regtest: align activations with Bitcoin Core

diff --git a/blockchain/chain_test.go b/blockchain/chain_test.go
index b3bccf5..a1fcb9a 100644
--- a/blockchain/chain_test.go
+++ b/blockchain/chain_test.go
@@ -1384,6 +1384,7 @@ func TestInvalidateBlock(t *testing.T) {
 						"invalidate-once")
 				// Grab the tip of the chain.
 				tip := btcutil.NewBlock(params.GenesisBlock)
+				tip.SetHeight(0)
 
 				// Create a chain with 11 blocks.
 				_, _, err := addBlocks(11, chain, tip, []*testhelper.SpendableOut{})
@@ -1407,6 +1408,7 @@ func TestInvalidateBlock(t *testing.T) {
 				chain, params, tearDown := utxoCacheTestChain("TestInvalidateBlock-invalidate-twice")
 				// Grab the tip of the chain.
 				tip := btcutil.NewBlock(params.GenesisBlock)
+				tip.SetHeight(0)
 
 				// Create a chain with 11 blocks.
 				_, spendableOuts, err := addBlocks(11, chain, tip, []*testhelper.SpendableOut{})
@@ -1453,6 +1455,7 @@ func TestInvalidateBlock(t *testing.T) {
 			chainGen: func() (*BlockChain, []*chainhash.Hash, func()) {
 				chain, params, tearDown := utxoCacheTestChain("TestInvalidateBlock-invalidate-side-branch")
 				tip := btcutil.NewBlock(params.GenesisBlock)
+				tip.SetHeight(0)
 
 				// Grab the tip of the chain.
 				tip, err := chain.BlockByHash(&chain.bestChain.Tip().hash)
@@ -1635,6 +1638,7 @@ func TestReconsiderBlock(t *testing.T) {
 
 				// Create a chain with 101 blocks.
 				tip := btcutil.NewBlock(params.GenesisBlock)
+				tip.SetHeight(0)
 				_, _, err := addBlocks(101, chain, tip, []*testhelper.SpendableOut{})
 				if err != nil {
 					t.Fatal(err)
@@ -1657,6 +1661,7 @@ func TestReconsiderBlock(t *testing.T) {
 
 				// Create a chain with 101 blocks.
 				tip := btcutil.NewBlock(params.GenesisBlock)
+				tip.SetHeight(0)
 				_, spendableOuts, err := addBlocks(101, chain, tip, []*testhelper.SpendableOut{})
 				if err != nil {
 					t.Fatal(err)
@@ -1689,6 +1694,7 @@ func TestReconsiderBlock(t *testing.T) {
 
 				// Create a chain with 101 blocks.
 				tip := btcutil.NewBlock(params.GenesisBlock)
+				tip.SetHeight(0)
 				_, spendableOuts, err := addBlocks(101, chain, tip, []*testhelper.SpendableOut{})
 				if err != nil {
 					t.Fatal(err)
@@ -1718,6 +1724,7 @@ func TestReconsiderBlock(t *testing.T) {
 				chain, params, tearDown := utxoCacheTestChain("TestReconsiderBlock-reconsider-an-invalid-side-branch-higher")
 
 				tip := btcutil.NewBlock(params.GenesisBlock)
+				tip.SetHeight(0)
 				_, spendableOuts, err := addBlocks(6, chain, tip, []*testhelper.SpendableOut{})
 				if err != nil {
 					t.Fatal(err)
@@ -1752,6 +1759,7 @@ func TestReconsiderBlock(t *testing.T) {
 				chain, params, tearDown := utxoCacheTestChain("TestReconsiderBlock-reconsider-an-invalid-side-branch-lower")
 
