Bitcoin Development Series, Part 4 of 4
The Lightning Network has had a quieter 2026 than its 2022–2023 growth years, but the development activity underneath the surface is some of the most consequential the protocol has seen. Two major releases dropped in the past two months: LND v0.21 from Lightning Labs and Core Lightning 26.06 from Blockstream, the latter shipping a feature whose name alone signals where the industry is heading, quantum-resistant Lightning channels.
This post covers what is new in both implementations, the technical foundations of how Lightning works, and what you need to know to build on the stack today.
How a Lightning Channel Works: The Basics for Developers
Before getting into what is new, let's have a grounding in the mechanics. A Lightning channel is a 2-of-2 multisig output on the Bitcoin base layer. The two parties hold a sequence of commitment transactions, off-chain transactions that reflect the current balance split between them. Neither party broadcasts these unless the channel is being closed.
When Alice pays Bob through a channel, they exchange new signed commitment transactions reflecting the updated balances, and invalidate the old ones using a penalty mechanism (revocation keys). No on-chain transaction happens. This is the core throughput gain: thousands of payments can flow through a channel with only two on-chain transactions (open and close).
# Simplified channel state model
from dataclasses import dataclass
@dataclass
class ChannelState:
channel_id: str
local_balance_sat: int # our side
remote_balance_sat: int # counterparty side
capacity_sat: int
commitment_number: int # increments with every update
def can_send(self, amount_sat: int) -> bool:
# Reserve 1% for channel reserve requirement
reserve = self.capacity_sat // 100
available = self.local_balance_sat - reserve
return available >= amount_sat
def update(self, amount_sat: int, direction: str) -> "ChannelState":
if direction == "send":
if not self.can_send(amount_sat):
raise ValueError(f"Insufficient local balance. Available: {self.local_balance_sat}")
return ChannelState(
channel_id=self.channel_id,
local_balance_sat=self.local_balance_sat - amount_sat,
remote_balance_sat=self.remote_balance_sat + amount_sat,
capacity_sat=self.capacity_sat,
commitment_number=self.commitment_number + 1,
)
elif direction == "receive":
return ChannelState(
channel_id=self.channel_id,
local_balance_sat=self.local_balance_sat + amount_sat,
remote_balance_sat=self.remote_balance_sat - amount_sat,
capacity_sat=self.capacity_sat,
commitment_number=self.commitment_number + 1,
)
raise ValueError(f"Unknown direction: {direction}")
# Example
ch = ChannelState(
channel_id="abc123",
local_balance_sat=500_000,
remote_balance_sat=500_000,
capacity_sat=1_000_000,
commitment_number=0,
)
ch = ch.update(100_000, "send")
print(f"After sending 100k sats:")
print(f" Local: {ch.local_balance_sat:,} sat")
print(f" Remote: {ch.remote_balance_sat:,} sat")
print(f" Commitment #: {ch.commitment_number}")
Payments across multiple hops use HTLCs (Hash Time-Locked Contracts) — conditional payments that are only claimable if the recipient reveals a preimage, and that expire if not claimed within a certain number of blocks.
What Is New: LND v0.21
Lightning Labs released LND v0.21 in June 2026. The headline themes were reliability, speed, and security hardening, a "grown up" release compared to the feature-heavy v0.20 cycle.
Key developer-relevant changes:
- HTLC CLTV delta improvements: More intelligent handling of expiry deltas to reduce the risk of force-close cascades during mempool fee spikes.
- Improved MPP (Multi-Path Payments): The router now better handles splitting payments across multiple paths, reducing failures on larger payments.
- gRPC streaming for channel events: Real-time channel state updates without polling.
-
AMP (Atomic Multi-Path) support in
QueryRoutesandBuildRoute: A long-awaited PR (#10801) adds AMP record support to these RPCs, making it possible to build AMP-aware routing tools without rewriting the payment lifecycle from scratch.
