graphdb: fix backwards-compat for channel edge feature deserialization

diff --git a/docs/release-notes/release-notes-0.20.1.md b/docs/release-notes/release-notes-0.20.1.md
index fddd81e..c5bb192 100644
--- a/docs/release-notes/release-notes-0.20.1.md
+++ b/docs/release-notes/release-notes-0.20.1.md
@@ -62,6 +62,13 @@
   due to validation checks in the graph Builder that were resurfaced after the
   graph refactor work.
 
+* [Fix backwards compatibility for channel edge feature
+  deserialization](https://github.com/lightningnetwork/lnd/pull/10529). Nodes
+  upgrading from pre-v0.20 versions could fail to read channel edges from their
+  graph database due to a format change in how channel features are serialized.
+  The fix adds automatic format detection to handle both legacy (raw feature
+  bits) and new (length-prefixed) formats.
+
 # New Features
 
 ## Functional Enhancements
diff --git a/graph/db/kv_store.go b/graph/db/kv_store.go
index 80e802e..34b21c9 100644
--- a/graph/db/kv_store.go
+++ b/graph/db/kv_store.go
@@ -4715,6 +4715,78 @@ func fetchChanEdgeInfo(edgeIndex kvdb.RBucket,
 	return deserializeChanEdgeInfo(edgeInfoReader)
 }
 
+// deserializeChanEdgeFeatures deserializes channel edge features from bytes,
+// handling both the legacy format (raw feature bits) and the current format
+// (2-byte length prefix followed by feature bits).
+//
+// Legacy format (pre-v0.20): VarBytes containing raw feature bits directly.
+// Current format (v0.20+): VarBytes containing a 2-byte big-endian length
+// followed by the feature bits.
+//
+// The format is detected by checking if the first 2 bytes, interpreted as a
+// big-endian uint16 length, equals len(featureBytes)-2. Since this length
+// check alone could have false positives (e.g., a 258-byte legacy vector
+// starting with 0x01, 0x00), we additionally verify by decoding and
+// re-encoding the payload to confirm it produces the exact same bytes
+// (canonical encoding check).
+func deserializeChanEdgeFeatures(featureBytes []byte) (*lnwire.FeatureVector,
+	error) {
+
+	features := lnwire.NewRawFeatureVector()
+
+	// Empty features are valid in both formats.
+	if len(featureBytes) == 0 {
+		return lnwire.NewFeatureVector(features, lnwire.Features), nil
+	}
+
+	// Check if this looks like the new format with a 2-byte length prefix.
+	// In the new format, the first 2 bytes encode the length of the
+	// remaining feature bytes.
+	if len(featureBytes) >= 2 {
+		encodedLen := binary.BigEndian.Uint16(featureBytes[:2])
+		if int(encodedLen) == len(featureBytes)-2 {
+			// This looks like it could be the new format. To be
+			// certain, we decode and re-encode to verify canonical
+			// encoding, as a legacy feature vector could
+			// accidentally match the length check (e.g., a 258-byte
+			// legacy vector starting with 0x01, 0x00 would have
+			// 256 == 258 - 2).
+			payload := featureBytes[2:]
+			tempFeatures := lnwire.NewRawFeatureVector()
+			err := tempFeatures.DecodeBase256(
+				bytes.NewReader(payload), int(encodedLen),
+			)
+
+			var checkBuf bytes.Buffer
+			if err == nil {
+				err = tempFeatures.EncodeBase256(&checkBuf)
+			}
+
+			// If there were no errors and the re-encoded payload
+			// matches the original, we are confident it's the new
+			// format.
+			isCanonical := bytes.Equal(checkBuf.Bytes(), payload)
+			if err == nil && isCanonical {
+				return lnwire.NewFeatureVector(
+					tempFeatures, lnwire.Features,
+				), nil
+			}
+		}
+	}
+
+	// Legacy format: the bytes are raw feature bits without a length
+	// prefix.
