Merge pull request #10396 from ziggie1984/enhance-lsp-heuritic

Enhance Lsp Heuristic when probing a payment

docs/estimate_route_fee.md

@@ -150,52 +150,58 @@ probing.
 
 The heuristic examines the structure of route hints provided in the invoice to
 identify characteristic LSP patterns. The detection operates on the principle
-that LSPs typically maintain private channels to their users and appear as the
-penultimate hop in payment routing.
+that LSPs typically maintain private channels to their users and appear as
+public nodes in the network, while the final destination is private.
 
 ```mermaid
 flowchart TD
     Start([Route Hints Received]) --> Empty{Empty Hints?}
     Empty -->|Yes| NotLSP([Not LSP])
-    Empty -->|No| GetFirst[Get First Hint's Last Hop]
-    
-    GetFirst --> CheckPub1{Is Channel<br/>Public?}
-    CheckPub1 -->|Yes| NotLSP
-    CheckPub1 -->|No| SaveNode[Save Node ID]
-    
-    SaveNode --> MoreHints{More Hints?}
-    MoreHints -->|No| IsLSP([Detected as LSP])
+    Empty -->|No| CheckTarget{Invoice Target<br/>in Graph?}
+
+    CheckTarget -->|Yes| NotLSP
+    CheckTarget -->|No| GetFirstDest[Get First Hint's<br/>Destination Hop]
+
+    GetFirstDest --> CheckPub1{Destination Node<br/>in Graph?}
+    CheckPub1 -->|Yes| IsLSP([Detected as LSP])
+    CheckPub1 -->|No| MoreHints{More Hints?}
+
+    MoreHints -->|No| NotLSP
     MoreHints -->|Yes| NextHint[Check Next Hint]
-    
-    NextHint --> GetLast[Get Last Hop]
-    GetLast --> CheckPub2{Is Channel<br/>Public?}
-    CheckPub2 -->|Yes| NotLSP
-    CheckPub2 -->|No| SameNode{Same Node ID<br/>as First?}
-    
-    SameNode -->|No| NotLSP
-    SameNode -->|Yes| MoreHints
+
+    NextHint --> GetNextDest[Get Destination Hop]
+    GetNextDest --> CheckPub2{Destination Node<br/>in Graph?}
+    CheckPub2 -->|Yes| IsLSP
+    CheckPub2 -->|No| MoreHints
 ```
 
-The detection criteria are:
+The detection follows three simple rules applied sequentially:
 
-- **All route hints must terminate at the same node ID** - This indicates a
-  single destination behind potentially multiple LSP entry points
+**Rule 1: Public Invoice Target → NOT an LSP**
+- If the invoice target (destination) is a public node that exists in the
+  channel graph, the payment can be routed directly to it
+- This means it's not an LSP setup, regardless of what route hints are provided
+- Example: A well-connected merchant node with route hints for liquidity
+  signaling
 
-- **Final hop channels must be private** - The channels in the last hop of
-  each route hint must not exist in the public channel graph
+**Rule 2: Public Destination Hop → IS an LSP**
+- If at least one route hint has a destination hop (last hop in the route hint)
+  that is a public node in the graph, LSP detection is triggered
+- This indicates the destination hop is an LSP serving a private client
+- The private client is reached through the LSP's private channel
 
-- **No public channels in final hops** - If any route hint contains a public
-  channel in its final hop, LSP detection is disabled entirely
-
-- **Multiple route hints strengthen detection** - While not required,
-  multiple hints converging on the same destination strongly suggest an LSP
-  configuration
+**Rule 3: All Private Destination Hops → NOT an LSP**
+- If all destination hops in all route hints are private nodes (not in the
+  public graph), this is not treated as an LSP setup
+- The payment will be routed directly to the invoice destination using the
+  route hints as additional path information
+- This is the standard case for private channel payments
 
 This pattern effectively distinguishes LSP configurations from other routing
 scenarios. For instance, some Lightning implementations like CLN include route
 hints even for public nodes to signal liquidity availability or preferred
-routing paths. The heuristic correctly identifies these as non-LSP scenarios by
-detecting the presence of public channels.
+routing paths. The heuristic correctly identifies these as non-LSP scenarios
+by Rule 1 (detecting that the invoice target itself is public).
 
 ### How Probing Differs When an LSP is Detected
 
@@ -432,10 +438,6 @@ appropriately.
 The `EstimateRouteFee` implementation continues to evolve based on real-world
 usage patterns. Ongoing discussions in the LND community focus on:
 
-**Improved LSP Detection**: Developing more sophisticated heuristics that
-accurately identify LSP configurations while avoiding false positives for
-regular private channels.
-
 **Multi-Path Payment Support**: Extending fee estimation to support MPP
 scenarios where payments split across multiple routes.
 

docs/release-notes/release-notes-0.20.1.md

@@ -55,6 +55,14 @@
 
 ## RPC Updates
 
+ * The `EstimateRouteFee` RPC now implements an [LSP detection 
+   heuristic](https://github.com/lightningnetwork/lnd/pull/10396) that probes up
+   to 3 unique Lightning Service Providers when route hints indicate an LSP
+   setup. The implementation returns worst-case (most expensive) fee estimates
+   for conservative budgeting and includes griefing protection by limiting the
+   number of probed LSPs. It enhances the previous LSP design by being more
+   generic and more flexible.
+
 ## lncli Updates
 
 ## Breaking Changes

graph/db/kv_store.go

@@ -3431,11 +3431,10 @@ func (c *KVStore) fetchLightningNode(tx kvdb.RTx,
 	return node, nil
 }
 
-// HasLightningNode determines if the graph has a vertex identified by the
-// target node identity public key. If the node exists in the database, a
-// timestamp of when the data for the node was lasted updated is returned along
-// with a true boolean. Otherwise, an empty time.Time is returned with a false
-// boolean.
+// HasNode determines if the graph has a vertex identified by the target node
+// identity public key. If the node exists in the database, a timestamp of when
+// the data for the node was lasted updated is returned along with a true
+// boolean. Otherwise, an empty time.Time is returned with a false boolean.
 func (c *KVStore) HasNode(_ context.Context,
 	nodePub [33]byte) (time.Time, bool, error) {
 

itest/lnd_estimate_route_fee_test.go

@@ -59,18 +59,38 @@ type estimateRouteFeeTestCase struct {
 }
 
 // testEstimateRouteFee tests the estimation of routing fees using either graph
-// data or sending out a probe payment.
+// data or sending out a probe payment. This test validates graph-based fee
+// estimation, probe-based fee estimation with single LSP, probe-based fee
+// estimation with multiple route hints to same LSP (worst-case fee selection),
+// probe-based fee estimation with multiple different public LSPs (worst-case
+// fee selection across LSPs, up to MaxLspsToProbe), and non-LSP probing (all
+// private destination hops).
+//
+// Note: We test with exactly MaxLspsToProbe (3) LSPs. Testing with more LSPs
+// is not feasible because the LSP selection uses map iteration, which has
+// non-deterministic order in Go, making it impossible to predict which LSPs
+// will be probed.
 func testEstimateRouteFee(ht *lntest.HarnessTest) {
+	// Ensure MaxLspsToProbe is set to 3 as expected by this test. The test
+	// uses exactly 3 LSPs in the multi-LSP test case. If MaxLspsToProbe
+	// changes, this assertion will fail as a reminder to update the test.
+	require.Equal(ht, 3, routerrpc.MaxLspsToProbe,
+		"MaxLspsToProbe should be 3")
+
 	mts := newMppTestScenario(ht)
 