 				tip := btcutil.NewBlock(params.GenesisBlock)
+				tip.SetHeight(0)
 				_, spendableOuts, err := addBlocks(6, chain, tip, []*testhelper.SpendableOut{})
 				if err != nil {
 					t.Fatal(err)
diff --git a/blockchain/common_test.go b/blockchain/common_test.go
index 12badd3..8c647c1 100644
--- a/blockchain/common_test.go
+++ b/blockchain/common_test.go
@@ -465,7 +465,7 @@ func newBlock(chain *BlockChain, prev *btcutil.Block,
 	// SolveBlock.
 	block := btcutil.NewBlock(&wire.MsgBlock{
 		Header: wire.BlockHeader{
-			Version:    1,
+			Version:    4,
 			PrevBlock:  *prev.Hash(),
 			MerkleRoot: calcMerkleRoot(txns),
 			Bits:       chain.chainParams.PowLimitBits,
diff --git a/blockchain/fullblocktests/generate.go b/blockchain/fullblocktests/generate.go
index 2b499c4..a17af45 100644
--- a/blockchain/fullblocktests/generate.go
+++ b/blockchain/fullblocktests/generate.go
@@ -286,6 +286,30 @@ func replaceCoinbaseSigScript(script []byte) func(*wire.MsgBlock) {
 	}
 }
 
+// padCoinbaseSigScript returns a function that pads the existing coinbase
+// signature script with OP_0 until it reaches the provided length. It keeps the
+// existing prefix (including the serialized height) intact.
+func padCoinbaseSigScript(targetLen int) func(*wire.MsgBlock) {
+	return func(b *wire.MsgBlock) {
+		sigScript := b.Transactions[0].TxIn[0].SignatureScript
+		if len(sigScript) > targetLen {
+			panic(fmt.Sprintf("padCoinbaseSigScript: script len "+
+				"%d > target %d", len(sigScript), targetLen))
+		}
+
+		if len(sigScript) == targetLen {
+			return
+		}
+
+		padding := bytes.Repeat(
+			[]byte{txscript.OP_0}, targetLen-len(sigScript),
+		)
+		b.Transactions[0].TxIn[0].SignatureScript = append(
+			sigScript, padding...,
+		)
+	}
+}
+
 // additionalTx returns a function that itself takes a block and modifies it by
 // adding the provided transaction.
 func additionalTx(tx *wire.MsgTx) func(*wire.MsgBlock) {
@@ -355,7 +379,7 @@ func (g *testGenerator) nextBlock(blockName string, spend *testhelper.SpendableO
 
 	block := wire.MsgBlock{
 		Header: wire.BlockHeader{
-			Version:    1,
+			Version:    4,
 			PrevBlock:  g.tip.BlockHash(),
 			MerkleRoot: calcMerkleRoot(txns),
 			Bits:       g.params.PowLimitBits,
@@ -1044,8 +1068,7 @@ func Generate(includeLargeReorg bool) (tests [][]TestInstance, err error) {
 	//
 	//   ... -> b23(6) -> b30(7)
 	g.setTip("b23")
-	maxSizeCbScript := repeatOpcode(0x00, maxCoinbaseScriptLen)
-	g.nextBlock("b30", outs[7], replaceCoinbaseSigScript(maxSizeCbScript))
+	g.nextBlock("b30", outs[7], padCoinbaseSigScript(maxCoinbaseScriptLen))
 	accepted()
 
 	// ---------------------------------------------------------------------
@@ -1576,6 +1599,19 @@ func Generate(includeLargeReorg bool) (tests [][]TestInstance, err error) {
 		parent := g.blocks[b.Header.PrevBlock]
 		b.Transactions[0] = parent.Transactions[0]
 	})
+	rejected(blockchain.ErrBadCoinbaseHeight)
+
+	// Create block that duplicates a non-coinbase transaction from an
+	// earlier block to trigger BIP30 rejection (duplicate txid in UTXO).
+	//
+	//   ... -> b60(17)
+	//                 \-> b61dup(18)
+	g.setTip("b60")
+	g.nextBlock("b61dup", outs[18], func(b *wire.MsgBlock) {
+		parent := g.blocks[b.Header.PrevBlock]
+		dupTx := parent.Transactions[1].Copy()
+		b.AddTransaction(dupTx)
+	})
 	rejected(blockchain.ErrOverwriteTx)
 
