqNumber
 		}
 		l1BlockInfo.BlobBaseFeeScalar = scalars.BlobBaseFeeScalar
 		l1BlockInfo.BaseFeeScalar = scalars.BaseFeeScalar
-		out, err := l1BlockInfo.marshalBinaryEcotone()
-		if err != nil {
-			return nil, fmt.Errorf("failed to marshal Ecotone l1 block info: %w", err)
+		if isInteropButNotFirstBlock(rollupCfg, l2BlockTime) {
+			out, err := l1BlockInfo.marshalBinaryIsthmus()
+			if err != nil {
+				return nil, fmt.Errorf("failed to marshal Isthmus l1 block info: %w", err)
+			}
+			data = out
+		} else {
+			out, err := l1BlockInfo.marshalBinaryEcotone()
+			if err != nil {
+				return nil, fmt.Errorf("failed to marshal Ecotone l1 block info: %w", err)
+			}
+			data = out
 		}
-		data = out
 	} else {
 		l1BlockInfo.L1FeeOverhead = sysCfg.Overhead
 		l1BlockInfo.L1FeeScalar = sysCfg.Scalar
@@ -335,3 +390,33 @@ func L1InfoDepositBytes(rollupCfg *rollup.Config, sysCfg eth.SystemConfig, seqNu
 	}
 	return opaqueL1Tx, nil
 }
+
+func DepositsCompleteDeposit(seqNumber uint64, block eth.BlockInfo) (*types.DepositTx, error) {
+	source := AfterForceIncludeSource{
+		L1BlockHash: block.Hash(),
+	}
+	out := &types.DepositTx{
+		SourceHash:          source.SourceHash(),
+		From:                L1InfoDepositerAddress,
+		To:                  &L1BlockAddress,
+		Mint:                nil,
+		Value:               big.NewInt(0),
+		Gas:                 DepositsCompleteGas,
+		IsSystemTransaction: false,
+		Data:                DepositsCompleteBytes4,
+	}
+	return out, nil
+}
+
+func DepositsCompleteBytes(seqNumber uint64, l1Info eth.BlockInfo) ([]byte, error) {
+	dep, err := DepositsCompleteDeposit(seqNumber, l1Info)
+	if err != nil {
+		return nil, fmt.Errorf("failed to create DepositsComplete tx: %w", err)
+	}
+	depositsCompleteTx := types.NewTx(dep)
+	opaqueDepositsCompleteTx, err := depositsCompleteTx.MarshalBinary()
+	if err != nil {
+		return nil, fmt.Errorf("failed to encode DepositsComplete tx: %w", err)
+	}
+	return opaqueDepositsCompleteTx, nil
+}
diff --git a/op-node/rollup/derive/l1_block_info_test.go b/op-node/rollup/derive/l1_block_info_test.go
index e5c9253ce1c..0f847309995 100644
--- a/op-node/rollup/derive/l1_block_info_test.go
+++ b/op-node/rollup/derive/l1_block_info_test.go
@@ -41,6 +41,7 @@ func TestParseL1InfoDepositTxData(t *testing.T) {
 	randomSeqNr := func(rng *rand.Rand) uint64 {
 		return rng.Uint64()
 	}
+
 	// Go 1.18 will have native fuzzing for us to use, until then, we cover just the below cases
 	cases := []infoTest{
 		{"random", testutils.MakeBlockInfo(nil), randomL1Cfg, randomSeqNr},
@@ -109,10 +110,8 @@ func TestParseL1InfoDepositTxData(t *testing.T) {
 	t.Run("regolith", func(t *testing.T) {
 		rng := rand.New(rand.NewSource(1234))
 		info := testutils.MakeBlockInfo(nil)(rng)
-		zero := uint64(0)
-		rollupCfg := rollup.Config{
-			RegolithTime: &zero,
-		}
+		rollupCfg := rollup.Config{}
+		rollupCfg.AtHardfork(rollup.Regolith)
 		depTx, err := L1InfoDeposit(&rollupCfg, randomL1Cfg(rng, info), randomSeqNr(rng), info, 0)
 		require.NoError(t, err)
 		require.False(t, depTx.IsSystemTransaction)
@@ -121,11 +120,8 @@ func TestParseL1InfoDepositTxData(t *testing.T) {
 	t.Run("ecotone", func(t *testing.T) {
 		rng := rand.New(rand.NewSource(1234))
 		info := testutils.MakeBlockInfo(nil)(rng)
-		zero := uint64(0)
-		rollupCfg := rollup.Config{
-			RegolithTime: &zero,
-			EcotoneTime:  &zero,
-		}
+		rollupCfg := rollup.Config{}
+		rollupCfg.AtHardfork(rollup.Ecotone)
 		depTx, err := L1InfoDeposit(&rollupCfg, randomL1Cfg(rng, info), randomSeqNr(rng), info, 1)
 		require.NoError(t, err)
 		require.False(t, depTx.IsSystemTransaction)
@@ -135,12 +131,8 @@ func TestParseL1InfoDepositTxData(t *testing.T) {
 	t.Run("first-block ecotone", func(t *testing.T) {
 		rng := rand.New(rand.NewSource(1234))
 		info := testutils.MakeBlockInfo(nil)(rng)
-		zero := uint64(2)
-		rollupCfg := rollup.Config{
-			RegolithTime: &zero,
-			EcotoneTime:  &zero,
-			BlockTime:    2,
-		}
+		rollupCfg := rollup.Config{}
+		rollupCfg.AfterHardfork(rollup.Ecotone, 2)
 		depTx, err := L1InfoDeposit(&rollupCfg, randomL1Cfg(rng, info), randomSeqNr(rng), info, 2)
 		require.NoError(t, err)
 		require.False(t, depTx.IsSystemTransaction)
@@ -150,16 +142,75 @@ func TestParseL1InfoDepositTxData(t *testing.T) {
 	t.Run("genesis-block ecotone", func(t *testing.T) {
 		rng := rand.New(rand.NewSource(1234))
 		info := testutils.MakeBlockInfo(nil)(rng)
-		zero := uint64(0)
-		rollupCfg := rollup.Config{
-			RegolithTime: &zero,
-			EcotoneTime:  &zero,
-			BlockTime:    2,
-		}
+		rollupCfg := rollup.Config{}
+		rollupCfg.AfterHardfork(rollup.Ecotone, 2)
+		depTx, err := L1InfoDeposit(&rollupCfg, randomL1Cfg(rng, info), randomSeqNr(rng), info, 0)
+		require.NoError(t, err)
+		require.False(t, depTx.IsSystemTransaction)
+		require.Equal(t, depTx.Gas, uint64(RegolithSystemTxGas))
+		require.Equal(t, L1InfoEcotoneLen, len(depTx.Data))
+	})
+	t.Run("Isthmus", func(t *testing.T) {
+		rng := rand.New(rand.NewSource(1234))
+		info := testutils.MakeBlockInfo(nil)(rng)
+		rollupCfg := rollup.Config{}
+		rollupCfg.AtHardfork(rollup.Interop)
+		depTx, err := L1InfoDeposit(&rollupCfg, randomL1Cfg(rng, info), randomSeqNr(rng), info, 1)
+		require.NoError(t, err)
+		require.False(t, depTx.IsSystemTransaction)
+		require.Equal(t, depTx.Gas, uint64(RegolithSystemTxGas))
+		require.Equal(t, L1InfoEcotoneLen, len(depTx.Data))
+		require.Equal(t, L1InfoFuncEcotoneBytes4, depTx.Data[:4])
+		// should still send Ecotone signature since upgrade happens after deposits
+	})
+	t.Run("first-block isthmus", func(t *testing.T) {
+		rng := rand.New(rand.NewSource(1234))
+		info := testutils.MakeBlockInfo(nil)(rng)
+		rollupCfg := rollup.Config{}
+		rollupCfg.AfterHardfork(rollup.Interop, 2)
+		depTx, err := L1InfoDeposit(&rollupCfg, randomL1Cfg(rng, info), randomSeqNr(rng), info, 2)
+		require.NoError(t, err)
+		require.False(t, depTx.IsSystemTransaction)
+		require.Equal(t, depTx.Gas, uint64(RegolithSystemTxGas))
+		require.Equal(t, L1InfoBedrockLen, len(depTx.Data))
+		require.Equal(t, L1InfoFuncIsthmusBytes4, depTx.Data[:4])
+	})
+	t.Run("genesis-block isthmus", func(t *testing.T) {
+		rng := rand.New(rand.NewSource(1234))
+		info := testutils.MakeBlockInfo(nil)(rng)
+		rollupCfg := rollup.Config{}
+		rollupCfg.AfterHardfork(rollup.Interop, 2)
 		depTx, err := L1InfoDeposit(&rollupCfg, randomL1Cfg(rng, info), randomSeqNr(rng), info, 0)
 		require.NoError(t, err)
 		require.False(t, depTx.IsSystemTransaction)
 		require.Equal(t, depTx.Gas, uint64(RegolithSystemTxGas))
 		require.Equal(t, L1InfoEcotoneLen, len(depTx.Data))
+		require.Equal(t, L1InfoBedrockLen, len(depTx.Data))
+	})
+}
+
+func TestDepositsCompleteBytes(t *testing.T) {
+	randomSeqNr := func(rng *rand.Rand) uint64 {
+		return rng.Uint64()
+	}
+	t.Run("valid return bytes", func(t *testing.T) {
+		rng := rand.New(rand.NewSource(1234))
+		info := testutils.MakeBlockInfo(nil)(rng)
+		depTxByes, err := DepositsCompleteBytes(randomSeqNr(rng), info)
+		require.NoError(t, err)
+		require.Equal(t, depTxByes, DepositsCompleteBytes4)
+		require.Equal(t, DepositsCompleteLen, len(depTxByes))
+	})
+	t.Run("valid return Transaction", func(t *testing.T) {
+		rng := rand.New(rand.NewSource(1234))
+		info := testutils.MakeBlockInfo(nil)(rng)
+		depTx, err := DepositsCompleteDeposit(randomSeqNr(rng), info)
+		require.NoError(t, err)
+		require.Equal(t, depTx.Data, DepositsCompleteBytes4)
+		require.Equal(t, DepositsCompleteLen, len(depTx.Data))
+		require.Equal(t, DepositsCompleteGas, depTx.Gas)
+		require.False(t, depTx.IsSystemTransaction)
+		require.Equal(t, depTx.Value, big.NewInt(0))
+		require.Equal(t, depTx.From, L1InfoDepositerAddress)
 	})
 }
diff --git a/op-node/rollup/types.go b/op-node/rollup/types.go
index fec118567c5..476a60fc32d 100644
--- a/op-node/rollup/types.go
+++ b/op-node/rollup/types.go
@@ -457,12 +457,51 @@ func (c *Config) IsHoloceneActivationBlock(l2BlockTime uint64) bool {
 		!c.IsHolocene(l2BlockTime-c.BlockTime)
 }
 