-	// We extend the regular mpp test scenario with a new node Paula. Paula
-	// is connected to Bob and Eve through private channels.
+	// We extend the regular mpp test scenario with two new nodes:
+	// - Paula: connected to Bob and Eve through private channels
+	// - Frank: connected to Dave through a private channel
+	//
 	//                  /-------------\
 	//              _ Eve _  (private) \
 	//             /       \            \
 	// Alice -- Carol ---- Bob --------- Paula
 	//      \              /   (private)
 	//       \__ Dave ____/
+	//             \
+	//              \__ Frank (private)
 	//
 	req := &mppOpenChannelRequest{
 		amtAliceCarol: 200_000,
@@ -88,6 +108,7 @@ func testEstimateRouteFee(ht *lntest.HarnessTest) {
 	probeInitiator = mts.alice
 
 	paula := ht.NewNode("Paula", nil)
+	frank := ht.NewNode("Frank", nil)
 
 	// The channel from Bob to Paula actually doesn't have enough liquidity
 	// to carry out the probe. We assume in normal operation that hop hints
@@ -106,6 +127,13 @@ func testEstimateRouteFee(ht *lntest.HarnessTest) {
 		Amt:     1_000_000,
 	})
 
+	// Frank is a private node connected to Dave (public LSP).
+	ht.EnsureConnected(mts.dave, frank)
+	ht.OpenChannel(mts.dave, frank, lntest.OpenChannelParams{
+		Private: true,
+		Amt:     1_000_000,
+	})
+
 	bobsPrivChannels := mts.bob.RPC.ListChannels(&lnrpc.ListChannelsRequest{
 		PrivateOnly: true,
 	})
@@ -118,6 +146,14 @@ func testEstimateRouteFee(ht *lntest.HarnessTest) {
 	require.Len(ht, evesPrivChannels.Channels, 1)
 	evePaulaChanID := evesPrivChannels.Channels[0].ChanId
 
+	davesPrivChannels := mts.dave.RPC.ListChannels(
+		&lnrpc.ListChannelsRequest{
+			PrivateOnly: true,
+		},
+	)
+	require.Len(ht, davesPrivChannels.Channels, 1)
+	daveFrankChanID := davesPrivChannels.Channels[0].ChanId
+
 	// Let's disable the paths from Alice to Bob through Dave and Eve with
 	// high fees. This ensures that the path estimates are based on Carol's
 	// channel to Bob for the first set of tests.
@@ -196,6 +232,33 @@ func testEstimateRouteFee(ht *lntest.HarnessTest) {
 				},
 			},
 		}
+
+		daveHopHint = &lnrpc.HopHint{
+			NodeId:                    mts.dave.PubKeyStr,
+			FeeBaseMsat:               3_000,
+			FeeProportionalMillionths: 3_000,
+			CltvExpiryDelta:           120,
+			ChanId:                    daveFrankChanID,
+		}
+
+		// Multiple different public LSPs (Bob, Eve, Dave).
+		multipleLspsRouteHints = []*lnrpc.RouteHint{
+			{
+				HopHints: []*lnrpc.HopHint{
+					bobHopHint,
+				},
+			},
+			{
+				HopHints: []*lnrpc.HopHint{
+					eveHopHint,
+				},
+			},
+			{
+				HopHints: []*lnrpc.HopHint{
+					daveHopHint,
+				},
+			},
+		}
 	)
 
 	defaultTimelock := int64(chainreg.DefaultBitcoinTimeLockDelta)
@@ -231,6 +294,14 @@ func testEstimateRouteFee(ht *lntest.HarnessTest) {
 	feeACEP := feeEP + feeCE
 	deltaACEP := deltaCE + deltaEP
 
+	// For multiple LSPs test, the route with the highest fee should be
+	// selected (Eve). Note that we return both fee and CLTV delta from
+	// the same route (the highest-fee route), not the max fee and max
+	// delta independently. This ensures the returned values represent an
+	// actual viable route.
+	highestFeeRouteFee := feeACEP
+	highestFeeRouteDelta := deltaACEP
+
 	initialBlockHeight := int64(mts.alice.RPC.GetInfo().BlockHeight)
 
 	// Locktime is always composed of the initial block height and the
@@ -271,6 +342,19 @@ func testEstimateRouteFee(ht *lntest.HarnessTest) {
 			expectedCltvDelta:       locktime + deltaCB,
 			expectedFailureReason:   failureReasonNone,
 		},
+		// Rule 1: Invoice target is public (Bob), even with public
+		// destination hop hints. Should route directly to Bob, NOT
+		// treat as LSP.
+		{
+			name: "probe based estimate, public " +
+				"target with public hop hints",
+			probing:                 true,
+			destination:             mts.bob,
+			routeHints:              singleRouteHint,
+			expectedRoutingFeesMsat: feeStandardSingleHop,
+			expectedCltvDelta:       locktime + deltaCB,
+			expectedFailureReason:   failureReasonNone,
+		},
 		// We expect the previous probing results adjusted by Paula's
 		// hop data.
 		{
@@ -340,6 +424,23 @@ func testEstimateRouteFee(ht *lntest.HarnessTest) {
 			expectedCltvDelta:       0,
 			expectedFailureReason:   failureReasonNoRoute,
 		},
+		// Test multiple different public LSPs. The worst-case (most
+		// expensive) route should be returned. Eve has the highest
+		// fees among the 3 LSPs tested. Note: We don't test with more
+		// than MaxLspsToProbe LSPs because map iteration order in Go
+		// is non-deterministic, making it impossible to predict which
+		// LSPs will be selected for probing.
+		{
+			name: "probe based estimate, " +
+				"multiple different public LSPs",
+			probing:                 true,
+			destination:             frank,
+			routeHints:              multipleLspsRouteHints,
+			expectedRoutingFeesMsat: highestFeeRouteFee,
+			expectedCltvDelta: locktime +
+				highestFeeRouteDelta,
+			expectedFailureReason: failureReasonNone,
+		},
 	}
 
 	for _, testCase := range testCases {

lnrpc/routerrpc/router_backend.go

@@ -64,6 +64,11 @@ type RouterBackend struct {
 	FetchChannelEndpoints func(chanID uint64) (route.Vertex,
 		route.Vertex, error)
 
+	// HasNode returns true if the node exists in the graph (i.e., has
+	// public channels), false otherwise. This means the node is a public
+	// node and should be reachable.
+	HasNode func(nodePub route.Vertex) (bool, error)
+
 	// FindRoute is a closure that abstracts away how we locate/query for
 	// routes.
 	FindRoute func(*routing.RouteRequest) (*route.Route, float64, error)

lnrpc/routerrpc/router_server.go

@@ -1,7 +1,6 @@
 package routerrpc
 
 import (
-	"bytes"
 	"context"
 	crand "crypto/rand"
 	"errors"
@@ -44,6 +43,12 @@ const (
 	// DefaultPaymentTimeout is the default value of time we should spend
 	// when attempting to fulfill the payment.
 	DefaultPaymentTimeout int32 = 60
+
+	// MaxLspsToProbe is the maximum number of LSPs to probe when
+	// estimating fees for worst-case fee estimation. This is a
+	// precautionary measure to prevent the estimation from taking too
+	// long, and it is also a griefing protection.
+	MaxLspsToProbe = 3
 )
 
 var (
@@ -171,10 +176,9 @@ var (
 	DefaultRouterMacFilename = "router.macaroon"
 )
 
-// FetchChannelEndpoints returns the pubkeys of both endpoints of the
-// given channel id if it exists in the graph.
-type FetchChannelEndpoints func(chanID uint64) (route.Vertex, route.Vertex,
-	error)
+// HasNode returns true if the node exists in the graph (i.e., has public
+// channels), false otherwise.
+type HasNode func(nodePub route.Vertex) (bool, error)
 
 // ServerShell is a shell struct holding a reference to the actual sub-server.
 // It is used to register the gRPC sub-server with the root server before we
@@ -561,7 +565,8 @@ func (s *Server) probePaymentRequest(ctx context.Context, paymentRequest string,
 	// If the hints don't indicate an LSP then chances are that our probe
 	// payment won't be blocked along the route to the destination. We send
 	// a probe payment with unmodified route hints.
-	if !isLSP(hints, s.cfg.RouterBackend.FetchChannelEndpoints) {
+	invoiceTargetCompressed := payReq.Destination.SerializeCompressed()
+	if !isLSP(hints, invoiceTargetCompressed, s.cfg.RouterBackend.HasNode) {
 		log.Infof("No LSP detected, probing destination %x",
 			probeRequest.Dest)
 