+	err := features.DecodeBase256(
+		bytes.NewReader(featureBytes), len(featureBytes),
+	)
+	if err != nil {
+		return nil, fmt.Errorf("unable to decode features "+
+			"(legacy format): %w", err)
+	}
+
+	return lnwire.NewFeatureVector(features, lnwire.Features), nil
+}
+
 func deserializeChanEdgeInfo(r io.Reader) (*models.ChannelEdgeInfo, error) {
 	var (
 		err      error
@@ -4739,13 +4811,10 @@ func deserializeChanEdgeInfo(r io.Reader) (*models.ChannelEdgeInfo, error) {
 		return nil, err
 	}
 
-	features := lnwire.NewRawFeatureVector()
-	err = features.Decode(bytes.NewReader(featureBytes))
+	edgeInfo.Features, err = deserializeChanEdgeFeatures(featureBytes)
 	if err != nil {
-		return nil, fmt.Errorf("unable to decode "+
-			"features: %w", err)
+		return nil, err
 	}
-	edgeInfo.Features = lnwire.NewFeatureVector(features, lnwire.Features)
 
 	proof := &models.ChannelAuthProof{}
 
diff --git a/graph/db/kv_store_features_test.go b/graph/db/kv_store_features_test.go
new file mode 100644
index 0000000..161ea59
--- /dev/null
+++ b/graph/db/kv_store_features_test.go
@@ -0,0 +1,449 @@
+package graphdb
+
+import (
+	"bytes"
+	"encoding/binary"
+	"testing"
+
+	"github.com/lightningnetwork/lnd/lnwire"
+	"github.com/stretchr/testify/require"
+	"pgregory.net/rapid"
+)
+
+// TestDeserializeChanEdgeFeaturesEmpty tests that empty feature bytes are
+// handled correctly for both legacy and new formats.
+func TestDeserializeChanEdgeFeaturesEmpty(t *testing.T) {
+	t.Parallel()
+
+	// Empty bytes should result in empty features.
+	features, err := deserializeChanEdgeFeatures(nil)
+	require.NoError(t, err)
+	require.True(t, features.IsEmpty())
+
+	features, err = deserializeChanEdgeFeatures([]byte{})
+	require.NoError(t, err)
+	require.True(t, features.IsEmpty())
+
+	// New format with zero-length features: [0x00, 0x00].
+	features, err = deserializeChanEdgeFeatures([]byte{0x00, 0x00})
+	require.NoError(t, err)
+	require.True(t, features.IsEmpty())
+}
+
+// TestDeserializeChanEdgeFeaturesLegacyFormat tests deserialization of
+// feature bytes written in the legacy format (pre-v0.20), which contains
+// raw feature bits without a 2-byte length prefix.
+func TestDeserializeChanEdgeFeaturesLegacyFormat(t *testing.T) {
+	t.Parallel()
+
+	testCases := []struct {
+		name          string
+		legacyBytes   []byte
+		expectedFeats []lnwire.FeatureBit
+	}{
+		{
+			name:          "single byte - bit 0",
+			legacyBytes:   []byte{0x01}, // bit 0 set
+			expectedFeats: []lnwire.FeatureBit{0},
+		},
+		{
+			name:          "single byte - bit 7",
+			legacyBytes:   []byte{0x80}, // bit 7 set
+			expectedFeats: []lnwire.FeatureBit{7},
+		},
+		{
+			name:          "single byte - multiple bits",
+			legacyBytes:   []byte{0x25}, // bits 0, 2, 5 set
+			expectedFeats: []lnwire.FeatureBit{0, 2, 5},
+		},
+		{
+			name:          "two bytes - bit 8",
+			legacyBytes:   []byte{0x01, 0x00}, // bit 8 set
+			expectedFeats: []lnwire.FeatureBit{8},
+		},
+		{
+			name:          "two bytes - bits 0 and 15",
+			legacyBytes:   []byte{0x80, 0x01}, // bits 0 and 15 set
+			expectedFeats: []lnwire.FeatureBit{0, 15},
+		},
+		{
+			// bit 1 (DataLossProtectOptional).