+// TODO rename to IsIsthmusActivationBlock (this will require quite a bit of renaming)
 func (c *Config) IsInteropActivationBlock(l2BlockTime uint64) bool {
 	return c.IsInterop(l2BlockTime) &&
 		l2BlockTime >= c.BlockTime &&
 		!c.IsInterop(l2BlockTime-c.BlockTime)
 }
 
+func (c *Config) AtHardfork(hardfork ForkName) {
+	zero := uint64(0)
+	// IMPORTANT! ordered from newest to oldest
+	switch hardfork {
+	case Interop:
+		c.InteropTime = &zero
+		fallthrough
+	case Holocene:
+		c.HoloceneTime = &zero
+		fallthrough
+	case Granite:
+		c.GraniteTime = &zero
+		fallthrough
+	case Fjord:
+		c.FjordTime = &zero
+		fallthrough
+	case Ecotone:
+		c.EcotoneTime = &zero
+		fallthrough
+	case Delta:
+		c.DeltaTime = &zero
+		fallthrough
+	case Canyon:
+		c.CanyonTime = &zero
+		fallthrough
+	case Regolith:
+		c.RegolithTime = &zero
+		fallthrough
+	case None:
+		break
+	}
+}
+func (c *Config) AfterHardfork(hardfork ForkName, timestamp uint64) {
+	c.AtHardfork(hardfork)
+	// required after hardfork time bump
+	c.BlockTime = timestamp
+}
+
 // ForkchoiceUpdatedVersion returns the EngineAPIMethod suitable for the chain hard fork version.
 func (c *Config) ForkchoiceUpdatedVersion(attr *eth.PayloadAttributes) eth.EngineAPIMethod {
 	if attr == nil {
diff --git a/packages/contracts-bedrock/.gas-snapshot b/packages/contracts-bedrock/.gas-snapshot
index b3ea3b88545..c34ed57a169 100644
--- a/packages/contracts-bedrock/.gas-snapshot
+++ b/packages/contracts-bedrock/.gas-snapshot
@@ -1,3 +1,9 @@
+GasBenchMark_L1BlockIsthmus_DepositsComplete:test_depositsComplete_benchmark() (gas: 7768)
+GasBenchMark_L1BlockIsthmus_DepositsComplete_Warm:test_depositsComplete_benchmark() (gas: 5768)
+GasBenchMark_L1BlockIsthmus_SetValuesIsthmus:test_setL1BlockValuesIsthmus_benchmark() (gas: 174050)
+GasBenchMark_L1BlockIsthmus_SetValuesIsthmus_Warm:test_setL1BlockValuesIsthmus_benchmark() (gas: 5185)
+GasBenchMark_L1Block_SetValuesEcotone:test_setL1BlockValuesEcotone_benchmark() (gas: 159033)
+GasBenchMark_L1Block_SetValuesEcotone_Warm:test_setL1BlockValuesEcotone_benchmark() (gas: 7539)
 GasBenchMark_L1CrossDomainMessenger:test_sendMessage_benchmark_0() (gas: 369356)
 GasBenchMark_L1CrossDomainMessenger:test_sendMessage_benchmark_1() (gas: 2967496)
 GasBenchMark_L1StandardBridge_Deposit:test_depositERC20_benchmark_0() (gas: 564483)
diff --git a/packages/contracts-bedrock/scripts/L2Genesis.s.sol b/packages/contracts-bedrock/scripts/L2Genesis.s.sol
index 7d456865393..073b4e9af28 100644
--- a/packages/contracts-bedrock/scripts/L2Genesis.s.sol
+++ b/packages/contracts-bedrock/scripts/L2Genesis.s.sol
@@ -357,7 +357,7 @@ contract L2Genesis is Deployer {
     /// @notice This predeploy is following the safety invariant #1.
     function setL1Block() public {
         if (cfg.useInterop()) {
-            string memory cname = "L1BlockInterop";
+            string memory cname = "L1BlockIsthmus";
             address impl = Predeploys.predeployToCodeNamespace(Predeploys.L1_BLOCK_ATTRIBUTES);
             console.log("Setting %s implementation at: %s", cname, impl);
             vm.etch(impl, vm.getDeployedCode(string.concat(cname, ".sol:", cname)));
diff --git a/packages/contracts-bedrock/scripts/periphery/drippie/DrippieConfig.s.sol b/packages/contracts-bedrock/scripts/periphery/drippie/DrippieConfig.s.sol
index 3ab115ba326..1b03d53a929 100644
--- a/packages/contracts-bedrock/scripts/periphery/drippie/DrippieConfig.s.sol
+++ b/packages/contracts-bedrock/scripts/periphery/drippie/DrippieConfig.s.sol
@@ -126,7 +126,8 @@ contract DrippieConfig is Script {
                 abi.decode(checkparams, (CheckSecrets.Params));
             } else if (strcmp(dripcheck, "CheckTrue")) {
                 // No parameters to decode.
-            } else {
+            }
+            else {
                 console.log("ERROR: unknown drip configuration %s", dripcheck);
                 revert UnknownDripCheck(dripcheck);
             }
diff --git a/packages/contracts-bedrock/semver-lock.json b/packages/contracts-bedrock/semver-lock.json
index 59def1ae86c..bc5df36ba3f 100644
--- a/packages/contracts-bedrock/semver-lock.json
+++ b/packages/contracts-bedrock/semver-lock.json
@@ -83,7 +83,7 @@
     "initCodeHash": "0xfd099da051edf13b147f4382ab4bed9db546d0c48157736ba298fb7e178b20d9",
     "sourceCodeHash": "0x24db623574743432626ed0d7dd938bbd2149b570a00328c772debd7eb179ff1d"
   },
-  "src/L2/L1BlockInterop.sol": {
+  "src/L2/L1BlockIsthmus.sol": {
     "initCodeHash": "0x6833a323934b3be1e5a5c7491c652b6e90bc5102416ddbb255b5f65aa6d5d4a1",
     "sourceCodeHash": "0xd8ec2f814690d1ffd55e5b8496bca5a179d6d1772d61f71cdf8296c9058dc2c6"
   },
diff --git a/packages/contracts-bedrock/src/L1/OptimismPortalInterop.sol b/packages/contracts-bedrock/src/L1/OptimismPortalInterop.sol
index 9fad94a0916..2b4bb3cd3e6 100644
--- a/packages/contracts-bedrock/src/L1/OptimismPortalInterop.sol
+++ b/packages/contracts-bedrock/src/L1/OptimismPortalInterop.sol
@@ -2,7 +2,7 @@
 pragma solidity 0.8.15;
 
 import { OptimismPortal2 } from "src/L1/OptimismPortal2.sol";
-import { L1BlockInterop, ConfigType } from "src/L2/L1BlockInterop.sol";
+import { L1BlockIsthmus, ConfigType } from "src/L2/L1BlockIsthmus.sol";
 import { Predeploys } from "src/libraries/Predeploys.sol";
 import { Constants } from "src/libraries/Constants.sol";
 
@@ -47,7 +47,7 @@ contract OptimismPortalInterop is OptimismPortal2 {
                 uint256(0), // value
                 uint64(SYSTEM_DEPOSIT_GAS_LIMIT), // gasLimit
                 false, // isCreation,
-                abi.encodeCall(L1BlockInterop.setConfig, (_type, _value))
+                abi.encodeCall(L1BlockIsthmus.setConfig, (_type, _value))
             )
         );
     }
diff --git a/packages/contracts-bedrock/src/L1/SystemConfigInterop.sol b/packages/contracts-bedrock/src/L1/SystemConfigInterop.sol
index 098b59e1f9f..eb5ec7bbf12 100644
--- a/packages/contracts-bedrock/src/L1/SystemConfigInterop.sol
+++ b/packages/contracts-bedrock/src/L1/SystemConfigInterop.sol
@@ -6,7 +6,7 @@ import { OptimismPortalInterop as OptimismPortal } from "src/L1/OptimismPortalIn
 import { GasPayingToken } from "src/libraries/GasPayingToken.sol";
 import { ERC20 } from "@openzeppelin/contracts/token/ERC20/ERC20.sol";
 import { SystemConfig } from "src/L1/SystemConfig.sol";
-import { ConfigType } from "src/L2/L1BlockInterop.sol";
+import { ConfigType } from "src/L2/L1BlockIsthmus.sol";
 import { StaticConfig } from "src/libraries/StaticConfig.sol";
 import { ResourceMetering } from "src/L1/ResourceMetering.sol";
 import { Storage } from "src/libraries/Storage.sol";
diff --git a/packages/contracts-bedrock/src/L2/CrossL2Inbox.sol b/packages/contracts-bedrock/src/L2/CrossL2Inbox.sol
index cb0ccfa7714..d55f2ca80df 100644
--- a/packages/contracts-bedrock/src/L2/CrossL2Inbox.sol
+++ b/packages/contracts-bedrock/src/L2/CrossL2Inbox.sol
@@ -17,6 +17,13 @@ interface IDependencySet {
     function isInDependencySet(uint256 _chainId) external view returns (bool);
 }
 
+/// @title IL1Block
+/// @notice Interface for L1Block with only `isDeposit()` method.
+interface IL1Block {
+    /// @notice Returns whether the call was triggered from a a deposit or not.
+    /// @return True if the current call was triggered by a deposit transaction, and false otherwise.
+    function isDeposit() external view returns (bool);
+}
 /// @notice Thrown when the caller is not DEPOSITOR_ACCOUNT when calling `setInteropStart()`
 error NotDepositor();
 