@@ -569,200 +574,342 @@ func (s *Server) probePaymentRequest(ctx context.Context, paymentRequest string,
 		return s.sendProbePayment(ctx, probeRequest)
 	}
 
-	// If the heuristic indicates an LSP we modify the route hints to allow
-	// probing the LSP.
-	lspAdjustedRouteHints, lspHint, err := prepareLspRouteHints(
-		hints, *payReq.MilliSat,
+	// If the heuristic indicates an LSP, we filter and group route hints by
+	// public LSP nodes, then probe each unique LSP separately and return
+	// the cheapest route.
+	lspGroups, err := prepareLspRouteHints(
+		hints, *payReq.MilliSat, s.cfg.RouterBackend.HasNode,
 	)
 	if err != nil {
 		return nil, err
 	}
 
-	// Set the destination to the LSP node ID.
-	lspDest := lspHint.NodeID.SerializeCompressed()
-	probeRequest.Dest = lspDest
+	log.Infof("LSP detected, found %d unique public LSP node(s) to probe",
+		len(lspGroups))
 
-	log.Infof("LSP detected, probing LSP with destination: %x", lspDest)
-
-	// The adjusted route hints serve the payment probe to find the last
-	// public hop to the LSP on the route.
-	if len(lspAdjustedRouteHints) > 0 {
-		probeRequest.RouteHints = invoicesrpc.CreateRPCRouteHints(
-			lspAdjustedRouteHints,
-		)
+	// Probe up to MaxLspsToProbe LSPs and track the most expensive route
+	// for worst-case fee estimation.
+	if len(lspGroups) > MaxLspsToProbe {
+		log.Debugf("Limiting LSP probes from %d to %d for worst-case "+
+			"fee estimation", len(lspGroups), MaxLspsToProbe)
 	}
+	var (
+		worstCaseResp    *RouteFeeResponse
+		worstCaseLspDest route.Vertex
+		probeCount       int
+	)
 
-	// The payment probe will be able to calculate the fee up until the LSP
-	// node. The fee of the last hop has to be calculated manually. Since
-	// the last hop's fee amount has to be sent across the payment path we
-	// have to add it to the original payment amount. Only then will the
-	// payment probe be able to determine the correct fee to the last hop
-	// prior to the private destination. For example, if the user wants to
-	// send 1000 sats to a private destination and the last hop's fee is 10
-	// sats, then 1010 sats will have to arrive at the last hop. This means
-	// that the probe has to be dispatched with 1010 sats to correctly
-	// calculate the routing fee.
-	//
-	// Calculate the hop fee for the last hop manually.
-	hopFee := lspHint.HopFee(*payReq.MilliSat)
-	if err != nil {
-		return nil, err
-	}
+	for lspKey, group := range lspGroups {
+		if probeCount >= MaxLspsToProbe {
+			break
+		}
+		probeCount++
 
-	// Add the last hop's fee to the requested payment amount that we want
-	// to get an estimate for.
-	probeRequest.AmtMsat += int64(hopFee)
+		lspHint := group.LspHopHint
 
-	// Use the hop hint's cltv delta as the payment request's final cltv
-	// delta. The actual final cltv delta of the invoice will be added to
-	// the payment probe's cltv delta.
-	probeRequest.FinalCltvDelta = int32(lspHint.CLTVExpiryDelta)
+		log.Infof("Probing LSP with destination: %v", lspKey)
 
-	// Dispatch the payment probe with adjusted fee amount.
-	resp, err := s.sendProbePayment(ctx, probeRequest)
-	if err != nil {
-		return nil, fmt.Errorf("failed to send probe payment to "+
-			"LSP with destination %x: %w", lspDest, err)
-	}
+		// Create a new probe request for this LSP.
+		lspProbeRequest := &SendPaymentRequest{
+			TimeoutSeconds:   probeRequest.TimeoutSeconds,
+			Dest:             lspKey[:],
+			MaxParts:         probeRequest.MaxParts,
+			AllowSelfPayment: probeRequest.AllowSelfPayment,
+			AmtMsat:          amtMsat,
+			PaymentHash:      probeRequest.PaymentHash,
+			FeeLimitSat:      probeRequest.FeeLimitSat,
+			FinalCltvDelta:   int32(lspHint.CLTVExpiryDelta),
+			DestFeatures:     probeRequest.DestFeatures,
+		}
 
-	// If the payment probe failed we only return the failure reason and
-	// leave the probe result params unaltered.
-	if resp.FailureReason != lnrpc.PaymentFailureReason_FAILURE_REASON_NONE { //nolint:ll
-		return resp, nil
-	}
+		// Copy the payment address if present.
+		if len(probeRequest.PaymentAddr) > 0 {
+			copy(
+				lspProbeRequest.PaymentAddr,
+				probeRequest.PaymentAddr,
+			)
+		}
 
-	// The probe succeeded, so we can add the last hop's fee to fee the
-	// payment probe returned.
-	resp.RoutingFeeMsat += int64(hopFee)
+		// Set the adjusted route hints for this LSP.
+		if len(group.AdjustedRouteHints) > 0 {
+			lspProbeRequest.RouteHints = invoicesrpc.
+				CreateRPCRouteHints(group.AdjustedRouteHints)
+		}
 
-	// Add the final cltv delta of the invoice to the payment probe's total
-	// cltv delta. This is the cltv delta for the hop behind the LSP.
-	resp.TimeLockDelay += int64(payReq.MinFinalCLTVExpiry())
+		// Calculate the hop fee for the last hop manually.
+		hopFee := lspHint.HopFee(*payReq.MilliSat)
 
-	return resp, nil
-}
+		// Add the last hop's fee to the probe amount.
+		lspProbeRequest.AmtMsat += int64(hopFee)
 
-// isLSP checks if the route hints indicate an LSP. An LSP is indicated with
-// true if the destination hop hint in each route hint has the same node id,
-// false otherwise. If the destination hop hint of any route hint contains a
-// public channel, the function returns false because we can directly send a
-// probe to the final destination.
-func isLSP(routeHints [][]zpay32.HopHint,
-	fetchChannelEndpoints FetchChannelEndpoints) bool {
-
-	if len(routeHints) == 0 || len(routeHints[0]) == 0 {
-		return false
-	}
-
-	destHopHint := routeHints[0][len(routeHints[0])-1]
-
-	// If the destination hop hint of the first route hint contains a public
-	// channel we can send a probe to it directly, hence we don't signal an
-	// LSP.
-	_, _, err := fetchChannelEndpoints(destHopHint.ChannelID)
-	if err == nil {
-		return false
-	}
-
-	for i := 1; i < len(routeHints); i++ {
-		// Skip empty route hints.
-		if len(routeHints[i]) == 0 {
+		// Dispatch the payment probe for this LSP.
+		resp, err := s.sendProbePayment(ctx, lspProbeRequest)
+		if err != nil {
+			log.Warnf("Failed to probe LSP %v: %v", lspKey, err)
 			continue
 		}
 
-		lastHop := routeHints[i][len(routeHints[i])-1]
+		// If the probe failed, skip this LSP.
+		if resp.FailureReason !=
+			lnrpc.PaymentFailureReason_FAILURE_REASON_NONE {
 
-		// If the last hop hint of any route hint contains a public
-		// channel we can send a probe to it directly, hence we don't
-		// signal an LSP.
-		_, _, err = fetchChannelEndpoints(lastHop.ChannelID)
-		if err == nil {
-			return false
+			log.Debugf("Probe to LSP %v failed with reason: %v",
+				lspKey, resp.FailureReason)
+
+			continue
 		}
 