Connecting to LND via gRPC (Go)
package main
import (
"context"
"encoding/hex"
"fmt"
"log"
"os"
"google.golang.org/grpc"
"google.golang.org/grpc/credentials"
"gopkg.in/macaroon.v2"
lnrpc "github.com/lightningnetwork/lnd/lnrpc"
)
func loadMacaroon(path string) (string, error) {
data, err := os.ReadFile(path)
if err != nil {
return "", err
}
mac := &macaroon.Macaroon{}
if err := mac.UnmarshalBinary(data); err != nil {
return "", err
}
macBytes, err := mac.MarshalBinary()
if err != nil {
return "", err
}
return hex.EncodeToString(macBytes), nil
}
func main() {
tlsCert, err := credentials.NewClientTLSFromFile("/home/user/.lnd/tls.cert", "")
if err != nil {
log.Fatalf("failed to load TLS cert: %v", err)
}
conn, err := grpc.Dial(
"localhost:10009",
grpc.WithTransportCredentials(tlsCert),
)
if err != nil {
log.Fatalf("failed to connect: %v", err)
}
defer conn.Close()
client := lnrpc.NewLightningClient(conn)
ctx := context.Background()
info, err := client.GetInfo(ctx, &lnrpc.GetInfoRequest{})
if err != nil {
log.Fatalf("GetInfo failed: %v", err)
}
fmt.Printf("Node alias: %s\n", info.Alias)
fmt.Printf("Public key: %s\n", info.IdentityPubkey)
fmt.Printf("Active channels: %d\n", info.NumActiveChannels)
fmt.Printf("Pending channels: %d\n", info.NumPendingChannels)
fmt.Printf("Block height: %d\n", info.BlockHeight)
fmt.Printf("Synced to chain: %v\n", info.SyncedToChain)
}
Subscribing to Channel Events (gRPC Streaming)
stream, err := client.SubscribeChannelEvents(ctx, &lnrpc.ChannelEventSubscription{})
if err != nil {
log.Fatalf("SubscribeChannelEvents failed: %v", err)
}
for {
event, err := stream.Recv()
if err != nil {
log.Printf("stream error: %v", err)
break
}
switch event.Type {
case lnrpc.ChannelEventUpdate_OPEN_CHANNEL:
ch := event.GetOpenChannel()
fmt.Printf("Channel opened: %s | capacity: %d sat\n",
ch.ChannelPoint, ch.Capacity)
case lnrpc.ChannelEventUpdate_CLOSED_CHANNEL:
ch := event.GetClosedChannel()
fmt.Printf("Channel closed: %s | type: %s\n",
ch.ChannelPoint, ch.CloseType)
case lnrpc.ChannelEventUpdate_ACTIVE_CHANNEL:
fmt.Printf("Channel became active: %v\n", event.GetActiveChannel())
case lnrpc.ChannelEventUpdate_INACTIVE_CHANNEL:
fmt.Printf("Channel went inactive: %v\n", event.GetInactiveChannel())
}
}
What Is New: Core Lightning 26.06 — Quantum-Resistant Channels
Blockstream's Core Lightning released v26.06 in June 2026 with the headline feature of quantum-resistant Lightning channels the first Lightning implementation to do so, and it follows NEAR Protocol's base-layer quantum-safe signing that shipped on July 13 (covered in Part 1 of this series).
The implementation uses a hybrid approach: existing ECDSA/Schnorr signatures are retained for backwards compatibility with the current network, while a post-quantum signature scheme is layered on top for new channel types. A channel opened with a quantum-resistant peer uses both signature schemes, the classical one for network-wide compatibility and the PQ scheme as an additional binding that quantum-safe nodes can verify.
This is the same hybrid pattern NIST recommends for post-quantum migration: do not rip out classical crypto, add PQ on top and transition gradually.
Other notable 26.06 changes:
-
gracefulcommand: prepares a node for shutdown by closing channels cooperatively before stopping, reducing force-close fees. - Improved channel policy automation - fee rate adjustments based on liquidity depth without manual intervention.
Sending a Payment via Core Lightning (Python + pyln-client)
pip install pyln-client
from pyln.client import LightningRpc
import json
# Core Lightning uses a Unix socket for its RPC interface
rpc = LightningRpc("/home/user/.lightning/bitcoin/lightning-rpc")
def get_node_info() -> dict:
info = rpc.getinfo()
return {
"id": info["id"],
"alias": info.get("alias", ""),
"num_peers": info["num_peers"],
"num_channels": info["num_active_channels"],
"block_height": info["blockheight"],
"version": info["version"],
}
def list_channels() -> list[dict]:
channels = rpc.listchannels()["channels"]
return [
{
"source": ch["source"],
"destination": ch["destination"],
"capacity_sat": ch["satoshis"],
"active": ch["active"],
"base_fee_msat": ch["base_fee_millisatoshi"],
"fee_ppm": ch["fee_per_millionth"],
}
for ch in channels[:10] # first 10 for brevity
]
def pay_invoice(bolt11: str) -> dict:
"""
Pay a BOLT-11 invoice.
CLN returns payment details including preimage on success.