+			name:        "common features - data loss protect",
+			legacyBytes: []byte{0x02},
+			expectedFeats: []lnwire.FeatureBit{
+				lnwire.DataLossProtectOptional,
+			},
+		},
+		{
+			// bits 1, 7, 9, 13, 15 = DataLossProtect,
+			// GossipQueries, TLVOnion, StaticRemoteKey,
+			// PaymentAddr.
+			name:          "multiple common features",
+			legacyBytes:   []byte{0xA2, 0x82},
+			expectedFeats: []lnwire.FeatureBit{1, 7, 9, 13, 15},
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			features, err := deserializeChanEdgeFeatures(
+				tc.legacyBytes,
+			)
+			require.NoError(t, err)
+
+			for _, bit := range tc.expectedFeats {
+				require.True(t, features.IsSet(bit),
+					"expected bit %d to be set", bit)
+			}
+
+			// Verify no extra bits are set by creating expected
+			// feature vector and comparing.
+			expectedRaw := lnwire.NewRawFeatureVector(
+				tc.expectedFeats...,
+			)
+			require.True(t, expectedRaw.Equals(
+				features.RawFeatureVector),
+				"feature vectors don't match")
+		})
+	}
+}
+
+// TestDeserializeChanEdgeFeaturesNewFormat tests deserialization of
+// feature bytes written in the new format (v0.20+), which contains
+// a 2-byte big-endian length prefix followed by raw feature bits.
+func TestDeserializeChanEdgeFeaturesNewFormat(t *testing.T) {
+	t.Parallel()
+
+	testCases := []struct {
+		name          string
+		expectedFeats []lnwire.FeatureBit
+	}{
+		{
+			name:          "empty features",
+			expectedFeats: nil,
+		},
+		{
+			name:          "single feature bit 0",
+			expectedFeats: []lnwire.FeatureBit{0},
+		},
+		{
+			name:          "single feature bit 15",
+			expectedFeats: []lnwire.FeatureBit{15},
+		},
+		{
+			name:          "multiple features",
+			expectedFeats: []lnwire.FeatureBit{1, 5, 9, 13, 17},
+		},
+		{
+			name: "common lightning features",
+			expectedFeats: []lnwire.FeatureBit{
+				lnwire.DataLossProtectOptional,
+				lnwire.GossipQueriesOptional,
+				lnwire.TLVOnionPayloadOptional,
+				lnwire.StaticRemoteKeyOptional,
+				lnwire.PaymentAddrOptional,
+			},
+		},
+		{
+			name: "high bit features",
+			expectedFeats: []lnwire.FeatureBit{
+				lnwire.AMPOptional,     // 31
+				lnwire.KeysendOptional, // 55
+			},
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			// Create feature vector and encode in new format.
+			rawFeatures := lnwire.NewRawFeatureVector(
+				tc.expectedFeats...,
+			)
+			fv := lnwire.NewFeatureVector(
+				rawFeatures, lnwire.Features,
+			)
+
+			// Encode using the new format (with length prefix).
+			var buf bytes.Buffer
+			err := fv.Encode(&buf)
+			require.NoError(t, err)
+
+			// Deserialize and verify.
+			features, err := deserializeChanEdgeFeatures(
+				buf.Bytes(),
+			)
+			require.NoError(t, err)
+
+			for _, bit := range tc.expectedFeats {
+				require.True(t, features.IsSet(bit),
+					"expected bit %d to be set", bit)
+			}
+
+			// Verify feature equality.
+			require.True(t, rawFeatures.Equals(
+				features.RawFeatureVector),
+			)
+		})
+	}
+}
+
+// TestDeserializeChanEdgeFeaturesFormatDetection tests that the format
+// detection correctly distinguishes between legacy and new formats.