-		matchesDestNode := bytes.Equal(
-			lastHop.NodeID.SerializeCompressed(),
-			destHopHint.NodeID.SerializeCompressed(),
-		)
-		if !matchesDestNode {
+		// The probe succeeded, add the last hop's fee.
+		resp.RoutingFeeMsat += int64(hopFee)
+
+		// Add the final cltv delta of the invoice.
+		resp.TimeLockDelay += int64(payReq.MinFinalCLTVExpiry())
+
+		log.Infof("Probe to LSP %v succeeded with fee: %d msat",
+			lspKey, resp.RoutingFeeMsat)
+
+		// Track the most expensive route for worst-case estimation.
+		// We solely consider the routing fee for the worst-case
+		// estimation.
+		if worstCaseResp == nil ||
+			resp.RoutingFeeMsat > worstCaseResp.RoutingFeeMsat {
+
+			if worstCaseResp != nil {
+				log.Debugf("LSP %v has higher fee "+
+					"(%d msat) than current worst-case "+
+					"%v (%d msat), updating worst-case "+
+					"estimate", lspKey,
+					resp.RoutingFeeMsat, worstCaseLspDest,
+					worstCaseResp.RoutingFeeMsat)
+			}
+
+			worstCaseResp = resp
+			worstCaseLspDest = lspKey
+		} else {
+			log.Debugf("LSP %v fee (%d msat) is lower than "+
+				"current worst-case %v (%d msat), keeping "+
+				"worst-case estimate", lspKey,
+				resp.RoutingFeeMsat, worstCaseLspDest,
+				worstCaseResp.RoutingFeeMsat)
+		}
+	}
+
+	// If no LSP probe succeeded, return an error.
+	if worstCaseResp == nil {
+		return nil, fmt.Errorf("all LSP probe payments failed")
+	}
+
+	log.Infof("Returning worst-case route via LSP %v with fee: %d msat, "+
+		"timelock: %d", worstCaseLspDest, worstCaseResp.RoutingFeeMsat,
+		worstCaseResp.TimeLockDelay)
+
+	return worstCaseResp, nil
+}
+
+// isLSP checks if the route hints indicate an LSP setup. An LSP setup is
+// identified when the invoice destination is private but the final hop in the
+// route hints is a public node (the LSP). This function implements three rules:
+//
+//  1. If the invoice target is a public node (exists in graph) => isLsp = false
+//     We can route directly to the target, so no LSP is involved.
+//
+//  2. If at least one destination hop hint (last hop in route hint) is public
+//     => isLsp = true. The public destination hop is the LSP, and the actual
+//     invoice target is a private node behind it.
+//
+//  3. If all destination hop hints are private nodes => isLsp = false.
+//     We assume this is NOT an LSP setup. Instead, we expect the route hints
+//     contain public nodes earlier in the path (not the final hop) that our
+//     pathfinder can route to. For example:
+//     The pathfinder will route to PublicNode and use the hints from there.
+//     Note: If no public nodes exist anywhere in the route hints, the
+//     destination would be unreachable (malformed invoice), but we don't
+//     validate that here.
+func isLSP(routeHints [][]zpay32.HopHint, invoiceTarget []byte,
+	hasNode HasNode) bool {
+
+	if len(routeHints) == 0 || len(routeHints[0]) == 0 {
+		log.Debugf("No route hints provided, this is not an LSP setup")
+		return false
+	}
+
+	// Rule 1: If the invoice target is a public node (exists in the graph),
+	// we can route directly to it, so it's not an LSP setup.
+	if len(invoiceTarget) > 0 {
+		var targetVertex route.Vertex
+		copy(targetVertex[:], invoiceTarget)
+
+		isPublic, err := hasNode(targetVertex)
+		if err != nil {
+			log.Warnf("Failed to check if invoice target %x is "+
+				"public: %v", invoiceTarget, err)
+
+			return false
+		}
+		if isPublic {
+			log.Infof("Invoice target %x is a public node in the "+
+				"graph, this is NOT an LSP setup",
+				invoiceTarget)
+
 			return false
 		}
 	}
 
-	// We ensured that the destination hop hint doesn't contain a public
-	// channel, and that all destination hop hints of all route hints match,
-	// so we signal an LSP.
-	return true
+	for _, hopHints := range routeHints {
+		// Skip empty route hints.
+		if len(hopHints) == 0 {
+			continue
+		}
+
+		lastHop := hopHints[len(hopHints)-1]
+		lastHopNodeCompressed := lastHop.NodeID.SerializeCompressed()
+
+		// Check if this destination hop hint node is public.
+		// Rule 2: If we find a public node, we can exit early.
+		var lastHopVertex route.Vertex
+		copy(lastHopVertex[:], lastHopNodeCompressed)
+
+		isPublic, err := hasNode(lastHopVertex)
+		if err != nil {
+			log.Warnf("Failed to check if destination hop "+
+				"hint %x is public: %v", lastHopNodeCompressed,
+				err)
+
+			continue
+		}
+		if isPublic {
+			log.Infof("Destination hop hint %x is a public node, "+
+				"this is an LSP setup", lastHopNodeCompressed)
+
+			return true
+		}
+	}
+
+	// Rule 3: If all destination hop hints are private nodes (not in the
+	// graph), this is NOT an LSP setup. We assume the route hints contain
+	// public nodes earlier in the path that we can route through using
+	// standard pathfinding with the hints.
+	log.Infof("All destination hop hints are private, this is NOT an " +
+		"LSP setup")
+
+	return false
+}
+
+// LspRouteGroup represents a group of route hints that share the same public
+// LSP destination node. This is needed when probing LSPs separately to find
+// the cheapest route.
+type LspRouteGroup struct {
+	// LspHopHint is the hop hint for the LSP node with worst-case fees and
+	// CLTV delta.
+	LspHopHint *zpay32.HopHint
+
+	// AdjustedRouteHints are the route hints with the LSP hop stripped off.
+	AdjustedRouteHints [][]zpay32.HopHint
 }
 
 // prepareLspRouteHints assumes that the isLsp heuristic returned true for the
-// route hints passed in here. It constructs a modified list of route hints that
-// allows the caller to probe the LSP, which itself is returned as a separate
-// hop hint.
+// route hints passed in here. It filters route hints to only include those with
+// public destination nodes, groups them by unique LSP node, and returns a map
+// of LSP groups keyed by the LSP node's compressed public key.
 func prepareLspRouteHints(routeHints [][]zpay32.HopHint,
-	amt lnwire.MilliSatoshi) ([][]zpay32.HopHint, *zpay32.HopHint, error) {
+	amt lnwire.MilliSatoshi,
+	hasNode HasNode) (map[route.Vertex]*LspRouteGroup, error) {
 
+	// This should never happen, but we check for it for completeness.
+	// Because the isLSP heuristic already checked that the route hints are
+	// not empty.
 	if len(routeHints) == 0 {
-		return nil, nil, fmt.Errorf("no route hints provided")
+		return nil, fmt.Errorf("no route hints provided")
 	}
 
-	// Create the LSP hop hint. We are probing for the worst case fee and
-	// cltv delta. So we look for the max values amongst all LSP hop hints.
-	refHint := routeHints[0][len(routeHints[0])-1]
-	refHint.CLTVExpiryDelta = maxLspCltvDelta(routeHints)
-	refHint.FeeBaseMSat, refHint.FeeProportionalMillionths = maxLspFee(
-		routeHints, amt,
-	)
+	// Map to group route hints by LSP node pubkey.
+	lspGroups := make(map[route.Vertex]*LspRouteGroup)
 
-	// We construct a modified list of route hints that allows the caller to
-	// probe the LSP.
-	adjustedHints := make([][]zpay32.HopHint, 0, len(routeHints))
+	for _, routeHint := range routeHints {
+		// Skip empty route hints.
+		if len(routeHint) == 0 {
+			continue
+		}
 