 	// ---------------------------------------------------------------------
diff --git a/blockchain/fullblocktests/params.go b/blockchain/fullblocktests/params.go
index 4679036..2e36dce 100644
--- a/blockchain/fullblocktests/params.go
+++ b/blockchain/fullblocktests/params.go
@@ -103,9 +103,9 @@ var regressionNetParams = &chaincfg.Params{
 	PowLimit:                 regressionPowLimit,
 	PowLimitBits:             0x207fffff,
 	CoinbaseMaturity:         100,
-	BIP0034Height:            100000000, // Not active - Permit ver 1 blocks
-	BIP0065Height:            1351,      // Used by regression tests
-	BIP0066Height:            1251,      // Used by regression tests
+	BIP0034Height:            1,
+	BIP0065Height:            1,
+	BIP0066Height:            1,
 	SubsidyReductionInterval: 150,
 	TargetTimespan:           time.Hour * 24 * 14, // 14 days
 	TargetTimePerBlock:       time.Minute * 10,    // 10 minutes
diff --git a/blockchain/regtest_test.go b/blockchain/regtest_test.go
new file mode 100644
index 0000000..1f554a5
--- /dev/null
+++ b/blockchain/regtest_test.go
@@ -0,0 +1,218 @@
+package blockchain
+
+import (
+	"testing"
+	"time"
+
+	"github.com/btcsuite/btcd/btcutil"
+	"github.com/btcsuite/btcd/chaincfg"
+	"github.com/btcsuite/btcd/chaincfg/chainhash"
+	"github.com/btcsuite/btcd/txscript"
+	"github.com/btcsuite/btcd/wire"
+	"github.com/stretchr/testify/require"
+)
+
+// stubChainCtx provides the minimal ChainCtx implementation needed for header
+// context checks in tests.
+type stubChainCtx struct {
+	params *chaincfg.Params
+}
+
+// ChainParams returns the active chain parameters.
+func (s stubChainCtx) ChainParams() *chaincfg.Params {
+	return s.params
+}
+
+// BlocksPerRetarget returns the blocks per difficulty retarget.
+func (s stubChainCtx) BlocksPerRetarget() int32 {
+	return int32(s.params.TargetTimespan / s.params.TargetTimePerBlock)
+}
+
+// MinRetargetTimespan returns the lower bound of the retarget timespan.
+func (s stubChainCtx) MinRetargetTimespan() int64 {
+	return int64(
+		s.params.TargetTimespan /
+			time.Duration(s.params.RetargetAdjustmentFactor),
+	)
+}
+
+// MaxRetargetTimespan returns the upper bound of the retarget timespan.
+func (s stubChainCtx) MaxRetargetTimespan() int64 {
+	return int64(
+		s.params.TargetTimespan *
+			time.Duration(s.params.RetargetAdjustmentFactor),
+	)
+}
+
+// VerifyCheckpoint reports whether a checkpoint matches.
+func (s stubChainCtx) VerifyCheckpoint(_ int32, _ *chainhash.Hash) bool {
+	return true
+}
+
+// FindPreviousCheckpoint returns the last known checkpoint.
+func (s stubChainCtx) FindPreviousCheckpoint() (HeaderCtx, error) {
+	return nil, nil
+}
+
+// regtestPrevNode returns a blockNode for the regtest genesis block to serve
+// as the parent of height-1 test headers.
+func regtestPrevNode(t *testing.T) *blockNode {
+	t.Helper()
+
+	params := chaincfg.RegressionNetParams
+
+	return newBlockNode(&params.GenesisBlock.Header, nil)
+}
+
+// TestRegtestBlockVersions ensures regtest enforces BIP34/66/65 version floors
+// from height 1.
+func TestRegtestBlockVersions(t *testing.T) {
+	params := chaincfg.RegressionNetParams
+	prevNode := regtestPrevNode(t)
+
+	testCases := []struct {
+		name    string
+		version int32
+		wantErr ErrorCode
+	}{
+		{
+			name:    "v1_rejected",
+			version: 1,
+			wantErr: ErrBlockVersionTooOld,
+		},
+		{
+			name:    "v2_rejected",
+			version: 2,
+			wantErr: ErrBlockVersionTooOld,
+		},
+		{
+			name:    "v3_rejected",
+			version: 3,
+			wantErr: ErrBlockVersionTooOld,
+		},
+		{
+			name:    "v4_allowed",
+			version: 4,
+			wantErr: ErrorCode(0),
+		},
+		{
+			name:    "vb_signal_allowed",
+			version: 0x20000000,
+			wantErr: ErrorCode(0),
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			block := &wire.BlockHeader{