@@ -35,6 +42,9 @@ error InvalidChainId();
 /// @notice Thrown when trying to execute a cross chain message and the target call fails.
 error TargetCallFailed();
 
+/// @notice Thrown when trying to execute a cross chain message on a deposit transaction.
+error NoExecutingDeposits();
+
 /// @custom:proxied
 /// @custom:predeploy 0x4200000000000000000000000000000000000022
 /// @title CrossL2Inbox
@@ -71,7 +81,7 @@ contract CrossL2Inbox is ICrossL2Inbox, ISemver, TransientReentrancyAware {
 
     /// @notice Semantic version.
     /// @custom:semver 1.0.0-beta.5
-    string public constant version = "1.0.0-beta.5";
+    string public constant version = "1.1.0-beta.5";
 
     /// @notice Emitted when a cross chain message is being executed.
     /// @param msgHash Hash of message payload being executed.
@@ -144,6 +154,9 @@ contract CrossL2Inbox is ICrossL2Inbox, ISemver, TransientReentrancyAware {
         payable
         reentrantAware
     {
+        // We need to know if this is being called on a depositTx
+        if (IL1Block(Predeploys.L1_BLOCK_ATTRIBUTES).isDeposit()) revert NoExecutingDeposits();
+
         // Check the Identifier.
         _checkIdentifier(_id);
 
diff --git a/packages/contracts-bedrock/src/L2/L1Block.sol b/packages/contracts-bedrock/src/L2/L1Block.sol
index 8cfbb391f0f..2476e686277 100644
--- a/packages/contracts-bedrock/src/L2/L1Block.sol
+++ b/packages/contracts-bedrock/src/L2/L1Block.sol
@@ -57,9 +57,9 @@ contract L1Block is ISemver, IGasToken {
     /// @notice The latest L1 blob base fee.
     uint256 public blobBaseFee;
 
-    /// @custom:semver 1.4.1-beta.1
+    /// @custom:semver 1.5.1-beta.1
     function version() public pure virtual returns (string memory) {
-        return "1.4.1-beta.1";
+        return "1.5.1-beta.1";
     }
 
     /// @notice Returns the gas paying token, its decimals, name and symbol.
@@ -133,7 +133,34 @@ contract L1Block is ISemver, IGasToken {
     ///   7. _blobBaseFee        L1 blob base fee.
     ///   8. _hash               L1 blockhash.
     ///   9. _batcherHash        Versioned hash to authenticate batcher by.
-    function setL1BlockValuesEcotone() external {
+    function setL1BlockValuesEcotone() public {
+        _setL1BlockValuesEcotone();
+    }
+
+    /// @notice Sets the gas paying token for the L2 system. Can only be called by the special
+    ///         depositor account. This function is not called on every L2 block but instead
+    ///         only called by specially crafted L1 deposit transactions.
+    function setGasPayingToken(address _token, uint8 _decimals, bytes32 _name, bytes32 _symbol) external {
+        if (msg.sender != DEPOSITOR_ACCOUNT()) revert NotDepositor();
+
+        GasPayingToken.set({ _token: _token, _decimals: _decimals, _name: _name, _symbol: _symbol });
+
+        emit GasPayingTokenSet({ token: _token, decimals: _decimals, name: _name, symbol: _symbol });
+    }
+
+    /// @notice Updates the L1 block values for an Ecotone upgraded chain.
+    /// Params are packed and passed in as raw msg.data instead of ABI to reduce calldata size.
+    /// Params are expected to be in the following order:
+    ///   1. _baseFeeScalar      L1 base fee scalar
+    ///   2. _blobBaseFeeScalar  L1 blob base fee scalar
+    ///   3. _sequenceNumber     Number of L2 blocks since epoch start.
+    ///   4. _timestamp          L1 timestamp.
+    ///   5. _number             L1 blocknumber.
+    ///   6. _basefee            L1 base fee.
+    ///   7. _blobBaseFee        L1 blob base fee.
+    ///   8. _hash               L1 blockhash.
+    ///   9. _batcherHash        Versioned hash to authenticate batcher by.
+    function _setL1BlockValuesEcotone() internal {
         address depositor = DEPOSITOR_ACCOUNT();
         assembly {
             // Revert if the caller is not the depositor account.
@@ -151,15 +178,4 @@ contract L1Block is ISemver, IGasToken {
             sstore(batcherHash.slot, calldataload(132)) // bytes32
         }
     }
-
-    /// @notice Sets the gas paying token for the L2 system. Can only be called by the special
-    ///         depositor account. This function is not called on every L2 block but instead
-    ///         only called by specially crafted L1 deposit transactions.
-    function setGasPayingToken(address _token, uint8 _decimals, bytes32 _name, bytes32 _symbol) external {
-        if (msg.sender != DEPOSITOR_ACCOUNT()) revert NotDepositor();
-
-        GasPayingToken.set({ _token: _token, _decimals: _decimals, _name: _name, _symbol: _symbol });
-
-        emit GasPayingTokenSet({ token: _token, decimals: _decimals, name: _name, symbol: _symbol });
-    }
 }
diff --git a/packages/contracts-bedrock/src/L2/L1BlockInterop.sol b/packages/contracts-bedrock/src/L2/L1BlockIsthmus.sol
similarity index 69%
rename from packages/contracts-bedrock/src/L2/L1BlockInterop.sol
rename to packages/contracts-bedrock/src/L2/L1BlockIsthmus.sol
index 8dbf986fb7e..b64c44646ab 100644
--- a/packages/contracts-bedrock/src/L2/L1BlockInterop.sol
+++ b/packages/contracts-bedrock/src/L2/L1BlockIsthmus.sol
@@ -5,9 +5,10 @@ import { L1Block } from "src/L2/L1Block.sol";
 import { EnumerableSet } from "@openzeppelin/contracts/utils/structs/EnumerableSet.sol";
 import { GasPayingToken } from "src/libraries/GasPayingToken.sol";
 import { StaticConfig } from "src/libraries/StaticConfig.sol";
+import { Predeploys } from "src/libraries/Predeploys.sol";
 import "src/libraries/L1BlockErrors.sol";
 
-/// @notice Enum representing different types of configurations that can be set on L1BlockInterop.
+/// @notice Enum representing different types of configurations that can be set on L1BlockIsthmus.
 /// @custom:value SET_GAS_PAYING_TOKEN  Represents the config type for setting the gas paying token.
 /// @custom:value ADD_DEPENDENCY        Represents the config type for adding a chain to the interop dependency set.
 /// @custom:value REMOVE_DEPENDENCY     Represents the config type for removing a chain from the interop dependency set.
@@ -19,9 +20,9 @@ enum ConfigType {
 
 /// @custom:proxied
 /// @custom:predeploy 0x4200000000000000000000000000000000000015
-/// @title L1BlockInterop
-/// @notice Interop extenstions of L1Block.
-contract L1BlockInterop is L1Block {
+/// @title L1BlockIsthmus
+/// @notice Isthmus extenstions of L1Block.
+contract L1BlockIsthmus is L1Block {
     using EnumerableSet for EnumerableSet.UintSet;
 
     /// @notice Event emitted when a new dependency is added to the interop dependency set.
@@ -33,9 +34,23 @@ contract L1BlockInterop is L1Block {
     /// @notice The interop dependency set, containing the chain IDs in it.
     EnumerableSet.UintSet dependencySet;
 
-    /// @custom:semver +interop
+    /// @notice Storage slot that the isDeposit is stored at.
+    ///         This is a custom slot that is not part of the standard storage layout.
+    /// keccak256(abi.encode(uint256(keccak256("l1Block.identifier.isDeposit")) - 1)) & ~bytes32(uint256(0xff))
+    uint256 internal constant IS_DEPOSIT_SLOT = 0x921bd3a089295c6e5540e8fba8195448d253efd6f2e3e495b499b627dc36a300;
+
+    /// @custom:semver +isthmus
     function version() public pure override returns (string memory) {
-        return string.concat(super.version(), "+interop");
+        return string.concat(super.version(), "+isthmus");
+    }
+
+    /// @notice Returns whether the call was triggered from a a deposit or not.
+    /// @notice This function is only callable by the CrossL2Inbox contract.
+    function isDeposit() external view returns (bool isDeposit_) {
+        if (msg.sender != Predeploys.CROSS_L2_INBOX) revert NotCrossL2Inbox();
+        assembly {
+            isDeposit_ := sload(IS_DEPOSIT_SLOT)
+        }
     }
 
     /// @notice Returns true if a chain ID is in the interop dependency set and false otherwise.
@@ -52,6 +67,29 @@ contract L1BlockInterop is L1Block {
         return uint8(dependencySet.length());
     }
 
+    /// @notice Updates the `isDeposit` flag and sets the L1 block values for an Isthmus upgraded chain.
+    ///         It updates the L1 block values through the `setL1BlockValuesEcotone` function.
+    ///         It forwards the calldata to the internally-used `setL1BlockValuesEcotone` function.
+    function setL1BlockValuesIsthmus() external {
+        // Set the isDeposit flag to true.
+        assembly {
+            sstore(IS_DEPOSIT_SLOT, 1)
+        }
+
+        _setL1BlockValuesEcotone();
+    }
+
+    /// @notice Resets the isDeposit flag.
+    ///         Should only be called by the depositor account after the deposits are complete.
+    function depositsComplete() external {
+        if (msg.sender != DEPOSITOR_ACCOUNT()) revert NotDepositor();
+
+        // Set the isDeposit flag to false.
+        assembly {
+            sstore(IS_DEPOSIT_SLOT, 0)
+        }
+    }
+
     /// @notice Sets static configuration options for the L2 system. Can only be called by the special
     ///         depositor account.
     /// @param _type  The type of configuration to set.
diff --git a/packages/contracts-bedrock/src/libraries/L1BlockErrors.sol b/packages/contracts-bedrock/src/libraries/L1BlockErrors.sol
index c9ef3903aeb..44e156e158c 100644
--- a/packages/contracts-bedrock/src/libraries/L1BlockErrors.sol
+++ b/packages/contracts-bedrock/src/libraries/L1BlockErrors.sol
@@ -4,6 +4,9 @@ pragma solidity ^0.8.0;
 /// @notice Error returns when a non-depositor account tries to set L1 block values.
 error NotDepositor();
 