+func TestDeserializeChanEdgeFeaturesFormatDetection(t *testing.T) {
+	t.Parallel()
+
+	// Test that legacy format bytes that could theoretically be confused
+	// with new format are handled correctly. This shouldn't happen in
+	// practice because in legacy format the first byte always has at least
+	// one bit set (the highest feature bit determines the byte length).
+
+	// Create a feature vector with bit 8 set (requires 2 bytes in legacy).
+	// Legacy format: [0x01, 0x00] (big-endian, high byte first).
+	// As a length, 0x0100 = 256, which != 0 (len-2), so correctly detected
+	// as legacy.
+	legacyBit8 := []byte{0x01, 0x00}
+	features, err := deserializeChanEdgeFeatures(legacyBit8)
+	require.NoError(t, err)
+	require.True(t, features.IsSet(8))
+	require.False(t, features.IsSet(0))
+
+	// New format with bit 8: [0x00, 0x02, 0x01, 0x00]
+	// Length prefix 0x0002 = 2, remaining 2 bytes = feature bits.
+	newFormatBit8 := []byte{0x00, 0x02, 0x01, 0x00}
+	features, err = deserializeChanEdgeFeatures(newFormatBit8)
+	require.NoError(t, err)
+	require.True(t, features.IsSet(8))
+	require.False(t, features.IsSet(0))
+
+	// Test single byte legacy format - cannot be confused with new format
+	// since new format minimum is 2 bytes (the length prefix).
+	legacyBit0 := []byte{0x01}
+	features, err = deserializeChanEdgeFeatures(legacyBit0)
+	require.NoError(t, err)
+	require.True(t, features.IsSet(0))
+}
+
+// TestDeserializeChanEdgeFeaturesRoundTrip tests that features can be
+// serialized and deserialized correctly using the new format.
+func TestDeserializeChanEdgeFeaturesRoundTrip(t *testing.T) {
+	t.Parallel()
+
+	testFeatureSets := [][]lnwire.FeatureBit{
+		{},
+		{0},
+		{7},
+		{8},
+		{15},
+		{0, 1, 2, 3, 4, 5, 6, 7},
+		{8, 9, 10, 11, 12, 13, 14, 15},
+		{0, 8, 16, 24, 32},
+		{
+			lnwire.DataLossProtectOptional,
+			lnwire.GossipQueriesOptional,
+			lnwire.TLVOnionPayloadOptional,
+			lnwire.StaticRemoteKeyOptional,
+			lnwire.PaymentAddrOptional,
+			lnwire.MPPOptional,
+			lnwire.AnchorsZeroFeeHtlcTxOptional,
+		},
+	}
+
+	for _, featureBits := range testFeatureSets {
+		// Create and encode.
+		rawFeatures := lnwire.NewRawFeatureVector(featureBits...)
+		fv := lnwire.NewFeatureVector(rawFeatures, lnwire.Features)
+
+		var buf bytes.Buffer
+		err := fv.Encode(&buf)
+		require.NoError(t, err)
+
+		// Deserialize.
+		decoded, err := deserializeChanEdgeFeatures(buf.Bytes())
+		require.NoError(t, err)
+
+		// Verify equality.
+		require.True(t, rawFeatures.Equals(decoded.RawFeatureVector),
+			"mismatch for features %v", featureBits)
+	}
+}
+
+// TestDeserializeChanEdgeFeaturesPropertyBased uses property-based testing
+// to verify that the deserialization works correctly for arbitrary feature
+// combinations in both legacy and new formats.
+func TestDeserializeChanEdgeFeaturesPropertyBased(t *testing.T) {
+	t.Parallel()
+
+	// Test legacy format: raw feature bytes without length prefix.
+	rapid.Check(t, func(t *rapid.T) {
+		// Generate random feature bits (max 256 to keep reasonable).