+				Version:   tc.version,
+				PrevBlock: *params.GenesisHash,
+				Bits:      params.PowLimitBits,
+				Timestamp: time.Unix(prevNode.Timestamp()+1, 0),
+			}
+
+			err := CheckBlockHeaderContext(
+				block, prevNode, BFFastAdd,
+				stubChainCtx{params: &params}, true,
+			)
+
+			if tc.wantErr == 0 {
+				require.NoError(t, err)
+
+				return
+			}
+
+			require.Error(t, err)
+
+			var rErr RuleError
+			require.ErrorAs(t, err, &rErr)
+			require.Equal(t, tc.wantErr, rErr.ErrorCode)
+		})
+	}
+}
+
+// TestRegtestBuriedDeploymentsAlwaysActive asserts CSV, SegWit, and Taproot
+// are always active on regtest, mirroring Bitcoin Core.
+func TestRegtestBuriedDeploymentsAlwaysActive(t *testing.T) {
+	chain := newFakeChain(&chaincfg.RegressionNetParams)
+	prevNode := chain.bestChain.Tip()
+
+	stateCSV, err := chain.deploymentState(prevNode, chaincfg.DeploymentCSV)
+	require.NoError(t, err)
+	require.Equal(t, ThresholdActive, stateCSV)
+
+	stateSegwit, err := chain.deploymentState(
+		prevNode, chaincfg.DeploymentSegwit,
+	)
+	require.NoError(t, err)
+	require.Equal(t, ThresholdActive, stateSegwit)
+
+	stateTaproot, err := chain.deploymentState(
+		prevNode, chaincfg.DeploymentTaproot,
+	)
+	require.NoError(t, err)
+	require.Equal(t, ThresholdActive, stateTaproot)
+}
+
+// TestRegtestRejectsCoinbaseMissingHeight ensures contextual validation
+// enforces coinbase height serialization on regtest.
+func TestRegtestRejectsCoinbaseMissingHeight(t *testing.T) {
+	chain := newFakeChain(&chaincfg.RegressionNetParams)
+	prevNode := chain.bestChain.Tip()
+
+	block := wire.MsgBlock{
+		Header: wire.BlockHeader{
+			Version:   4,
+			PrevBlock: prevNode.hash,
+			Bits:      chain.chainParams.PowLimitBits,
+			Timestamp: time.Unix(prevNode.timestamp+1, 0),
+		},
+	}
+
+	coinbase := wire.NewMsgTx(wire.TxVersion)
+	coinbase.AddTxIn(&wire.TxIn{SignatureScript: []byte{}})
+	coinbase.AddTxOut(&wire.TxOut{Value: 0})
+	block.AddTransaction(coinbase)
+	block.Header.MerkleRoot = block.Transactions[0].TxHash()
+
+	btcBlock := btcutil.NewBlock(&block)
+
+	// The block without a height in coinbase is not considered valid.
+	err := chain.checkBlockContext(btcBlock, prevNode, BFNone)
+	require.Error(t, err)
+
+	// Make sure the error is in coinbase height record.
+	var rErr RuleError
+	require.ErrorAs(t, err, &rErr)
+	require.Equal(t, ErrMissingCoinbaseHeight, rErr.ErrorCode)
+}
+
+// TestRegtestAcceptsCoinbaseHeight ensures a properly encoded coinbase height
+// passes contextual checks on regtest.
+func TestRegtestAcceptsCoinbaseHeight(t *testing.T) {
+	chain := newFakeChain(&chaincfg.RegressionNetParams)
+	prevNode := chain.bestChain.Tip()
+
+	coinbaseScript, err := txscript.NewScriptBuilder().AddInt64(1).Script()
+	require.NoError(t, err)
+
+	block := wire.MsgBlock{
+		Header: wire.BlockHeader{
+			Version:   4,
+			PrevBlock: prevNode.hash,
+			Bits:      chain.chainParams.PowLimitBits,
+			Timestamp: time.Unix(prevNode.timestamp+1, 0),
+		},
+	}
+
+	coinbase := wire.NewMsgTx(wire.TxVersion)
+	coinbase.AddTxIn(&wire.TxIn{SignatureScript: coinbaseScript})
+	coinbase.AddTxOut(&wire.TxOut{Value: 0})
+	block.AddTransaction(coinbase)
+	block.Header.MerkleRoot = block.Transactions[0].TxHash()
+
+	btcBlock := btcutil.NewBlock(&block)
+
+	// Make sure the block with the height in coinbase is considered valid.
+	require.NoError(t, chain.checkBlockContext(btcBlock, prevNode, BFNone))
+}
diff --git a/blockchain/utxocache_test.go b/blockchain/utxocache_test.go
index 0f410cc..9b3bc99 100644
--- a/blockchain/utxocache_test.go
+++ b/blockchain/utxocache_test.go
@@ -405,6 +405,7 @@ func TestUtxoCacheFlush(t *testing.T) {
 	defer tearDown()
 	cache := chain.utxoCache
 	tip := btcutil.NewBlock(params.GenesisBlock)
+	tip.SetHeight(0)
 