+/// @notice Error when a non-cross L2 Inbox sender tries to call the `isDeposit()` method.
+error NotCrossL2Inbox();
+
 /// @notice Error when a chain ID is not in the interop dependency set.
 error NotDependency();
 
diff --git a/packages/contracts-bedrock/test/BenchmarkTest.t.sol b/packages/contracts-bedrock/test/BenchmarkTest.t.sol
index 4722107a314..ca1f1c623ea 100644
--- a/packages/contracts-bedrock/test/BenchmarkTest.t.sol
+++ b/packages/contracts-bedrock/test/BenchmarkTest.t.sol
@@ -9,6 +9,7 @@ import { Bridge_Initializer } from "test/setup/Bridge_Initializer.sol";
 import { CrossDomainMessenger } from "src/universal/CrossDomainMessenger.sol";
 import { ResourceMetering } from "src/L1/ResourceMetering.sol";
 import { Types } from "src/libraries/Types.sol";
+import { L1BlockIsthmus } from "src/L2/L1BlockIsthmus.sol";
 
 // Free function for setting the prevBaseFee param in the OptimismPortal.
 function setPrevBaseFee(Vm _vm, address _op, uint128 _prevBaseFee) {
@@ -209,3 +210,105 @@ contract GasBenchMark_L2OutputOracle is CommonTest {
         l2OutputOracle.proposeL2Output(nonZeroHash, nextBlockNumber, 0, 0);
     }
 }
+
+contract GasBenchMark_L1Block is CommonTest {
+    address depositor;
+    bytes setValuesEcotoneCalldata;
+
+    function setUp() public virtual override {
+        super.setUp();
+        depositor = l1Block.DEPOSITOR_ACCOUNT();
+        setValuesEcotoneCalldata = abi.encodePacked(
+            bytes4(keccak256("setL1BlockValuesEcotone()")),
+            type(uint32).max,
+            type(uint32).max,
+            type(uint64).max,
+            type(uint64).max,
+            type(uint64).max,
+            type(uint256).max,
+            type(uint256).max,
+            keccak256(abi.encode(1)),
+            bytes32(type(uint256).max)
+        );
+        vm.startPrank(depositor);
+    }
+}
+
+contract GasBenchMark_L1Block_SetValuesEcotone is GasBenchMark_L1Block {
+    function test_setL1BlockValuesEcotone_benchmark() external {
+        address(l1Block).call(abi.encodePacked(setValuesEcotoneCalldata));
+    }
+}
+
+contract GasBenchMark_L1Block_SetValuesEcotone_Warm is GasBenchMark_L1Block {
+    function setUp() public virtual override {
+        address(l1Block).call(abi.encodePacked(setValuesEcotoneCalldata));
+    }
+
+    function test_setL1BlockValuesEcotone_benchmark() external {
+        address(l1Block).call(abi.encodePacked(setValuesEcotoneCalldata));
+    }
+}
+
+contract GasBenchMark_L1BlockIsthmus is GasBenchMark_L1Block {
+    L1BlockIsthmus l1BlockIsthmus;
+
+    function setUp() public virtual override {
+        super.setUp();
+        l1BlockIsthmus = new L1BlockIsthmus();
+        setValuesEcotoneCalldata = abi.encodePacked(
+            type(uint32).max,
+            type(uint32).max,
+            type(uint64).max,
+            type(uint64).max,
+            type(uint64).max,
+            type(uint256).max,
+            type(uint256).max,
+            keccak256(abi.encode(1)),
+            bytes32(type(uint256).max)
+        );
+    }
+}
+
+contract GasBenchMark_L1BlockIsthmus_SetValuesIsthmus is GasBenchMark_L1BlockIsthmus {
+    function test_setL1BlockValuesIsthmus_benchmark() external {
+        address(l1BlockIsthmus).call(
+            abi.encodePacked(l1BlockIsthmus.setL1BlockValuesIsthmus.selector, setValuesEcotoneCalldata)
+        );
+    }
+}
+
+contract GasBenchMark_L1BlockIsthmus_SetValuesIsthmus_Warm is GasBenchMark_L1BlockIsthmus {
+    function setUp() public virtual override {
+        address(l1BlockIsthmus).call(
+            abi.encodePacked(l1BlockIsthmus.setL1BlockValuesIsthmus.selector, setValuesEcotoneCalldata)
+        );
+    }
+
+    function test_setL1BlockValuesIsthmus_benchmark() external {
+        address(l1BlockIsthmus).call(
+            abi.encodePacked(l1BlockIsthmus.setL1BlockValuesIsthmus.selector, setValuesEcotoneCalldata)
+        );
+    }
+}
+
+contract GasBenchMark_L1BlockIsthmus_DepositsComplete is GasBenchMark_L1BlockIsthmus {
+    function test_depositsComplete_benchmark() external {
+        address(l1BlockIsthmus).call(abi.encodeWithSelector(l1BlockIsthmus.depositsComplete.selector));
+    }
+}
+
+contract GasBenchMark_L1BlockIsthmus_DepositsComplete_Warm is GasBenchMark_L1BlockIsthmus {
+    function setUp() public virtual override {
+        super.setUp();
+        // Set the isDeposit flag to true so then we can benchmark when it is reset.
+        address(l1BlockIsthmus).call(
+            abi.encodePacked(l1BlockIsthmus.setL1BlockValuesIsthmus.selector, setValuesEcotoneCalldata)
+        );
+    }
+
+    function test_depositsComplete_benchmark() external {
+        address(l1BlockIsthmus).call(abi.encodeWithSelector(l1BlockIsthmus.depositsComplete.selector));
+    }
+
+}
diff --git a/packages/contracts-bedrock/test/L1/OptimismPortalInterop.t.sol b/packages/contracts-bedrock/test/L1/OptimismPortalInterop.t.sol
index 1d9c2b01a24..6fb9a1aab6e 100644
--- a/packages/contracts-bedrock/test/L1/OptimismPortalInterop.t.sol
+++ b/packages/contracts-bedrock/test/L1/OptimismPortalInterop.t.sol
@@ -11,7 +11,7 @@ import { Predeploys } from "src/libraries/Predeploys.sol";
 // Target contract dependencies
 import "src/libraries/PortalErrors.sol";
 import { OptimismPortalInterop } from "src/L1/OptimismPortalInterop.sol";
-import { L1BlockInterop, ConfigType } from "src/L2/L1BlockInterop.sol";
+import { L1BlockIsthmus, ConfigType } from "src/L2/L1BlockIsthmus.sol";
 
 contract OptimismPortalInterop_Test is CommonTest {
     /// @notice Marked virtual to be overridden in
@@ -31,7 +31,7 @@ contract OptimismPortalInterop_Test is CommonTest {
             _mint: 0,
             _gasLimit: 200_000,
             _isCreation: false,
-            _data: abi.encodeCall(L1BlockInterop.setConfig, (ConfigType.SET_GAS_PAYING_TOKEN, _value))
+            _data: abi.encodeCall(L1BlockIsthmus.setConfig, (ConfigType.SET_GAS_PAYING_TOKEN, _value))
         });
 
         vm.prank(address(_optimismPortalInterop().systemConfig()));
@@ -54,7 +54,7 @@ contract OptimismPortalInterop_Test is CommonTest {
             _mint: 0,
             _gasLimit: 200_000,
             _isCreation: false,
-            _data: abi.encodeCall(L1BlockInterop.setConfig, (ConfigType.ADD_DEPENDENCY, _value))
+            _data: abi.encodeCall(L1BlockIsthmus.setConfig, (ConfigType.ADD_DEPENDENCY, _value))
         });
 
         vm.prank(address(_optimismPortalInterop().systemConfig()));
@@ -77,7 +77,7 @@ contract OptimismPortalInterop_Test is CommonTest {
             _mint: 0,
             _gasLimit: 200_000,
             _isCreation: false,
-            _data: abi.encodeCall(L1BlockInterop.setConfig, (ConfigType.REMOVE_DEPENDENCY, _value))
+            _data: abi.encodeCall(L1BlockIsthmus.setConfig, (ConfigType.REMOVE_DEPENDENCY, _value))
         });
 
         vm.prank(address(_optimismPortalInterop().systemConfig()));
diff --git a/packages/contracts-bedrock/test/L1/SystemConfigInterop.t.sol b/packages/contracts-bedrock/test/L1/SystemConfigInterop.t.sol
index 36887fa3368..c861331b138 100644
--- a/packages/contracts-bedrock/test/L1/SystemConfigInterop.t.sol
+++ b/packages/contracts-bedrock/test/L1/SystemConfigInterop.t.sol
@@ -14,7 +14,7 @@ import { SystemConfig } from "src/L1/SystemConfig.sol";
 import { SystemConfigInterop } from "src/L1/SystemConfigInterop.sol";
 import { OptimismPortalInterop } from "src/L1/OptimismPortalInterop.sol";
 import { ERC20 } from "@openzeppelin/contracts/token/ERC20/ERC20.sol";
-import { ConfigType } from "src/L2/L1BlockInterop.sol";
+import { ConfigType } from "src/L2/L1BlockIsthmus.sol";
 
 contract SystemConfigInterop_Test is CommonTest {
     /// @notice Marked virtual to be overridden in
diff --git a/packages/contracts-bedrock/test/L2/CrossL2Inbox.t.sol b/packages/contracts-bedrock/test/L2/CrossL2Inbox.t.sol
index 12b24ae1d47..ff4b4d80f03 100644
--- a/packages/contracts-bedrock/test/L2/CrossL2Inbox.t.sol
+++ b/packages/contracts-bedrock/test/L2/CrossL2Inbox.t.sol
@@ -11,7 +11,9 @@ import { TransientContext } from "src/libraries/TransientContext.sol";
 // Target contracts
 import {
     CrossL2Inbox,
+    IL1Block,
     NotEntered,
+    NoExecutingDeposits,
     InvalidTimestamp,
     InvalidChainId,
     TargetCallFailed,
@@ -155,6 +157,13 @@ contract CrossL2InboxTest is Test {
         // Ensure that the target call is payable if value is sent
         if (_value > 0) assumePayable(_target);
 