-	// Strip off the LSP hop hint from all route hints.
-	for i := 0; i < len(routeHints); i++ {
-		hint := routeHints[i]
-		if len(hint) > 1 {
-			adjustedHints = append(
-				adjustedHints, hint[:len(hint)-1],
+		// Get the destination hop hint (last hop in the route).
+		destHop := routeHint[len(routeHint)-1]
+		destNodeCompressed := destHop.NodeID.SerializeCompressed()
+
+		// Check if this destination node is public.
+		var destVertex route.Vertex
+		copy(destVertex[:], destNodeCompressed)
+
+		isPublic, err := hasNode(destVertex)
+		if err != nil {
+			log.Warnf("Failed to check if dest hop hint %x is "+
+				"public: %v", destNodeCompressed, err)
+
+			continue
+		}
+
+		// Skip private destination nodes - we only probe public LSPs.
+		if !isPublic {
+			log.Debugf("Skipping route hint with private dest "+
+				"node %x", destNodeCompressed)
+
+			continue
+		}
+
+		// Use the compressed pubkey as the map key.
+		var lspKey route.Vertex
+		copy(lspKey[:], destNodeCompressed)
+
+		// Get or create the LSP group for this node.
+		group, exists := lspGroups[lspKey]
+		if !exists {
+			//nolint:ll
+			lspHop := zpay32.HopHint{
+				NodeID:                    destHop.NodeID,
+				ChannelID:                 destHop.ChannelID,
+				FeeBaseMSat:               destHop.FeeBaseMSat,
+				FeeProportionalMillionths: destHop.FeeProportionalMillionths,
+				CLTVExpiryDelta:           destHop.CLTVExpiryDelta,
+			}
+			group = &LspRouteGroup{
+				LspHopHint:         &lspHop,
+				AdjustedRouteHints: make([][]zpay32.HopHint, 0),
+			}
+			lspGroups[lspKey] = group
+		}
+
+		// Update the LSP hop hint with worst-case (max) fees and CLTV.
+		hopFee := destHop.HopFee(amt)
+		currentMaxFee := group.LspHopHint.HopFee(amt)
+		if hopFee > currentMaxFee {
+			group.LspHopHint.FeeBaseMSat = destHop.FeeBaseMSat
+			group.LspHopHint.FeeProportionalMillionths = destHop.
+				FeeProportionalMillionths
+		}
+
+		if destHop.CLTVExpiryDelta > group.LspHopHint.CLTVExpiryDelta {
+			group.LspHopHint.CLTVExpiryDelta = destHop.
+				CLTVExpiryDelta
+		}
+
+		// Add the route hint with the LSP hop stripped off (if there
+		// are hops before the LSP).
+		if len(routeHint) > 1 {
+			group.AdjustedRouteHints = append(
+				group.AdjustedRouteHints,
+				routeHint[:len(routeHint)-1],
 			)
 		}
 	}
 
-	return adjustedHints, &refHint, nil
-}
-
-// maxLspFee returns base fee and fee rate amongst all LSP route hints that
-// results in the overall highest fee for the given amount.
-func maxLspFee(routeHints [][]zpay32.HopHint, amt lnwire.MilliSatoshi) (uint32,
-	uint32) {
-
-	var maxFeePpm uint32
-	var maxBaseFee uint32
-	var maxTotalFee lnwire.MilliSatoshi
-	for _, rh := range routeHints {
-		lastHop := rh[len(rh)-1]
-		lastHopFee := lastHop.HopFee(amt)
-		if lastHopFee > maxTotalFee {
-			maxTotalFee = lastHopFee
-			maxBaseFee = lastHop.FeeBaseMSat
-			maxFeePpm = lastHop.FeeProportionalMillionths
-		}
+	if len(lspGroups) == 0 {
+		return nil, fmt.Errorf("no public LSP nodes found in " +
+			"route hints")
 	}
 
-	return maxBaseFee, maxFeePpm
-}
+	log.Infof("Found %d unique public LSP node(s) in route hints",
+		len(lspGroups))
 
-// maxLspCltvDelta returns the maximum cltv delta amongst all LSP route hints.
-func maxLspCltvDelta(routeHints [][]zpay32.HopHint) uint16 {
-	var maxCltvDelta uint16
-	for _, rh := range routeHints {
-		rhLastHop := rh[len(rh)-1]
-		if rhLastHop.CLTVExpiryDelta > maxCltvDelta {
-			maxCltvDelta = rhLastHop.CLTVExpiryDelta
-		}
-	}
-
-	return maxCltvDelta
+	return lspGroups, nil
 }
 
 // probePaymentStream is a custom implementation of the grpc.ServerStream

lnrpc/routerrpc/router_server_test.go

@@ -1,12 +1,12 @@
 package routerrpc
 
 import (
+	"bytes"
 	"context"
 	"testing"
 	"time"
 
 	"github.com/btcsuite/btcd/btcec/v2"
-	graphdb "github.com/lightningnetwork/lnd/graph/db"
 	"github.com/lightningnetwork/lnd/lnrpc"
 	"github.com/lightningnetwork/lnd/lnwire"
 	paymentsdb "github.com/lightningnetwork/lnd/payments/db"
@@ -220,12 +220,18 @@ func TestTrackPaymentsNoInflightUpdates(t *testing.T) {
 	require.Equal(t, lnrpc.Payment_SUCCEEDED, payment.Status)
 }
 
-// TestIsLsp tests the isLSP heuristic. Combinations of different route hints
-// with different fees and cltv deltas are tested to ensure that the heuristic
-// correctly identifies whether a route leads to an LSP or not.
+// TestIsLsp tests the isLSP heuristic. It validates all three LSP detection
+// rules:
+// Rule 1: Invoice target is public => not LSP.
+// Rule 2: All destination hop hints are private => not LSP (Boltz case).
+// Rule 3: At least one destination hop hint is public => LSP (Muun case).
 func TestIsLsp(t *testing.T) {
-	probeAmtMsat := lnwire.MilliSatoshi(1_000_000)
-
+	// Setup test nodes:
+	// - Alice: public node (in graph)
+	// - Bob: private node
+	// - Carol: private node
+	// - Dave: public node (in graph)
+	// - Eve: private node
 	alicePrivKey, err := btcec.NewPrivateKey()
 	require.NoError(t, err)
 	alicePubKey := alicePrivKey.PubKey()
@@ -242,216 +248,519 @@ func TestIsLsp(t *testing.T) {
 	require.NoError(t, err)
 	davePubKey := davePrivKey.PubKey()
 
-	var (
-		aliceHopHint = zpay32.HopHint{
-			NodeID:                    alicePubKey,
-			FeeBaseMSat:               100,
-			FeeProportionalMillionths: 1_000,
-			ChannelID:                 421337,
-		}
+	evePrivKey, err := btcec.NewPrivateKey()
+	require.NoError(t, err)
+	evePubKey := evePrivKey.PubKey()
 
-		bobHopHint = zpay32.HopHint{
-			NodeID:                    bobPubKey,
-			FeeBaseMSat:               2_000,
-			FeeProportionalMillionths: 2_000,
-			CLTVExpiryDelta:           288,
-			ChannelID:                 815,
-		}
+	// Create hop hints for each node.
+	aliceHopHint := zpay32.HopHint{
+		NodeID:                    alicePubKey,
+		FeeBaseMSat:               100,
+		FeeProportionalMillionths: 1_000,
+		CLTVExpiryDelta:           40,
+		ChannelID:                 1,
+	}
 
-		carolHopHint = zpay32.HopHint{
-			NodeID:                    carolPubKey,
-			FeeBaseMSat:               2_000,
-			FeeProportionalMillionths: 2_000,
-			ChannelID:                 815,
-		}
+	bobHopHint := zpay32.HopHint{
+		NodeID:                    bobPubKey,
+		FeeBaseMSat:               2_000,
+		FeeProportionalMillionths: 2_000,
+		CLTVExpiryDelta:           144,
+		ChannelID:                 2,
+	}
 
-		daveHopHint = zpay32.HopHint{
-			NodeID:                    davePubKey,
-			FeeBaseMSat:               2_000,
-			FeeProportionalMillionths: 2_000,
-			ChannelID:                 815,
-		}
+	carolHopHint := zpay32.HopHint{
+		NodeID:                    carolPubKey,
+		FeeBaseMSat:               1_500,
+		FeeProportionalMillionths: 1_500,
+		CLTVExpiryDelta:           144,
+		ChannelID:                 3,
+	}
 
