lsp_plugin: check that featurebit is set and that

diff --git a/plugins/lsps-plugin/src/client.rs b/plugins/lsps-plugin/src/client.rs
index b100e5f4..52f1da61 100644
--- a/plugins/lsps-plugin/src/client.rs
+++ b/plugins/lsps-plugin/src/client.rs
@@ -1,13 +1,19 @@
-use anyhow::Context;
+use anyhow::{anyhow, Context};
 use cln_lsps::jsonrpc::client::JsonRpcClient;
 use cln_lsps::lsps0::{
     self,
     transport::{Bolt8Transport, CustomMessageHookManager, WithCustomMessageHookManager},
 };
+use cln_lsps::util;
+use cln_lsps::LSP_FEATURE_BIT;
 use cln_plugin::options;
+use cln_rpc::model::requests::ListpeersRequest;
+use cln_rpc::primitives::PublicKey;
+use cln_rpc::ClnRpc;
 use log::debug;
 use serde::Deserialize;
 use std::path::Path;
+use std::str::FromStr as _;
 
 /// An option to enable this service.
 const OPTION_ENABLED: options::FlagConfigOption = options::ConfigOption::new_flag(
@@ -66,9 +72,14 @@ async fn on_lsps_listprotocols(
     }
     let dir = p.configuration().lightning_dir;
     let rpc_path = Path::new(&dir).join(&p.configuration().rpc_file);
+    let mut cln_client = cln_rpc::ClnRpc::new(rpc_path.clone()).await?;
 
     let req: Request = serde_json::from_value(v).context("Failed to parse request JSON")?;
 
+    // Fail early: Check that we are connected to the peer and that it has the
+    // LSP feature bit set.
+    ensure_lsp_connected(&mut cln_client, &req.lsp_id).await?;
+
     // Create the transport first and handle potential errors
     let transport = Bolt8Transport::new(
         &req.lsp_id,
@@ -90,3 +101,43 @@ async fn on_lsps_listprotocols(
     debug!("Received lsps0.list_protocols response: {:?}", res);
     Ok(serde_json::to_value(res)?)
 }
+
+/// Checks that the node is connected to the peer and that it has the LSP
+/// feature bit set.
+async fn ensure_lsp_connected(cln_client: &mut ClnRpc, lsp_id: &str) -> Result<(), anyhow::Error> {
+    let res = cln_client
+        .call_typed(&ListpeersRequest {
+            id: Some(PublicKey::from_str(lsp_id)?),
+            level: None,
+        })
+        .await?;
+
+    // unwrap in next line is safe as we checked that an item exists before.
+    if res.peers.is_empty() || !res.peers.first().unwrap().connected {
+        debug!("Node isn't connected to lsp {lsp_id}");
+        return Err(anyhow!("not connected to lsp"));
+    }
+
+    res.peers
+        .first()
+        .filter(|peer| {
+            // Check that feature bit is set
+            peer.features.as_deref().map_or(false, |f_str| {
+                if let Some(feature_bits) = hex::decode(f_str).ok() {
+                    let mut fb = feature_bits.clone();
+                    fb.reverse();
+                    util::is_feature_bit_set(&fb, LSP_FEATURE_BIT)
+                } else {
+                    false
+                }
+            })
+        })
+        .ok_or_else(|| {
+            anyhow!(
+                "peer is not an lsp, feature bit {} is missing",
+                LSP_FEATURE_BIT,
+            )
+        })?;
+
+    Ok(())
+}
diff --git a/plugins/lsps-plugin/src/lib.rs b/plugins/lsps-plugin/src/lib.rs
index 2eb605d9..aa93d0ac 100644
--- a/plugins/lsps-plugin/src/lib.rs
+++ b/plugins/lsps-plugin/src/lib.rs
@@ -1,3 +1,5 @@
 pub mod jsonrpc;
 pub mod lsps0;
 pub mod util;
+
+pub const LSP_FEATURE_BIT: usize = 729;
diff --git a/plugins/lsps-plugin/src/service.rs b/plugins/lsps-plugin/src/service.rs
index a350a3d9..6e3a578f 100644
--- a/plugins/lsps-plugin/src/service.rs
+++ b/plugins/lsps-plugin/src/service.rs
@@ -3,10 +3,10 @@ use async_trait::async_trait;
 use cln_lsps::jsonrpc::server::{JsonRpcResponseWriter, RequestHandler};
 use cln_lsps::jsonrpc::{server::JsonRpcServer, JsonRpcRequest};
 use cln_lsps::jsonrpc::{JsonRpcResponse, RequestObject, RpcError, TransportError};
-use cln_lsps::lsps0;
 use cln_lsps::lsps0::model::{Lsps0listProtocolsRequest, Lsps0listProtocolsResponse};
 use cln_lsps::lsps0::transport::{self, CustomMsg};
 use cln_lsps::util::wrap_payload_with_peer_id;
+use cln_lsps::{lsps0, util, LSP_FEATURE_BIT};
 use cln_plugin::options::ConfigOption;
 use cln_plugin::{options, Plugin};
 use cln_rpc::notifications::CustomMsgNotification;
@@ -31,6 +31,14 @@ struct State {
 async fn main() -> Result<(), anyhow::Error> {
     if let Some(plugin) = cln_plugin::Builder::new(tokio::io::stdin(), tokio::io::stdout())
         .option(OPTION_ENABLED)
+        .featurebits(
+            cln_plugin::FeatureBitsKind::Node,
+            util::feature_bit_to_hex(LSP_FEATURE_BIT),
+        )
+        .featurebits(
+            cln_plugin::FeatureBitsKind::Init,
+            util::feature_bit_to_hex(LSP_FEATURE_BIT),
+        )
         .hook("custommsg", on_custommsg)
         .configure()
         .await?
diff --git a/plugins/lsps-plugin/src/util.rs b/plugins/lsps-plugin/src/util.rs
index 89f308ae..fe61bb37 100644
--- a/plugins/lsps-plugin/src/util.rs
+++ b/plugins/lsps-plugin/src/util.rs
@@ -5,6 +5,42 @@ use core::fmt;
 use serde_json::Value;
 use std::str::FromStr;
 