 	// The chainSetup init triggers the consistency status write.
 	err := assertConsistencyState(chain, params.GenesisHash)
diff --git a/chaincfg/params.go b/chaincfg/params.go
index 1abe7fc..8aa6c11 100644
--- a/chaincfg/params.go
+++ b/chaincfg/params.go
@@ -481,9 +481,9 @@ var RegressionNetParams = Params{
 	PowLimitBits:             0x207fffff,
 	PoWNoRetargeting:         true,
 	CoinbaseMaturity:         100,
-	BIP0034Height:            100000000, // Not active - Permit ver 1 blocks
-	BIP0065Height:            1351,      // Used by regression tests
-	BIP0066Height:            1251,      // Used by regression tests
+	BIP0034Height:            1,
+	BIP0065Height:            1,
+	BIP0066Height:            1,
 	SubsidyReductionInterval: 150,
 	TargetTimespan:           time.Hour * 24 * 14, // 14 days
 	TargetTimePerBlock:       time.Minute * 10,    // 10 minutes
@@ -540,6 +540,7 @@ var RegressionNetParams = Params{
 			DeploymentEnder: NewMedianTimeDeploymentEnder(
 				time.Time{}, // Never expires
 			),
+			AlwaysActiveHeight: 1,
 		},
 		DeploymentSegwit: {
 			BitNumber: 1,
@@ -549,6 +550,7 @@ var RegressionNetParams = Params{
 			DeploymentEnder: NewMedianTimeDeploymentEnder(
 				time.Time{}, // Never expires.
 			),
+			AlwaysActiveHeight: 1,
 		},
 		DeploymentTaproot: {
 			BitNumber: 2,
@@ -558,6 +560,8 @@ var RegressionNetParams = Params{
 			DeploymentEnder: NewMedianTimeDeploymentEnder(
 				time.Time{}, // Never expires.
 			),
+			MinActivationHeight: 0,
+			AlwaysActiveHeight:  1,
 			CustomActivationThreshold: 108, // Only needs 75% hash rate.
 		},
 	},
diff --git a/integration/bip0009_test.go b/integration/bip0009_test.go
index 8f8b59a..5d443d7 100644
--- a/integration/bip0009_test.go
+++ b/integration/bip0009_test.go
@@ -139,6 +139,16 @@ func testBIP0009(t *testing.T, forkKey string, deploymentID uint32) {
 	}
 	defer r.TearDown()
 
+	// Short-circuit deployments that are configured as always active.
+	if deploymentID < uint32(len(r.ActiveNet.Deployments)) {
+		dep := &r.ActiveNet.Deployments[deploymentID]
+		if dep.AlwaysActiveHeight != 0 {
+			assertChainHeight(r, t, 0)
+			assertSoftForkStatus(r, t, forkKey, blockchain.ThresholdActive)
+			return
+		}
+	}
+
 	// If the deployment is meant to be always active, then it should be
 	// active from the very first block.
 	if deploymentID == chaincfg.DeploymentTestDummyAlwaysActive {