-		publicChannelID       = uint64(42)
-		daveHopHintPublicChan = zpay32.HopHint{
-			NodeID:                    davePubKey,
-			FeeBaseMSat:               2_000,
-			FeeProportionalMillionths: 2_000,
-			ChannelID:                 publicChannelID,
-		}
-	)
+	daveHopHint := zpay32.HopHint{
+		NodeID:                    davePubKey,
+		FeeBaseMSat:               3_000,
+		FeeProportionalMillionths: 3_000,
+		CLTVExpiryDelta:           288,
+		ChannelID:                 4,
+	}
 
-	bobExpensiveCopy := bobHopHint.Copy()
-	bobExpensiveCopy.FeeBaseMSat = 1_000_000
-	bobExpensiveCopy.FeeProportionalMillionths = 1_000_000
-	bobExpensiveCopy.CLTVExpiryDelta = bobHopHint.CLTVExpiryDelta - 1
+	eveHopHint := zpay32.HopHint{
+		NodeID:                    evePubKey,
+		FeeBaseMSat:               500,
+		FeeProportionalMillionths: 500,
+		CLTVExpiryDelta:           40,
+		ChannelID:                 5,
+	}
 
-	//nolint:ll
-	lspTestCases := []struct {
-		name           string
-		routeHints     [][]zpay32.HopHint
-		probeAmtMsat   lnwire.MilliSatoshi
-		isLsp          bool
-		expectedHints  [][]zpay32.HopHint
-		expectedLspHop *zpay32.HopHint
+	// Mock hasNode: returns true only for alice and dave.
+	hasNode := func(nodePub route.Vertex) (bool, error) {
+		aliceVertex := route.NewVertex(alicePubKey)
+		daveVertex := route.NewVertex(davePubKey)
+		return bytes.Equal(nodePub[:], aliceVertex[:]) ||
+			bytes.Equal(nodePub[:], daveVertex[:]), nil
+	}
+
+	tests := []struct {
+		name          string
+		routeHints    [][]zpay32.HopHint
+		invoiceTarget []byte
+		expectLSP     bool
 	}{
+		// Edge cases.
 		{
-			name:           "empty route hints",
-			routeHints:     [][]zpay32.HopHint{{}},
-			probeAmtMsat:   probeAmtMsat,
-			isLsp:          false,
-			expectedHints:  [][]zpay32.HopHint{},
-			expectedLspHop: nil,
+			name:          "no route hints",
+			routeHints:    [][]zpay32.HopHint{},
+			invoiceTarget: nil,
+			expectLSP:     false,
 		},
 		{
-			name:           "single route hint",
-			routeHints:     [][]zpay32.HopHint{{daveHopHint}},
-			probeAmtMsat:   probeAmtMsat,
-			isLsp:          true,
-			expectedHints:  [][]zpay32.HopHint{},
-			expectedLspHop: &daveHopHint,
+			name:          "empty route hint array",
+			routeHints:    [][]zpay32.HopHint{{}},
+			invoiceTarget: nil,
+			expectLSP:     false,
 		},
+
+		// Rule 1: Invoice target is public => NOT an LSP.
+		// Rationale: Can route directly to public target.
 		{
-			name: "single route, multiple hints",
-			routeHints: [][]zpay32.HopHint{{
-				aliceHopHint, bobHopHint,
-			}},
-			probeAmtMsat:   probeAmtMsat,
-			isLsp:          true,
-			expectedHints:  [][]zpay32.HopHint{{aliceHopHint}},
-			expectedLspHop: &bobHopHint,
-		},
-		{
-			name: "multiple routes, multiple hints",
+			name: "invoice target is public (alice)",
 			routeHints: [][]zpay32.HopHint{
-				{
-					aliceHopHint, bobHopHint,
-				},
-				{
-					carolHopHint, bobHopHint,
-				},
+				{bobHopHint, carolHopHint},
 			},
-			probeAmtMsat: probeAmtMsat,
-			isLsp:        true,
-			expectedHints: [][]zpay32.HopHint{
-				{aliceHopHint}, {carolHopHint},
-			},
-			expectedLspHop: &bobHopHint,
+			invoiceTarget: alicePubKey.SerializeCompressed(),
+			expectLSP:     false,
 		},
 		{
-			name: "multiple routes, multiple hints with min length",
+			name: "invoice target is public with public dest hop",
 			routeHints: [][]zpay32.HopHint{
-				{
-					bobHopHint,
-				},
-				{
-					carolHopHint, bobHopHint,
-				},
+				{bobHopHint, daveHopHint},
 			},
-			probeAmtMsat: probeAmtMsat,
-			isLsp:        true,
-			expectedHints: [][]zpay32.HopHint{
-				{carolHopHint},
-			},
-			expectedLspHop: &bobHopHint,
+			invoiceTarget: davePubKey.SerializeCompressed(),
+			expectLSP:     false,
 		},
 		{
-			name: "multiple routes, multiple hints, diff fees+cltv",
+			name: "invoice target is public with multiple routes",
 			routeHints: [][]zpay32.HopHint{
-				{
-					bobHopHint,
-				},
-				{
-					carolHopHint, bobExpensiveCopy,
-				},
+				{bobHopHint, carolHopHint},
+				{aliceHopHint, daveHopHint},
 			},
-			probeAmtMsat: probeAmtMsat,
-			isLsp:        true,
-			expectedHints: [][]zpay32.HopHint{
-				{carolHopHint},
-			},
-			expectedLspHop: &zpay32.HopHint{
-				NodeID:                    bobHopHint.NodeID,
-				ChannelID:                 bobHopHint.ChannelID,
-				FeeBaseMSat:               bobExpensiveCopy.FeeBaseMSat,
-				FeeProportionalMillionths: bobExpensiveCopy.FeeProportionalMillionths,
-				CLTVExpiryDelta:           bobHopHint.CLTVExpiryDelta,
+			invoiceTarget: alicePubKey.SerializeCompressed(),
+			expectLSP:     false,
+		},
+
+		// Rule 2: All destination hop hints are private => NOT an LSP.
+		// Rationale: The destination hop hint is private so it cannot
+		// be probed so we default to NOT an LSP.
+		{
+			name: "single route to private dest",
+			routeHints: [][]zpay32.HopHint{
+				{aliceHopHint, bobHopHint},
 			},
+			invoiceTarget: bobPubKey.SerializeCompressed(),
+			expectLSP:     false,
 		},
 		{
-			name: "multiple routes, different final hops",
+			name: "multiple routes, all to private dests",
 			routeHints: [][]zpay32.HopHint{
-				{
-					aliceHopHint, bobHopHint,
-				},
-				{
-					carolHopHint, daveHopHint,
-				},
+				{aliceHopHint, bobHopHint},
+				{daveHopHint, carolHopHint},
 			},
-			probeAmtMsat:   probeAmtMsat,
-			isLsp:          false,
-			expectedHints:  [][]zpay32.HopHint{},
-			expectedLspHop: nil,
+			invoiceTarget: nil,
+			expectLSP:     false,
 		},
 		{
-			name: "multiple routes, same public hops",
+			name: "single hop to private node",
 			routeHints: [][]zpay32.HopHint{
-				{
-					aliceHopHint, daveHopHintPublicChan,
-				},
-				{
-					carolHopHint, daveHopHintPublicChan,
-				},
+				{eveHopHint},
 			},
-			probeAmtMsat:   probeAmtMsat,
-			isLsp:          false,
-			expectedHints:  [][]zpay32.HopHint{},
-			expectedLspHop: nil,
+			invoiceTarget: evePubKey.SerializeCompressed(),
+			expectLSP:     false,
 		},
 		{
-			name: "multiple routes, same public hops",
+			name: "all routes to same private node",
 			routeHints: [][]zpay32.HopHint{
-				{
-					aliceHopHint, daveHopHint,
-				},
-				{
-					carolHopHint, daveHopHintPublicChan,
-				},
-				{
-					aliceHopHint, daveHopHintPublicChan,
-				},
+				{aliceHopHint, bobHopHint},
+				{daveHopHint, bobHopHint},
+				{carolHopHint, bobHopHint},
 			},
-			probeAmtMsat:   probeAmtMsat,
-			isLsp:          false,
-			expectedHints:  [][]zpay32.HopHint{},
-			expectedLspHop: nil,
+			invoiceTarget: nil,
+			expectLSP:     false,
+		},
+
+		// Rule 3: At least one destination hop is public => IS an LSP.
+		// Rationale: As long as there is at least one public
+		// destination route hint, it is an LSP setup and can be probed.
+		{
+			name: "single route to public dest (dave)",
+			routeHints: [][]zpay32.HopHint{
+				{bobHopHint, daveHopHint},
+			},
+			invoiceTarget: evePubKey.SerializeCompressed(),
+			expectLSP:     true,
+		},
+		{
+			name: "direct hop to public LSP (alice)",
+			routeHints: [][]zpay32.HopHint{
+				{aliceHopHint},
+			},
+			invoiceTarget: bobPubKey.SerializeCompressed(),
+			expectLSP:     true,
+		},
+		{
+			name: "multiple routes to same public LSP (dave)",
+			routeHints: [][]zpay32.HopHint{
+				{bobHopHint, daveHopHint},
+				{carolHopHint, daveHopHint},
+				{eveHopHint, daveHopHint},
+			},
+			invoiceTarget: nil,
+			expectLSP:     true,
+		},
+		{
+			name: "multiple routes to different public LSPs",
+			routeHints: [][]zpay32.HopHint{
+				{bobHopHint, aliceHopHint},
+				{carolHopHint, daveHopHint},
+			},
+			invoiceTarget: nil,
+			expectLSP:     true,
+		},
+		{
+			name: "mixed public and private dest hops",
+			routeHints: [][]zpay32.HopHint{
+				{aliceHopHint, bobHopHint},
+				{carolHopHint, daveHopHint},
+				{bobHopHint, eveHopHint},
+			},
+			invoiceTarget: nil,
+			expectLSP:     true,
+		},
+		{
+			name: "first route has public dest, rest private",
+			routeHints: [][]zpay32.HopHint{
+				{bobHopHint, aliceHopHint},
+				{carolHopHint, eveHopHint},
+			},
+			invoiceTarget: nil,
+			expectLSP:     true,
 		},
 	}
 