+        // Ensure is not a deposit transaction
+        vm.mockCall({
+            callee: Predeploys.L1_BLOCK_ATTRIBUTES,
+            data: abi.encodeWithSelector(IL1Block.isDeposit.selector),
+            returnData: abi.encode(false)
+        });
+
         // Ensure that the target call does not revert
         vm.mockCall({ callee: _target, msgValue: _value, data: _message, returnData: abi.encode(true) });
 
@@ -210,6 +219,13 @@ contract CrossL2InboxTest is Test {
         _id1.timestamp = bound(_id1.timestamp, interopStartTime + 1, block.timestamp);
         _id2.timestamp = bound(_id2.timestamp, interopStartTime + 1, block.timestamp);
 
+        // Ensure is not a deposit transaction
+        vm.mockCall({
+            callee: Predeploys.L1_BLOCK_ATTRIBUTES,
+            data: abi.encodeWithSelector(IL1Block.isDeposit.selector),
+            returnData: abi.encode(false)
+        });
+
         // Ensure that id1's chain ID is in the dependency set
         vm.mockCall({
             callee: Predeploys.L1_BLOCK_ATTRIBUTES,
@@ -254,6 +270,32 @@ contract CrossL2InboxTest is Test {
         assertEq(crossL2Inbox.chainId(), _id2.chainId);
     }
 
+    /// @dev Tests that the `executeMessage` function reverts if the transaction comes from a deposit.
+    function testFuzz_executeMessage_isDeposit_reverts(
+        ICrossL2Inbox.Identifier calldata _id,
+        address _target,
+        bytes calldata _message,
+        uint256 _value
+    )
+        external
+    {
+        // Ensure it is a deposit transaction
+        vm.mockCall({
+            callee: Predeploys.L1_BLOCK_ATTRIBUTES,
+            data: abi.encodeWithSelector(IL1Block.isDeposit.selector),
+            returnData: abi.encode(true)
+        });
+
+        // Ensure that the contract has enough balance to send with value
+        vm.deal(address(this), _value);
+
+        // Expect a revert with the NoExecutingDeposits selector
+        vm.expectRevert(NoExecutingDeposits.selector);
+
+        // Call the executeMessage function
+        crossL2Inbox.executeMessage{ value: _value }({ _id: _id, _target: _target, _message: _message });
+    }
+
     /// @dev Tests that the `executeMessage` function reverts when called with an identifier with an invalid timestamp.
     function testFuzz_executeMessage_invalidTimestamp_reverts(
         ICrossL2Inbox.Identifier calldata _id,
@@ -267,6 +309,13 @@ contract CrossL2InboxTest is Test {
         // Ensure that the id's timestamp is invalid (greater than the current block timestamp)
         vm.assume(_id.timestamp > block.timestamp);
 
+        // Ensure is not a deposit transaction
+        vm.mockCall({
+            callee: Predeploys.L1_BLOCK_ATTRIBUTES,
+            data: abi.encodeWithSelector(IL1Block.isDeposit.selector),
+            returnData: abi.encode(false)
+        });
+
         // Ensure that the contract has enough balance to send with value
         vm.deal(address(this), _value);
 
@@ -316,6 +365,13 @@ contract CrossL2InboxTest is Test {
         // interop start time)
         _id.timestamp = bound(_id.timestamp, interopStartTime + 1, block.timestamp);
 
+        // Ensure is not a deposit transaction
+        vm.mockCall({
+            callee: Predeploys.L1_BLOCK_ATTRIBUTES,
+            data: abi.encodeWithSelector(IL1Block.isDeposit.selector),
+            returnData: abi.encode(false)
+        });
+
         // Ensure that the chain ID is NOT in the dependency set
         vm.mockCall({
             callee: Predeploys.L1_BLOCK_ATTRIBUTES,
@@ -353,6 +409,13 @@ contract CrossL2InboxTest is Test {
         // Ensure that the target call reverts
         vm.mockCallRevert({ callee: _target, msgValue: _value, data: _message, revertData: abi.encode(false) });
 
+        // Ensure is not a deposit transaction
+        vm.mockCall({
+            callee: Predeploys.L1_BLOCK_ATTRIBUTES,
+            data: abi.encodeWithSelector(IL1Block.isDeposit.selector),
+            returnData: abi.encode(false)
+        });
+
         // Ensure that the chain ID is in the dependency set
         vm.mockCall({
             callee: Predeploys.L1_BLOCK_ATTRIBUTES,
diff --git a/packages/contracts-bedrock/test/L2/L1Block.t.sol b/packages/contracts-bedrock/test/L2/L1Block.t.sol
index ce67a669222..1c004a6d429 100644
--- a/packages/contracts-bedrock/test/L2/L1Block.t.sol
+++ b/packages/contracts-bedrock/test/L2/L1Block.t.sol
@@ -8,7 +8,7 @@ import { CommonTest } from "test/setup/CommonTest.sol";
 import { Encoding } from "src/libraries/Encoding.sol";
 import { Constants } from "src/libraries/Constants.sol";
 
-// Target contract
+// Target contract dependencies
 import { L1Block } from "src/L2/L1Block.sol";
 import "src/libraries/L1BlockErrors.sol";
 
diff --git a/packages/contracts-bedrock/test/L2/L1BlockInterop.t.sol b/packages/contracts-bedrock/test/L2/L1BlockInterop.t.sol
index 159021e77dc..1c2407dd73a 100644
--- a/packages/contracts-bedrock/test/L2/L1BlockInterop.t.sol
+++ b/packages/contracts-bedrock/test/L2/L1BlockInterop.t.sol
@@ -8,16 +8,17 @@ import { CommonTest } from "test/setup/CommonTest.sol";
 import { StaticConfig } from "src/libraries/StaticConfig.sol";
 
 // Target contract dependencies
-import { L1BlockInterop, ConfigType } from "src/L2/L1BlockInterop.sol";
+import { L1BlockIsthmus, ConfigType } from "src/L2/L1BlockIsthmus.sol";
+import { Predeploys } from "src/libraries/Predeploys.sol";
 import "src/libraries/L1BlockErrors.sol";
 
-contract L1BlockInteropTest is CommonTest {
+contract L1BlockIsthmusTest is CommonTest {
     event GasPayingTokenSet(address indexed token, uint8 indexed decimals, bytes32 name, bytes32 symbol);
     event DependencyAdded(uint256 indexed chainId);
     event DependencyRemoved(uint256 indexed chainId);
 
     modifier prankDepositor() {
-        vm.startPrank(l1Block.DEPOSITOR_ACCOUNT());
+        vm.startPrank(_l1BlockIsthmus().DEPOSITOR_ACCOUNT());
         _;
         vm.stopPrank();
     }
@@ -33,14 +34,14 @@ contract L1BlockInteropTest is CommonTest {
     function testFuzz_isInDependencySet_succeeds(uint256 _chainId) public prankDepositor {
         vm.assume(_chainId != block.chainid);
 
-        _l1BlockInterop().setConfig(ConfigType.ADD_DEPENDENCY, StaticConfig.encodeAddDependency(_chainId));
+        _l1BlockIsthmus().setConfig(ConfigType.ADD_DEPENDENCY, StaticConfig.encodeAddDependency(_chainId));
 
-        assertTrue(_l1BlockInterop().isInDependencySet(_chainId));
+        assertTrue(_l1BlockIsthmus().isInDependencySet(_chainId));
     }
 
     /// @dev Tests that `isInDependencySet` returns true when the chain's chain ID is passed as the input.
     function test_isInDependencySet_chainChainId_succeeds() public view {
-        assertTrue(_l1BlockInterop().isInDependencySet(block.chainid));
+        assertTrue(_l1BlockIsthmus().isInDependencySet(block.chainid));
     }
 
     /// @dev Tests that `isInDependencySet` reverts when the input chain ID is not in the dependency set
@@ -49,16 +50,16 @@ contract L1BlockInteropTest is CommonTest {
         vm.assume(_chainId != block.chainid);
 
         // Check that the chain ID is not in the dependency set
-        assertFalse(_l1BlockInterop().isInDependencySet(_chainId));
+        assertFalse(_l1BlockIsthmus().isInDependencySet(_chainId));
     }
 
     /// @dev Tests that `isInDependencySet` returns false when the dependency set is empty.
     function testFuzz_isInDependencySet_dependencySetEmpty_succeeds(uint256 _chainId) public view {
         vm.assume(_chainId != block.chainid);
 
-        assertEq(_l1BlockInterop().dependencySetSize(), 0);
+        assertEq(_l1BlockIsthmus().dependencySetSize(), 0);
 
-        assertFalse(_l1BlockInterop().isInDependencySet(_chainId));
+        assertFalse(_l1BlockIsthmus().isInDependencySet(_chainId));
     }
 
     /// @dev Tests that the dependency set size is correct when adding an arbitrary number of chain IDs.
@@ -69,16 +70,16 @@ contract L1BlockInteropTest is CommonTest {
 
         for (uint256 i = 0; i < _dependencySetSize; i++) {
             if (i == block.chainid) continue;
-            _l1BlockInterop().setConfig(ConfigType.ADD_DEPENDENCY, StaticConfig.encodeAddDependency(i));
+            _l1BlockIsthmus().setConfig(ConfigType.ADD_DEPENDENCY, StaticConfig.encodeAddDependency(i));
             uniqueCount++;
         }
 
-        assertEq(_l1BlockInterop().dependencySetSize(), uniqueCount);
+        assertEq(_l1BlockIsthmus().dependencySetSize(), uniqueCount);
     }
 
     /// @dev Tests that the dependency set size is correct when the dependency set is empty.
     function test_dependencySetSize_dependencySetEmpty_succeeds() public view {
-        assertEq(_l1BlockInterop().dependencySetSize(), 0);
+        assertEq(_l1BlockIsthmus().dependencySetSize(), 0);
     }
 
     /// @dev Tests that the config for setting the gas paying token succeeds.
@@ -96,7 +97,7 @@ contract L1BlockInteropTest is CommonTest {
         vm.expectEmit(address(l1Block));
         emit GasPayingTokenSet({ token: _token, decimals: _decimals, name: _name, symbol: _symbol });
 