+		numFeatures := rapid.IntRange(0, 20).Draw(t, "numFeatures")
+		featureBits := make([]lnwire.FeatureBit, numFeatures)
+		for i := 0; i < numFeatures; i++ {
+			featureBits[i] = lnwire.FeatureBit(
+				rapid.IntRange(0, 255).Draw(t, "featureBit"),
+			)
+		}
+
+		// Create feature vector.
+		rawFeatures := lnwire.NewRawFeatureVector(featureBits...)
+
+		// Encode without length prefix (legacy format).
+		var buf bytes.Buffer
+		err := rawFeatures.EncodeBase256(&buf)
+		require.NoError(t, err)
+
+		// Deserialize.
+		decoded, err := deserializeChanEdgeFeatures(buf.Bytes())
+		require.NoError(t, err)
+
+		// Verify equality.
+		require.True(t, rawFeatures.Equals(decoded.RawFeatureVector))
+	})
+
+	// Test new format: with length prefix.
+	rapid.Check(t, func(t *rapid.T) {
+		// Generate random feature bits.
+		numFeatures := rapid.IntRange(0, 20).Draw(t, "numFeatures")
+		featureBits := make([]lnwire.FeatureBit, numFeatures)
+		for i := 0; i < numFeatures; i++ {
+			featureBits[i] = lnwire.FeatureBit(
+				rapid.IntRange(0, 255).Draw(t, "featureBit"),
+			)
+		}
+
+		// Create feature vector.
+		rawFeatures := lnwire.NewRawFeatureVector(featureBits...)
+		fv := lnwire.NewFeatureVector(rawFeatures, lnwire.Features)
+
+		// Encode with length prefix (new format).
+		var buf bytes.Buffer
+		err := fv.Encode(&buf)
+		require.NoError(t, err)
+
+		// Deserialize.
+		decoded, err := deserializeChanEdgeFeatures(buf.Bytes())
+		require.NoError(t, err)
+
+		// Verify equality.
+		require.True(t, rawFeatures.Equals(decoded.RawFeatureVector))
+	})
+}
+
+// TestDeserializeChanEdgeFeaturesLegacyFormatNoCollision verifies that
+// the format detection cannot have false positives where legacy format
+// bytes are incorrectly detected as new format.
+func TestDeserializeChanEdgeFeaturesLegacyFormatNoCollision(t *testing.T) {
+	t.Parallel()
+
+	// The detection works through canonical encoding verification.
+	// Even if a legacy vector accidentally matches the length check
+	// (e.g., a 258-byte vector starting with 0x01, 0x00), the decode/
+	// re-encode check will fail because the legacy encoding won't be
+	// canonical when interpreted as new format payload.
+
+	rapid.Check(t, func(t *rapid.T) {
+		// Generate feature bits with higher range to catch more edge
+		// cases, including vectors that could match the length check.
+		maxBit := rapid.IntRange(0, 2200).Draw(t, "maxBit")
+		numExtra := rapid.IntRange(0, 10).Draw(t, "numExtra")
+
+		featureBits := []lnwire.FeatureBit{lnwire.FeatureBit(maxBit)}
+		for i := 0; i < numExtra; i++ {
+			bit := rapid.IntRange(0, maxBit).Draw(t, "extraBit")
+			featureBits = append(featureBits,
+				lnwire.FeatureBit(bit))
+		}
+
+		rawFeatures := lnwire.NewRawFeatureVector(featureBits...)
+
+		// Encode in legacy format.
+		var buf bytes.Buffer
+		err := rawFeatures.EncodeBase256(&buf)
+		require.NoError(t, err)
+
+		legacyBytes := buf.Bytes()
+		if len(legacyBytes) < 2 {
+			// Single byte can't be confused with new format.
+			return
+		}
+
+		// Verify deserialization still works correctly regardless of
+		// whether the length check happens to match.