-	// Returns ErrEdgeNotFound for private channels.
-	fetchChannelEndpoints := func(chanID uint64) (route.Vertex,
-		route.Vertex, error) {
-
-		if chanID == publicChannelID {
-			return route.Vertex{}, route.Vertex{}, nil
-		}
-
-		return route.Vertex{}, route.Vertex{}, graphdb.ErrEdgeNotFound
-	}
-
-	for _, tc := range lspTestCases {
-		t.Run(tc.name, func(t *testing.T) {
-			isLsp := isLSP(tc.routeHints, fetchChannelEndpoints)
-			require.Equal(t, tc.isLsp, isLsp)
-			if !tc.isLsp {
-				return
-			}
-
-			adjustedHints, lspHint, _ := prepareLspRouteHints(
-				tc.routeHints, tc.probeAmtMsat,
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			result := isLSP(
+				tt.routeHints, tt.invoiceTarget, hasNode,
 			)
-			require.Equal(t, tc.expectedHints, adjustedHints)
-			require.Equal(t, tc.expectedLspHop, lspHint)
+			require.Equal(t, tt.expectLSP, result)
 		})
 	}
 }
+
+// TestPrepareLspRouteHints tests the prepareLspRouteHints function to ensure
+// it correctly filters, groups, and calculates worst-case fees for LSP routes.
+func TestPrepareLspRouteHints(t *testing.T) {
+	// Setup test nodes:
+	// - Alice: public LSP node (in graph)
+	// - Bob: private node
+	// - Carol: private node
+	// - Dave: public LSP node (in graph)
+	// - Eve: private node
+	alicePrivKey, err := btcec.NewPrivateKey()
+	require.NoError(t, err)
+	alicePubKey := alicePrivKey.PubKey()
+
+	bobPrivKey, err := btcec.NewPrivateKey()
+	require.NoError(t, err)
+	bobPubKey := bobPrivKey.PubKey()
+
+	carolPrivKey, err := btcec.NewPrivateKey()
+	require.NoError(t, err)
+	carolPubKey := carolPrivKey.PubKey()
+
+	davePrivKey, err := btcec.NewPrivateKey()
+	require.NoError(t, err)
+	davePubKey := davePrivKey.PubKey()
+
+	evePrivKey, err := btcec.NewPrivateKey()
+	require.NoError(t, err)
+	evePubKey := evePrivKey.PubKey()
+
+	// Create hop hints with varying fees and CLTV deltas.
+	aliceHopHint1 := zpay32.HopHint{
+		NodeID:                    alicePubKey,
+		FeeBaseMSat:               100,
+		FeeProportionalMillionths: 1_000,
+		CLTVExpiryDelta:           40,
+		ChannelID:                 1,
+	}
+
+	aliceHopHint2 := zpay32.HopHint{
+		NodeID:                    alicePubKey,
+		FeeBaseMSat:               200,
+		FeeProportionalMillionths: 2_000,
+		CLTVExpiryDelta:           80,
+		ChannelID:                 2,
+	}
+
+	bobHopHint := zpay32.HopHint{
+		NodeID:                    bobPubKey,
+		FeeBaseMSat:               500,
+		FeeProportionalMillionths: 500,
+		CLTVExpiryDelta:           144,
+		ChannelID:                 3,
+	}
+
+	carolHopHint := zpay32.HopHint{
+		NodeID:                    carolPubKey,
+		FeeBaseMSat:               300,
+		FeeProportionalMillionths: 300,
+		CLTVExpiryDelta:           40,
+		ChannelID:                 4,
+	}
+
+	daveHopHint1 := zpay32.HopHint{
+		NodeID:                    davePubKey,
+		FeeBaseMSat:               1_000,
+		FeeProportionalMillionths: 1_000,
+		CLTVExpiryDelta:           144,
+		ChannelID:                 5,
+	}
+
+	daveHopHint2 := zpay32.HopHint{
+		NodeID:                    davePubKey,
+		FeeBaseMSat:               2_000,
+		FeeProportionalMillionths: 500,
+		CLTVExpiryDelta:           288,
+		ChannelID:                 6,
+	}
+
+	eveHopHint := zpay32.HopHint{
+		NodeID:                    evePubKey,
+		FeeBaseMSat:               100,
+		FeeProportionalMillionths: 100,
+		CLTVExpiryDelta:           40,
+		ChannelID:                 7,
+	}
+
+	// Mock hasNode: returns true only for alice and dave.
+	hasNode := func(nodePub route.Vertex) (bool, error) {
+		aliceVertex := route.NewVertex(alicePubKey)
+		daveVertex := route.NewVertex(davePubKey)
+		return bytes.Equal(nodePub[:], aliceVertex[:]) ||
+			bytes.Equal(nodePub[:], daveVertex[:]), nil
+	}
+
+	amt := lnwire.MilliSatoshi(1_000_000)
+
+	tests := []struct {
+		name         string
+		routeHints   [][]zpay32.HopHint
+		expectedGrps int
+		validateFunc func(t *testing.T,
+			groups map[route.Vertex]*LspRouteGroup)
+	}{
+		{
+			name: "single public LSP with one route",
+			routeHints: [][]zpay32.HopHint{
+				{bobHopHint, aliceHopHint1},
+			},
+			expectedGrps: 1,
+			validateFunc: func(t *testing.T,
+				groups map[route.Vertex]*LspRouteGroup) {
+
+				require.Len(t, groups, 1)
+
+				// Find alice's group.
+				aliceKey := route.NewVertex(alicePubKey)
+				group, ok := groups[aliceKey]
+				require.True(t, ok, "alice group not found")
+
+				// Verify LSP hop hint.
+				require.Equal(t, aliceHopHint1.FeeBaseMSat,
+					group.LspHopHint.FeeBaseMSat)
+				require.Equal(t, aliceHopHint1.CLTVExpiryDelta,
+					group.LspHopHint.CLTVExpiryDelta)
+
+				// Verify adjusted route hints.
+				require.Len(t, group.AdjustedRouteHints, 1)
+				require.Len(t, group.AdjustedRouteHints[0], 1)
+				require.Equal(t, bobHopHint.NodeID,
+					group.AdjustedRouteHints[0][0].NodeID)
+			},
+		},
+		{
+			name: "single LSP with multiple routes, same fees",
+			routeHints: [][]zpay32.HopHint{
+				{bobHopHint, aliceHopHint1},
+				{carolHopHint, aliceHopHint1},
+			},
+			expectedGrps: 1,
+			validateFunc: func(t *testing.T,
+				groups map[route.Vertex]*LspRouteGroup) {
+
+				aliceKey := route.NewVertex(alicePubKey)
+				group, ok := groups[aliceKey]
+				require.True(t, ok, "alice group not found")
+
+				// Should have 2 adjusted route hints.
+				require.Len(t, group.AdjustedRouteHints, 2)
+
+				// Fees should match the single hop hint.