-        _l1BlockInterop().setConfig(
+        _l1BlockIsthmus().setConfig(
             ConfigType.SET_GAS_PAYING_TOKEN,
             StaticConfig.encodeSetGasPayingToken({ _token: _token, _decimals: _decimals, _name: _name, _symbol: _symbol })
         );
@@ -114,7 +115,7 @@ contract L1BlockInteropTest is CommonTest {
         vm.assume(_token != address(vm));
 
         vm.expectRevert(NotDepositor.selector);
-        _l1BlockInterop().setConfig(
+        _l1BlockIsthmus().setConfig(
             ConfigType.SET_GAS_PAYING_TOKEN,
             StaticConfig.encodeSetGasPayingToken({ _token: _token, _decimals: _decimals, _name: _name, _symbol: _symbol })
         );
@@ -127,41 +128,41 @@ contract L1BlockInteropTest is CommonTest {
         vm.expectEmit(address(l1Block));
         emit DependencyAdded(_chainId);
 
-        _l1BlockInterop().setConfig(ConfigType.ADD_DEPENDENCY, StaticConfig.encodeAddDependency(_chainId));
+        _l1BlockIsthmus().setConfig(ConfigType.ADD_DEPENDENCY, StaticConfig.encodeAddDependency(_chainId));
     }
 
     /// @dev Tests that adding a dependency reverts if it's the chain's chain id
     function test_setConfig_addDependency_chainChainId_reverts() public prankDepositor {
         vm.expectRevert(AlreadyDependency.selector);
-        _l1BlockInterop().setConfig(ConfigType.ADD_DEPENDENCY, StaticConfig.encodeAddDependency(block.chainid));
+        _l1BlockIsthmus().setConfig(ConfigType.ADD_DEPENDENCY, StaticConfig.encodeAddDependency(block.chainid));
     }
 
     /// @dev Tests that adding a dependency already in the set reverts
     function test_setConfig_addDependency_alreadyDependency_reverts(uint256 _chainId) public prankDepositor {
         vm.assume(_chainId != block.chainid);
 
-        _l1BlockInterop().setConfig(ConfigType.ADD_DEPENDENCY, StaticConfig.encodeAddDependency(_chainId));
+        _l1BlockIsthmus().setConfig(ConfigType.ADD_DEPENDENCY, StaticConfig.encodeAddDependency(_chainId));
 
         vm.expectRevert(AlreadyDependency.selector);
-        _l1BlockInterop().setConfig(ConfigType.ADD_DEPENDENCY, StaticConfig.encodeAddDependency(_chainId));
+        _l1BlockIsthmus().setConfig(ConfigType.ADD_DEPENDENCY, StaticConfig.encodeAddDependency(_chainId));
     }
 
     /// @dev Tests that setting the add dependency config as not the depositor reverts.
     function testFuzz_setConfig_addDependency_notDepositor_reverts(uint256 _chainId) public {
         vm.expectRevert(NotDepositor.selector);
-        _l1BlockInterop().setConfig(ConfigType.ADD_DEPENDENCY, StaticConfig.encodeAddDependency(_chainId));
+        _l1BlockIsthmus().setConfig(ConfigType.ADD_DEPENDENCY, StaticConfig.encodeAddDependency(_chainId));
     }
 
     /// @dev Tests that setting the add dependency config when the dependency set size is too large reverts.
     function test_setConfig_addDependency_dependencySetSizeTooLarge_reverts() public prankDepositor {
         for (uint256 i = 0; i < type(uint8).max; i++) {
-            _l1BlockInterop().setConfig(ConfigType.ADD_DEPENDENCY, StaticConfig.encodeAddDependency(i));
+            _l1BlockIsthmus().setConfig(ConfigType.ADD_DEPENDENCY, StaticConfig.encodeAddDependency(i));
         }
 
-        assertEq(_l1BlockInterop().dependencySetSize(), type(uint8).max);
+        assertEq(_l1BlockIsthmus().dependencySetSize(), type(uint8).max);
 
         vm.expectRevert(DependencySetSizeTooLarge.selector);
-        _l1BlockInterop().setConfig(ConfigType.ADD_DEPENDENCY, StaticConfig.encodeAddDependency(1));
+        _l1BlockIsthmus().setConfig(ConfigType.ADD_DEPENDENCY, StaticConfig.encodeAddDependency(1));
     }
 
     /// @dev Tests that the config for removing a dependency can be set.
@@ -169,24 +170,24 @@ contract L1BlockInteropTest is CommonTest {
         vm.assume(_chainId != block.chainid);
 
         // Add the chain ID to the dependency set before removing it
-        _l1BlockInterop().setConfig(ConfigType.ADD_DEPENDENCY, StaticConfig.encodeAddDependency(_chainId));
+        _l1BlockIsthmus().setConfig(ConfigType.ADD_DEPENDENCY, StaticConfig.encodeAddDependency(_chainId));
 
         vm.expectEmit(address(l1Block));
         emit DependencyRemoved(_chainId);
 
-        _l1BlockInterop().setConfig(ConfigType.REMOVE_DEPENDENCY, StaticConfig.encodeRemoveDependency(_chainId));
+        _l1BlockIsthmus().setConfig(ConfigType.REMOVE_DEPENDENCY, StaticConfig.encodeRemoveDependency(_chainId));
     }
 
     /// @dev Tests that setting the remove dependency config as not the depositor reverts.
     function testFuzz_setConfig_removeDependency_notDepositor_reverts(uint256 _chainId) public {
         vm.expectRevert(NotDepositor.selector);
-        _l1BlockInterop().setConfig(ConfigType.REMOVE_DEPENDENCY, StaticConfig.encodeRemoveDependency(_chainId));
+        _l1BlockIsthmus().setConfig(ConfigType.REMOVE_DEPENDENCY, StaticConfig.encodeRemoveDependency(_chainId));
     }
 
     /// @dev Tests that setting the remove dependency config for the chain's chain ID reverts.
     function test_setConfig_removeDependency_chainChainId_reverts() public prankDepositor {
         vm.expectRevert(CantRemovedDependency.selector);
-        _l1BlockInterop().setConfig(ConfigType.REMOVE_DEPENDENCY, StaticConfig.encodeRemoveDependency(block.chainid));
+        _l1BlockIsthmus().setConfig(ConfigType.REMOVE_DEPENDENCY, StaticConfig.encodeRemoveDependency(block.chainid));
     }
 
     /// @dev Tests that setting the remove dependency config for a chain ID that is not in the dependency set reverts.
@@ -194,11 +195,118 @@ contract L1BlockInteropTest is CommonTest {
         vm.assume(_chainId != block.chainid);
 
         vm.expectRevert(NotDependency.selector);
-        _l1BlockInterop().setConfig(ConfigType.REMOVE_DEPENDENCY, StaticConfig.encodeRemoveDependency(_chainId));
+        _l1BlockIsthmus().setConfig(ConfigType.REMOVE_DEPENDENCY, StaticConfig.encodeRemoveDependency(_chainId));
     }
 