+		decoded, err := deserializeChanEdgeFeatures(legacyBytes)
+		require.NoError(t, err)
+		require.True(t, rawFeatures.Equals(decoded.RawFeatureVector),
+			"mismatch for legacy bytes %x with maxBit %d",
+			legacyBytes, maxBit)
+	})
+}
+
+// TestDeserializeChanEdgeFeaturesLengthCheckCollision specifically tests the
+// edge case where a legacy feature vector accidentally satisfies the length
+// check condition (first 2 bytes as uint16 == len - 2). This can happen with
+// a 258-byte vector starting with 0x01, 0x00, where 256 == 258 - 2.
+// The canonical encoding verification should correctly identify this as legacy
+// format.
+func TestDeserializeChanEdgeFeaturesLengthCheckCollision(t *testing.T) {
+	t.Parallel()
+
+	// Create a legacy feature vector that will produce bytes where the
+	// first two bytes, interpreted as a length, equal len - 2.
+	//
+	// To get 258 bytes in legacy format, we need bit 2063 set (258*8-1).
+	// The first byte will be 0x01 (bit 2056 is in byte 0, and we need
+	// bit 2063 which is 0x80, but the bytes are big-endian so byte 0
+	// contains the high bits). Actually let's work this out:
+	//
+	// For 258 bytes, bits 2056-2063 are in byte 0.
+	// Setting bit 2056 gives byte[0] = 0x01.
+	// If byte[0] = 0x01 and byte[1] = 0x00, then as uint16 = 256 = 258-2.
+	//
+	// So we need: bit 2056 set (gives 0x01 in byte 0), and no bits in
+	// byte 1 set (bits 2048-2055), and at least one bit set below to
+	// ensure we have full 258 bytes (bit 0 to ensure byte 257 is non-zero
+	// won't work since it affects the last byte...).
+	//
+	// Actually the encoding is that the first byte contains the HIGHEST
+	// bits. So for 258 bytes:
+	// - byte[0] contains bits 2056-2063
+	// - byte[1] contains bits 2048-2055
+	// - ...
+	// - byte[257] contains bits 0-7
+	//
+	// To get byte[0] = 0x01 and byte[1] = 0x00:
+	// - Set bit 2056 (gives 0x01 in byte 0)
+	// - Don't set bits 2048-2055 (keeps byte 1 = 0x00)
+	//
+	// We also need to set some lower bit to have meaningful features.
+	featureBits := []lnwire.FeatureBit{
+		2056, // This gives 0x01 in first byte (258 bytes total)
+		0,    // Set bit 0 for a meaningful feature
+	}
+
+	rawFeatures := lnwire.NewRawFeatureVector(featureBits...)
+
+	// Encode in legacy format.
+	var buf bytes.Buffer
+	err := rawFeatures.EncodeBase256(&buf)
+	require.NoError(t, err)
+
+	legacyBytes := buf.Bytes()
+	require.Len(t, legacyBytes, 258, "expected 258 bytes for bit 2056")
+
+	// Verify the collision condition: first 2 bytes as uint16 == len - 2.
+	encodedLen := binary.BigEndian.Uint16(legacyBytes[:2])
+	require.Equal(t, uint16(256), encodedLen,
+		"expected first 2 bytes to encode 256")
+	require.Equal(t, 256, len(legacyBytes)-2,
+		"expected length check to match")
+
+	// Despite the length check matching, deserialization should still
+	// correctly identify this as legacy format (via canonical encoding
+	// verification) and decode it properly.
+	decoded, err := deserializeChanEdgeFeatures(legacyBytes)
+	require.NoError(t, err)
+	require.True(t, decoded.IsSet(2056), "bit 2056 should be set")
+	require.True(t, decoded.IsSet(0), "bit 0 should be set")
+	require.True(t, rawFeatures.Equals(decoded.RawFeatureVector),
+		"feature vectors should match")
+}