+				require.Equal(t, aliceHopHint1.FeeBaseMSat,
+					group.LspHopHint.FeeBaseMSat)
+				require.Equal(t, aliceHopHint1.CLTVExpiryDelta,
+					group.LspHopHint.CLTVExpiryDelta)
+			},
+		},
+		{
+			name: "single LSP with different fees, uses worst case",
+			routeHints: [][]zpay32.HopHint{
+				{bobHopHint, aliceHopHint1},
+				{carolHopHint, aliceHopHint2},
+			},
+			expectedGrps: 1,
+			validateFunc: func(t *testing.T,
+				groups map[route.Vertex]*LspRouteGroup) {
+
+				aliceKey := route.NewVertex(alicePubKey)
+				group, ok := groups[aliceKey]
+				require.True(t, ok, "alice group not found")
+
+				// Should use worst-case (higher) fees.
+				fee1 := aliceHopHint1.HopFee(amt)
+				fee2 := aliceHopHint2.HopFee(amt)
+				require.Greater(t, fee2, fee1,
+					"hint2 should have higher fees")
+
+				// Group should have hint2's fees.
+				require.Equal(t, aliceHopHint2.FeeBaseMSat,
+					group.LspHopHint.FeeBaseMSat)
+
+				//nolint:ll
+				require.Equal(t,
+					aliceHopHint2.FeeProportionalMillionths,
+					group.LspHopHint.FeeProportionalMillionths)
+
+				// Should use worst-case CLTV delta.
+				require.Equal(t, aliceHopHint2.CLTVExpiryDelta,
+					group.LspHopHint.CLTVExpiryDelta)
+			},
+		},
+		{
+			name: "multiple public LSPs",
+			routeHints: [][]zpay32.HopHint{
+				{bobHopHint, aliceHopHint1},
+				{carolHopHint, daveHopHint1},
+			},
+			expectedGrps: 2,
+			validateFunc: func(t *testing.T,
+				groups map[route.Vertex]*LspRouteGroup) {
+
+				require.Len(t, groups, 2)
+
+				aliceKey := route.NewVertex(alicePubKey)
+				daveKey := route.NewVertex(davePubKey)
+
+				_, hasAlice := groups[aliceKey]
+				_, hasDave := groups[daveKey]
+				require.True(t, hasAlice, "alice group missing")
+				require.True(t, hasDave, "dave group missing")
+			},
+		},
+		{
+			name: "filters out private dest hops",
+			routeHints: [][]zpay32.HopHint{
+				{aliceHopHint1, bobHopHint},
+				{carolHopHint, daveHopHint1},
+				{bobHopHint, eveHopHint},
+			},
+			expectedGrps: 1,
+			validateFunc: func(t *testing.T,
+				groups map[route.Vertex]*LspRouteGroup) {
+
+				require.Len(t, groups, 1)
+
+				daveKey := route.NewVertex(davePubKey)
+				group, ok := groups[daveKey]
+				require.True(t, ok, "dave group not found")
+
+				// Only one route hint should remain
+				require.Len(t, group.AdjustedRouteHints, 1)
+			},
+		},
+		{
+			name: "multiple routes to same LSP with varying CLTV",
+			routeHints: [][]zpay32.HopHint{
+				{bobHopHint, daveHopHint1},
+				{carolHopHint, daveHopHint2},
+			},
+			expectedGrps: 1,
+			validateFunc: func(t *testing.T,
+				groups map[route.Vertex]*LspRouteGroup) {
+
+				daveKey := route.NewVertex(davePubKey)
+				group, ok := groups[daveKey]
+				require.True(t, ok, "dave group not found")
+
+				// Should use maximum CLTV delta.
+				require.Equal(t, daveHopHint2.CLTVExpiryDelta,
+					group.LspHopHint.CLTVExpiryDelta)
+			},
+		},
+		{
+			name: "single hop to public LSP",
+			routeHints: [][]zpay32.HopHint{
+				{aliceHopHint1},
+			},
+			expectedGrps: 1,
+			validateFunc: func(t *testing.T,
+				groups map[route.Vertex]*LspRouteGroup) {
+
+				aliceKey := route.NewVertex(alicePubKey)
+				group, ok := groups[aliceKey]
+				require.True(t, ok, "alice group not found")
+
+				// No adjusted hints since it's a direct hop
+				require.Len(t, group.AdjustedRouteHints, 0)
+			},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			groups, err := prepareLspRouteHints(
+				tt.routeHints, amt, hasNode,
+			)
+			require.NoError(t, err)
+			require.Len(t, groups, tt.expectedGrps)
+
+			// Run custom validation if provided.
+			if tt.validateFunc != nil {
+				tt.validateFunc(t, groups)
+			}
+		})
+	}
+
+	// Error cases which in operation should never happen because we always
+	// call isLSP first to check if the route hints are an LSP setup.
+	t.Run("error: no route hints", func(t *testing.T) {
+		_, err := prepareLspRouteHints(
+			[][]zpay32.HopHint{}, amt, hasNode,
+		)
+		require.Error(t, err)
+		require.Contains(t, err.Error(), "no route hints")
+	})
+
+	t.Run("error: no public LSP nodes found", func(t *testing.T) {
+		// All private destination hops. If all destination hops are
+		// private we cannot probe any LSPs so we return an error.
+		routeHints := [][]zpay32.HopHint{
+			{aliceHopHint1, bobHopHint},
+			{daveHopHint1, carolHopHint},
+		}
+		_, err := prepareLspRouteHints(routeHints, amt, hasNode)
+		require.Error(t, err)
+		require.Contains(t, err.Error(), "no public LSP nodes found")
+	})
+}

rpcserver.go

@@ -695,6 +695,8 @@ func newRPCServer(cfg *Config, interceptorChain *rpcperms.InterceptorChain,
 // addDeps populates all dependencies needed by the RPC server, and any
 // of the sub-servers that it maintains. When this is done, the RPC server can
 // be started, and start accepting RPC calls.
+//
+//nolint:funlen
 func (r *rpcServer) addDeps(ctx context.Context, s *server,
 	macService *macaroons.Service,
 	subServerCgs *subRPCServerConfigs, atpl *autopilot.Manager,
@@ -744,6 +746,11 @@ func (r *rpcServer) addDeps(ctx context.Context, s *server,
 
 			return info.NodeKey1Bytes, info.NodeKey2Bytes, nil
 		},
+		HasNode: func(nodePub route.Vertex) (bool, error) {
+			_, exists, err := graph.HasNode(ctx, nodePub)
+
+			return exists, err
+		},
 		FindRoute:              s.chanRouter.FindRoute,
 		MissionControl:         s.defaultMC,
 		ActiveNetParams:        r.cfg.ActiveNetParams.Params,