"""
result = rpc.pay(bolt11)
return {
"status": result["status"],
"amount_sent_msat": result.get("amount_sent_msat"),
"preimage": result.get("payment_preimage"),
"destination": result.get("destination"),
}
print(get_node_info())
Creating and Paying Invoices Programmatically
def create_invoice(amount_msat: int, description: str, expiry_seconds: int = 3600) -> dict:
"""
Generate a BOLT-11 invoice on this node.
amount_msat: amount in millisatoshis (1 sat = 1000 msat)
"""
label = f"invoice-{hash(description)}"
result = rpc.invoice(
amount_msat=amount_msat,
label=label,
description=description,
expiry=expiry_seconds,
)
return {
"bolt11": result["bolt11"],
"payment_hash": result["payment_hash"],
"expires_at": result["expires_at"],
}
# Create a 10,000 sat invoice
invoice = create_invoice(
amount_msat=10_000_000, # 10,000 sats = 10,000,000 msat
description="Test payment from post",
)
print("Invoice:", invoice["bolt11"][:60], "...")
Routing Fees and Liquidity Management
One of the persistent engineering challenges in Lightning is liquidity management, channels become unbalanced over time as payments flow preferentially in one direction. A channel where all the liquidity is on one side cannot route payments in the other direction, making it useless for half the traffic.
def analyze_channel_balance_health(rpc_conn: LightningRpc) -> list[dict]:
"""
Flags channels where local balance is heavily skewed,
which indicates a need for rebalancing.
"""
funds = rpc_conn.listfunds()
channels = funds.get("channels", [])
issues = []
for ch in channels:
total = ch["our_amount_msat"] + ch["amount_msat"]
if total == 0:
continue
local_pct = (ch["our_amount_msat"] / total) * 100
if local_pct < 10:
status = "depleted" # almost no outbound capacity
elif local_pct > 90:
status = "saturated" # almost no inbound capacity
else:
status = "healthy"
issues.append({
"peer_id": ch.get("peer_id", "unknown")[:20],
"local_pct": round(local_pct, 1),
"status": status,
"local_sat": ch["our_amount_msat"] // 1000,
"remote_sat": ch["amount_msat"] // 1000,
})
return sorted(issues, key=lambda x: x["local_pct"])
# Channels where local_pct < 10 or > 90 need rebalancing
Rebalancing is done by routing a circular payment: paying yourself through a path that exits on the saturated channel and returns on the depleted one. Both LND and CLN have built-in or plugin-based rebalancing tools, but understanding the mechanics matters when you are building systems that monitor channel health automatically.
The BOLT Specification: Where Protocol Changes Come From
Lightning's cross-implementation compatibility is maintained through the BOLT (Basis of Lightning Technology) specifications. Any change that affects how two different implementations interoperate: LND, CLN, Eclair, LDK, must go through the BOLT process.
Current active areas of BOLT development in 2026:
- Taproot channels (BOLT 3 update): Upgrading the on-chain commitment transaction structure to use Taproot outputs, which reduces the on-chain footprint of cooperative closes and improves privacy by making Lightning channels indistinguishable from ordinary Taproot spends.
- Route blinding (BOLT 4 update): Allows a recipient to hide their node identity from the sender while still receiving payments, critical for payment privacy.
- BOLT 12 (Offers): A more expressive invoice format that supports recurring payments, refunds, and metadata, without requiring a new invoice for every payment. Slowly rolling out across implementations.
# Decode a BOLT 12 offer (requires CLN's decode RPC)
def decode_bolt12_offer(rpc_conn: LightningRpc, offer_string: str) -> dict:
"""
BOLT 12 offers are reusable payment endpoints — unlike BOLT 11 invoices,
the same offer string can be used to request multiple invoices over time.
"""
result = rpc_conn.decode(offer_string)
return {
"type": result.get("type"),
"valid": result.get("valid"),
"description": result.get("description"),
"node_id": result.get("node_id", "")[:20] + "...",
"currency": result.get("currency", "bitcoin"),
"amount_msat": result.get("amount_msat"),
}
The Bigger Picture: Why Lightning Development Matters Now
Lightning is the primary answer to Bitcoin's throughput ceiling. The base layer processes roughly 7 transactions per second. Lightning, in theory, scales to millions of payments per second, the constraint is liquidity routing and channel management, not raw throughput.
With institutional adoption accelerating (Strategy's banking index, Morgan Stanley's ETF filings, DTCC's tokenization push), the pressure for Bitcoin to function as a payment network, not just a store of value, is growing. Lightning is the only production-ready answer to that. The quantum-resistance work in CLN 26.06 is particularly significant: it is the protocol community taking a 10–15 year horizon seriously, rather than just shipping features for the current market cycle.
If you are building anything that touches Bitcoin payments at the application layer, Lightning is no longer optional reading.
Further Reading
- LND v0.21 release notes — Lightning Labs
- Core Lightning 26.06 — Blockstream
- BOLT specifications — lightning/bolts GitHub
- pyln-client documentation
- LND gRPC API reference
This is Part 4 of 4 in the Bitcoin Development Series. Parts 1–3 cover BIP-110 and soft fork mechanics, how Ordinals work technically, and post-halving mining economics.
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