-    /// @dev Returns the L1BlockInterop instance.
-    function _l1BlockInterop() internal view returns (L1BlockInterop) {
-        return L1BlockInterop(address(l1Block));
+    /// @dev Returns the L1BlockIsthmus instance.
+    function _l1BlockIsthmus() internal view returns (L1BlockIsthmus) {
+        return L1BlockIsthmus(address(l1Block));
+    }
+}
+
+contract L1BlockIsthmusIsDeposit_Test is L1BlockIsthmusTest {
+    /// @dev Tests that `isDeposit` reverts if the caller is not the cross L2 inbox.
+    function test_isDeposit_notCrossL2Inbox_reverts(address _caller) external {
+        vm.assume(_caller != Predeploys.CROSS_L2_INBOX);
+        vm.expectRevert(NotCrossL2Inbox.selector);
+        _l1BlockIsthmus().isDeposit();
+    }
+
+    /// @dev Tests that `isDeposit` always returns the correct value.
+    function test_isDeposit_succeeds() external {
+        // Assert is false if the value is not updated
+        vm.prank(Predeploys.CROSS_L2_INBOX);
+        assertEq(_l1BlockIsthmus().isDeposit(), false);
+
+        /// @dev Assuming that `setL1BlockValuesIsthmus` will set the proper value. That function is tested as well
+        vm.prank(_l1BlockIsthmus().DEPOSITOR_ACCOUNT());
+        _l1BlockIsthmus().setL1BlockValuesIsthmus();
+
+        // Assert is true if the value is updated
+        vm.prank(Predeploys.CROSS_L2_INBOX);
+        assertEq(_l1BlockIsthmus().isDeposit(), true);
+    }
+}
+
+contract L1BlockIsthmusSetL1BlockValuesIsthmus_Test is L1BlockIsthmusTest {
+    /// @dev Tests that `setL1BlockValuesIsthmus` reverts if sender address is not the depositor
+    function test_setL1BlockValuesIsthmus_notDepositor_reverts(address _caller) external {
+        vm.assume(_caller != _l1BlockIsthmus().DEPOSITOR_ACCOUNT());
+        vm.prank(_caller);
+        vm.expectRevert(NotDepositor.selector);
+        _l1BlockIsthmus().setL1BlockValuesIsthmus();
+    }
+
+    /// @dev Tests that `setL1BlockValuesIsthmus` succeeds if sender address is the depositor
+    function test_setL1BlockValuesIsthmus_succeeds(
+        uint32 baseFeeScalar,
+        uint32 blobBaseFeeScalar,
+        uint64 sequenceNumber,
+        uint64 timestamp,
+        uint64 number,
+        uint256 baseFee,
+        uint256 blobBaseFee,
+        bytes32 hash,
+        bytes32 batcherHash
+    )
+        external
+    {
+        // Ensure the `isDepositTransaction` flag is false before calling `setL1BlockValuesIsthmus`
+        vm.prank(Predeploys.CROSS_L2_INBOX);
+        assertEq(_l1BlockIsthmus().isDeposit(), false);
+
+        bytes memory setValuesEcotoneCalldata = abi.encodePacked(
+            baseFeeScalar, blobBaseFeeScalar, sequenceNumber, timestamp, number, baseFee, blobBaseFee, hash, batcherHash
+        );
+
+        vm.prank(_l1BlockIsthmus().DEPOSITOR_ACCOUNT());
+        (bool success,) = address(l1Block).call(
+            abi.encodePacked(L1BlockIsthmus.setL1BlockValuesIsthmus.selector, setValuesEcotoneCalldata)
+        );
+        assertTrue(success, "function call failed");
+
+        // Assert that the `isDepositTransaction` flag was properly set to true
+        vm.prank(Predeploys.CROSS_L2_INBOX);
+        assertEq(_l1BlockIsthmus().isDeposit(), true);
+
+        // Assert `setL1BlockValuesEcotone` was properly called, forwarding the calldata to it
+        assertEq(_l1BlockIsthmus().baseFeeScalar(), baseFeeScalar, "base fee scalar not properly set");
+        assertEq(_l1BlockIsthmus().blobBaseFeeScalar(), blobBaseFeeScalar, "blob base fee scalar not properly set");
+        assertEq(_l1BlockIsthmus().sequenceNumber(), sequenceNumber, "sequence number not properly set");
+        assertEq(_l1BlockIsthmus().timestamp(), timestamp, "timestamp not properly set");
+        assertEq(_l1BlockIsthmus().number(), number, "number not properly set");
+        assertEq(_l1BlockIsthmus().basefee(), baseFee, "base fee not properly set");
+        assertEq(_l1BlockIsthmus().blobBaseFee(), blobBaseFee, "blob base fee not properly set");
+        assertEq(_l1BlockIsthmus().hash(), hash, "hash not properly set");
+        assertEq(_l1BlockIsthmus().batcherHash(), batcherHash, "batcher hash not properly set");
+    }
+}
+
+contract L1BlockDepositsComplete_Test is L1BlockIsthmusTest {
+    // @dev Tests that `depositsComplete` reverts if the caller is not the depositor.
+    function test_deposits_is_depositor_reverts(address _caller) external {
+        vm.assume(_caller != _l1BlockIsthmus().DEPOSITOR_ACCOUNT());
+        vm.expectRevert(NotDepositor.selector);
+        _l1BlockIsthmus().depositsComplete();
+    }
+
+    // @dev Tests that `depositsComplete` succeeds if the caller is the depositor.
+    function test_depositsComplete_succeeds() external {
+        // Set the `isDeposit` flag to true
+        vm.prank(_l1BlockIsthmus().DEPOSITOR_ACCOUNT());
+        _l1BlockIsthmus().setL1BlockValuesIsthmus();
+
+        // Assert that the `isDeposit` flag was properly set to true
+        vm.prank(Predeploys.CROSS_L2_INBOX);
+        assertTrue(_l1BlockIsthmus().isDeposit());
+
+        // Call `depositsComplete`
+        vm.prank(_l1BlockIsthmus().DEPOSITOR_ACCOUNT());
+        _l1BlockIsthmus().depositsComplete();
+
+        // Assert that the `isDeposit` flag was properly set to false
+        /// @dev Assuming that `isDeposit()` wil return the proper value. That function is tested as well
+        vm.prank(Predeploys.CROSS_L2_INBOX);
+        assertEq(_l1BlockIsthmus().isDeposit(), false);
     }
 }
diff --git a/packages/contracts-bedrock/test/L2/L1BlockIsthmus.t.sol b/packages/contracts-bedrock/test/L2/L1BlockIsthmus.t.sol
new file mode 100644
index 00000000000..1c2407dd73a
--- /dev/null
+++ b/packages/contracts-bedrock/test/L2/L1BlockIsthmus.t.sol
@@ -0,0 +1,312 @@
+// SPDX-License-Identifier: MIT
+pragma solidity 0.8.15;
+
+// Testing utilities
+import { CommonTest } from "test/setup/CommonTest.sol";
+
+// Libraries
+import { StaticConfig } from "src/libraries/StaticConfig.sol";
+
+// Target contract dependencies
+import { L1BlockIsthmus, ConfigType } from "src/L2/L1BlockIsthmus.sol";
+import { Predeploys } from "src/libraries/Predeploys.sol";
+import "src/libraries/L1BlockErrors.sol";
+
+contract L1BlockIsthmusTest is CommonTest {
+    event GasPayingTokenSet(address indexed token, uint8 indexed decimals, bytes32 name, bytes32 symbol);
+    event DependencyAdded(uint256 indexed chainId);
+    event DependencyRemoved(uint256 indexed chainId);
+
+    modifier prankDepositor() {
+        vm.startPrank(_l1BlockIsthmus().DEPOSITOR_ACCOUNT());
+        _;
+        vm.stopPrank();
+    }
+
+    /// @notice Marked virtual to be overridden in
+    ///         test/kontrol/deployment/DeploymentSummary.t.sol
+    function setUp() public virtual override {
+        super.enableInterop();
+        super.setUp();
+    }
+
+    /// @dev Tests that an arbitrary chain ID can be added to the dependency set.
+    function testFuzz_isInDependencySet_succeeds(uint256 _chainId) public prankDepositor {
+        vm.assume(_chainId != block.chainid);
+
+        _l1BlockIsthmus().setConfig(ConfigType.ADD_DEPENDENCY, StaticConfig.encodeAddDependency(_chainId));
+
+        assertTrue(_l1BlockIsthmus().isInDependencySet(_chainId));
+    }
+
+    /// @dev Tests that `isInDependencySet` returns true when the chain's chain ID is passed as the input.
+    function test_isInDependencySet_chainChainId_succeeds() public view {
+        assertTrue(_l1BlockIsthmus().isInDependencySet(block.chainid));
+    }
+
+    /// @dev Tests that `isInDependencySet` reverts when the input chain ID is not in the dependency set
+    ///      and is not the chain's chain ID.
+    function testFuzz_isInDependencySet_notDependency_reverts(uint256 _chainId) public view {
+        vm.assume(_chainId != block.chainid);
+
+        // Check that the chain ID is not in the dependency set
+        assertFalse(_l1BlockIsthmus().isInDependencySet(_chainId));
+    }
+
+    /// @dev Tests that `isInDependencySet` returns false when the dependency set is empty.
+    function testFuzz_isInDependencySet_dependencySetEmpty_succeeds(uint256 _chainId) public view {
+        vm.assume(_chainId != block.chainid);
+
+        assertEq(_l1BlockIsthmus().dependencySetSize(), 0);
+
+        assertFalse(_l1BlockIsthmus().isInDependencySet(_chainId));
+    }
+
+    /// @dev Tests that the dependency set size is correct when adding an arbitrary number of chain IDs.
+    function testFuzz_dependencySetSize_succeeds(uint8 _dependencySetSize) public prankDepositor {
+        vm.assume(_dependencySetSize <= type(uint8).max);
+
+        uint256 uniqueCount = 0;
+
+        for (uint256 i = 0; i < _dependencySetSize; i++) {
+            if (i == block.chainid) continue;
+            _l1BlockIsthmus().setConfig(ConfigType.ADD_DEPENDENCY, StaticConfig.encodeAddDependency(i));
+            uniqueCount++;
+        }
+
+        assertEq(_l1BlockIsthmus().dependencySetSize(), uniqueCount);
+    }
+
+    /// @dev Tests that the dependency set size is correct when the dependency set is empty.
+    function test_dependencySetSize_dependencySetEmpty_succeeds() public view {
+        assertEq(_l1BlockIsthmus().dependencySetSize(), 0);
+    }
+
+    /// @dev Tests that the config for setting the gas paying token succeeds.
+    function testFuzz_setConfig_gasPayingToken_succeeds(
+        address _token,
+        uint8 _decimals,
+        bytes32 _name,
+        bytes32 _symbol
+    )
+        public
+        prankDepositor
+    {
+        vm.assume(_token != address(vm));
+
+        vm.expectEmit(address(l1Block));
+        emit GasPayingTokenSet({ token: _token, decimals: _decimals, name: _name, symbol: _symbol });
+
+        _l1BlockIsthmus().setConfig(
+            ConfigType.SET_GAS_PAYING_TOKEN,
+            StaticConfig.encodeSetGasPayingToken({ _token: _token, _decimals: _decimals, _name: _name, _symbol: _symbol })
+        );
+    }
+
+    /// @dev Tests that setting the gas paying token config as not the depositor reverts.
+    function testFuzz_setConfig_gasPayingToken_notDepositor_reverts(
+        address _token,
+        uint8 _decimals,
+        bytes32 _name,
+        bytes32 _symbol
+    )
+        public
+    {
+        vm.assume(_token != address(vm));
+
+        vm.expectRevert(NotDepositor.selector);
+        _l1BlockIsthmus().setConfig(
+            ConfigType.SET_GAS_PAYING_TOKEN,
+            StaticConfig.encodeSetGasPayingToken({ _token: _token, _decimals: _decimals, _name: _name, _symbol: _symbol })
+        );
+    }
+
+    /// @dev Tests that the config for adding a dependency can be set.
+    function testFuzz_setConfig_addDependency_succeeds(uint256 _chainId) public prankDepositor {
+        vm.assume(_chainId != block.chainid);
+
+        vm.expectEmit(address(l1Block));
+        emit DependencyAdded(_chainId);
+
+        _l1BlockIsthmus().setConfig(ConfigType.ADD_DEPENDENCY, StaticConfig.encodeAddDependency(_chainId));
+    }
+
+    /// @dev Tests that adding a dependency reverts if it's the chain's chain id
+    function test_setConfig_addDependency_chainChainId_reverts() public prankDepositor {
+        vm.expectRevert(AlreadyDependency.selector);
+        _l1BlockIsthmus().setConfig(ConfigType.ADD_DEPENDENCY, StaticConfig.encodeAddDependency(block.chainid));
+    }
+
+    /// @dev Tests that adding a dependency already in the set reverts
+    function test_setConfig_addDependency_alreadyDependency_reverts(uint256 _chainId) public prankDepositor {
+        vm.assume(_chainId != block.chainid);
+
+        _l1BlockIsthmus().setConfig(ConfigType.ADD_DEPENDENCY, StaticConfig.encodeAddDependency(_chainId));
+
+        vm.expectRevert(AlreadyDependency.selector);
+        _l1BlockIsthmus().setConfig(ConfigType.ADD_DEPENDENCY, StaticConfig.encodeAddDependency(_chainId));
+    }
+
+    /// @dev Tests that setting the add dependency config as not the depositor reverts.
+    function testFuzz_setConfig_addDependency_notDepositor_reverts(uint256 _chainId) public {
+        vm.expectRevert(NotDepositor.selector);
+        _l1BlockIsthmus().setConfig(ConfigType.ADD_DEPENDENCY, StaticConfig.encodeAddDependency(_chainId));
+    }
+
+    /// @dev Tests that setting the add dependency config when the dependency set size is too large reverts.
+    function test_setConfig_addDependency_dependencySetSizeTooLarge_reverts() public prankDepositor {
+        for (uint256 i = 0; i < type(uint8).max; i++) {
+            _l1BlockIsthmus().setConfig(ConfigType.ADD_DEPENDENCY, StaticConfig.encodeAddDependency(i));
+        }
+
+        assertEq(_l1BlockIsthmus().dependencySetSize(), type(uint8).max);
+
+        vm.expectRevert(DependencySetSizeTooLarge.selector);
+        _l1BlockIsthmus().setConfig(ConfigType.ADD_DEPENDENCY, StaticConfig.encodeAddDependency(1));
+    }
+
+    /// @dev Tests that the config for removing a dependency can be set.
+    function testFuzz_setConfig_removeDependency_succeeds(uint256 _chainId) public prankDepositor {
+        vm.assume(_chainId != block.chainid);
+
+        // Add the chain ID to the dependency set before removing it
+        _l1BlockIsthmus().setConfig(ConfigType.ADD_DEPENDENCY, StaticConfig.encodeAddDependency(_chainId));
+
+        vm.expectEmit(address(l1Block));
+        emit DependencyRemoved(_chainId);
+
+        _l1BlockIsthmus().setConfig(ConfigType.REMOVE_DEPENDENCY, StaticConfig.encodeRemoveDependency(_chainId));
+    }
+
+    /// @dev Tests that setting the remove dependency config as not the depositor reverts.
+    function testFuzz_setConfig_removeDependency_notDepositor_reverts(uint256 _chainId) public {
+        vm.expectRevert(NotDepositor.selector);
+        _l1BlockIsthmus().setConfig(ConfigType.REMOVE_DEPENDENCY, StaticConfig.encodeRemoveDependency(_chainId));
+    }
+
+    /// @dev Tests that setting the remove dependency config for the chain's chain ID reverts.
+    function test_setConfig_removeDependency_chainChainId_reverts() public prankDepositor {
+        vm.expectRevert(CantRemovedDependency.selector);
+        _l1BlockIsthmus().setConfig(ConfigType.REMOVE_DEPENDENCY, StaticConfig.encodeRemoveDependency(block.chainid));
+    }
+
+    /// @dev Tests that setting the remove dependency config for a chain ID that is not in the dependency set reverts.
+    function testFuzz_setConfig_removeDependency_notDependency_reverts(uint256 _chainId) public prankDepositor {
+        vm.assume(_chainId != block.chainid);
+
+        vm.expectRevert(NotDependency.selector);
+        _l1BlockIsthmus().setConfig(ConfigType.REMOVE_DEPENDENCY, StaticConfig.encodeRemoveDependency(_chainId));
+    }
+
+    /// @dev Returns the L1BlockIsthmus instance.
+    function _l1BlockIsthmus() internal view returns (L1BlockIsthmus) {
+        return L1BlockIsthmus(address(l1Block));
+    }
+}
+
+contract L1BlockIsthmusIsDeposit_Test is L1BlockIsthmusTest {
+    /// @dev Tests that `isDeposit` reverts if the caller is not the cross L2 inbox.
+    function test_isDeposit_notCrossL2Inbox_reverts(address _caller) external {
+        vm.assume(_caller != Predeploys.CROSS_L2_INBOX);
+        vm.expectRevert(NotCrossL2Inbox.selector);
+        _l1BlockIsthmus().isDeposit();
+    }
+
+    /// @dev Tests that `isDeposit` always returns the correct value.
+    function test_isDeposit_succeeds() external {
+        // Assert is false if the value is not updated
+        vm.prank(Predeploys.CROSS_L2_INBOX);
+        assertEq(_l1BlockIsthmus().isDeposit(), false);
+
+        /// @dev Assuming that `setL1BlockValuesIsthmus` will set the proper value. That function is tested as well
+        vm.prank(_l1BlockIsthmus().DEPOSITOR_ACCOUNT());
+        _l1BlockIsthmus().setL1BlockValuesIsthmus();
+
+        // Assert is true if the value is updated
+        vm.prank(Predeploys.CROSS_L2_INBOX);
+        assertEq(_l1BlockIsthmus().isDeposit(), true);
+    }
+}
+
+contract L1BlockIsthmusSetL1BlockValuesIsthmus_Test is L1BlockIsthmusTest {
+    /// @dev Tests that `setL1BlockValuesIsthmus` reverts if sender address is not the depositor
+    function test_setL1BlockValuesIsthmus_notDepositor_reverts(address _caller) external {
+        vm.assume(_caller != _l1BlockIsthmus().DEPOSITOR_ACCOUNT());
+        vm.prank(_caller);
+        vm.expectRevert(NotDepositor.selector);
+        _l1BlockIsthmus().setL1BlockValuesIsthmus();
+    }
+
+    /// @dev Tests that `setL1BlockValuesIsthmus` succeeds if sender address is the depositor
+    function test_setL1BlockValuesIsthmus_succeeds(
+        uint32 baseFeeScalar,
+        uint32 blobBaseFeeScalar,
+        uint64 sequenceNumber,
+        uint64 timestamp,
+        uint64 number,
+        uint256 baseFee,
+        uint256 blobBaseFee,
+        bytes32 hash,
+        bytes32 batcherHash
+    )
+        external
+    {
+        // Ensure the `isDepositTransaction` flag is false before calling `setL1BlockValuesIsthmus`
+        vm.prank(Predeploys.CROSS_L2_INBOX);
+        assertEq(_l1BlockIsthmus().isDeposit(), false);
+
+        bytes memory setValuesEcotoneCalldata = abi.encodePacked(
+            baseFeeScalar, blobBaseFeeScalar, sequenceNumber, timestamp, number, baseFee, blobBaseFee, hash, batcherHash
+        );
+
+        vm.prank(_l1BlockIsthmus().DEPOSITOR_ACCOUNT());
+        (bool success,) = address(l1Block).call(
+            abi.encodePacked(L1BlockIsthmus.setL1BlockValuesIsthmus.selector, setValuesEcotoneCalldata)
+        );
+        assertTrue(success, "function call failed");
+
+        // Assert that the `isDepositTransaction` flag was properly set to true
+        vm.prank(Predeploys.CROSS_L2_INBOX);
+        assertEq(_l1BlockIsthmus().isDeposit(), true);
+
+        // Assert `setL1BlockValuesEcotone` was properly called, forwarding the calldata to it
+        assertEq(_l1BlockIsthmus().baseFeeScalar(), baseFeeScalar, "base fee scalar not properly set");
+        assertEq(_l1BlockIsthmus().blobBaseFeeScalar(), blobBaseFeeScalar, "blob base fee scalar not properly set");
+        assertEq(_l1BlockIsthmus().sequenceNumber(), sequenceNumber, "sequence number not properly set");
+        assertEq(_l1BlockIsthmus().timestamp(), timestamp, "timestamp not properly set");
+        assertEq(_l1BlockIsthmus().number(), number, "number not properly set");
+        assertEq(_l1BlockIsthmus().basefee(), baseFee, "base fee not properly set");
+        assertEq(_l1BlockIsthmus().blobBaseFee(), blobBaseFee, "blob base fee not properly set");
+        assertEq(_l1BlockIsthmus().hash(), hash, "hash not properly set");
+        assertEq(_l1BlockIsthmus().batcherHash(), batcherHash, "batcher hash not properly set");
+    }
+}
+
+contract L1BlockDepositsComplete_Test is L1BlockIsthmusTest {
+    // @dev Tests that `depositsComplete` reverts if the caller is not the depositor.
+    function test_deposits_is_depositor_reverts(address _caller) external {
+        vm.assume(_caller != _l1BlockIsthmus().DEPOSITOR_ACCOUNT());
+        vm.expectRevert(NotDepositor.selector);
+        _l1BlockIsthmus().depositsComplete();
+    }
+
+    // @dev Tests that `depositsComplete` succeeds if the caller is the depositor.
+    function test_depositsComplete_succeeds() external {
+        // Set the `isDeposit` flag to true
+        vm.prank(_l1BlockIsthmus().DEPOSITOR_ACCOUNT());
+        _l1BlockIsthmus().setL1BlockValuesIsthmus();
+
+        // Assert that the `isDeposit` flag was properly set to true
+        vm.prank(Predeploys.CROSS_L2_INBOX);
+        assertTrue(_l1BlockIsthmus().isDeposit());
+
+        // Call `depositsComplete`
+        vm.prank(_l1BlockIsthmus().DEPOSITOR_ACCOUNT());
+        _l1BlockIsthmus().depositsComplete();
+
+        // Assert that the `isDeposit` flag was properly set to false
+        /// @dev Assuming that `isDeposit()` wil return the proper value. That function is tested as well
+        vm.prank(Predeploys.CROSS_L2_INBOX);
+        assertEq(_l1BlockIsthmus().isDeposit(), false);
+    }
+}
diff --git a/packages/contracts-bedrock/test/universal/ProxyAdmin.t.sol b/packages/contracts-bedrock/test/universal/ProxyAdmin.t.sol
index 4de72c87257..15e806d3e52 100644
--- a/packages/contracts-bedrock/test/universal/ProxyAdmin.t.sol
+++ b/packages/contracts-bedrock/test/universal/ProxyAdmin.t.sol
@@ -164,9 +164,8 @@ contract ProxyAdmin_Test is Test {
             admin.getProxyAdmin(_proxy);
         } else if (proxyType == ProxyAdmin.ProxyType.RESOLVED) {
             // Just an empty block to show that all cases are covered
-        } else {
-            vm.expectRevert("ProxyAdmin: unknown proxy type");
         }
+        else vm.expectRevert("ProxyAdmin: unknown proxy type");
 
         // Call the proxy contract directly to get the admin.
         // Different proxy types have different interfaces.