+/// Checks if the feature bit is set in the provided bitmap.
+/// Returns true if the `feature_bit` is set in the `bitmap`. Returns false if
+/// the `feature_bit` is unset or our ouf bounds.
+///
+/// # Arguments
+///
+/// * `bitmap`: A slice of bytes representing the feature bitmap.
+/// * `feature_bit`: The 0-based index of the bit to check across the bitmap.
+///
+pub fn is_feature_bit_set(bitmap: &[u8], feature_bit: usize) -> bool {
+    let byte_index = feature_bit >> 3; // Equivalent to feature_bit / 8
+    let bit_index = feature_bit & 7; // Equivalent to feature_bit % 8
+
+    if let Some(&target_byte) = bitmap.get(byte_index) {
+        let mask = 1 << bit_index;
+        (target_byte & mask) != 0
+    } else {
+        false
+    }
+}
+
+/// Returns a single feature_bit in hex representation, least-significant bit
+/// first.
+///
+/// # Arguments
+///
+/// * `feature_bit`: The 0-based index of the bit to check across the bitmap.
+///
+pub fn feature_bit_to_hex(feature_bit: usize) -> String {
+    let byte_index = feature_bit >> 3; // Equivalent to feature_bit / 8
+    let mask = 1 << (feature_bit & 7); // Equivalent to feature_bit % 8
+    let mut map = vec![0u8; byte_index + 1];
+    map[0] |= mask; // least-significant bit first ordering.
+    hex::encode(&map)
+}
+
 /// Errors that can occur when unwrapping payload data
 #[derive(Debug, Clone, PartialEq)]
 pub enum UnwrapError {
@@ -131,4 +167,81 @@ mod tests {
             Some(UnwrapError::InvalidPublicKey(_))
         ));
     }
+
+    #[test]
+    fn test_basic_bit_checks() {
+        // Example bitmap:
+        // Byte 0: 0b10100101 (165) -> Bits 0, 2, 5, 7 set
+        // Byte 1: 0b01101010 (106) -> Bits 1, 3, 5, 6 set (indices 9, 11, 13, 14)
+        let bitmap: &[u8] = &[0b10100101, 0b01101010];
+
+        // Check bits in byte 0 (indices 0-7)
+        assert_eq!(is_feature_bit_set(bitmap, 0), true); // Bit 0
+        assert_eq!(is_feature_bit_set(bitmap, 1), false); // Bit 1
+        assert_eq!(is_feature_bit_set(bitmap, 2), true); // Bit 2
+        assert_eq!(is_feature_bit_set(bitmap, 3), false); // Bit 3
+        assert_eq!(is_feature_bit_set(bitmap, 4), false); // Bit 4
+        assert_eq!(is_feature_bit_set(bitmap, 5), true); // Bit 5
+        assert_eq!(is_feature_bit_set(bitmap, 6), false); // Bit 6
+        assert_eq!(is_feature_bit_set(bitmap, 7), true); // Bit 7
+
+        // Check bits in byte 1 (indices 8-15)
+        assert_eq!(is_feature_bit_set(bitmap, 8), false); // Bit 8 (Byte 1, bit 0)
+        assert_eq!(is_feature_bit_set(bitmap, 9), true); // Bit 9 (Byte 1, bit 1)
+        assert_eq!(is_feature_bit_set(bitmap, 10), false); // Bit 10 (Byte 1, bit 2)
+        assert_eq!(is_feature_bit_set(bitmap, 11), true); // Bit 11 (Byte 1, bit 3)
+        assert_eq!(is_feature_bit_set(bitmap, 12), false); // Bit 12 (Byte 1, bit 4)
+        assert_eq!(is_feature_bit_set(bitmap, 13), true); // Bit 13 (Byte 1, bit 5)
+        assert_eq!(is_feature_bit_set(bitmap, 14), true); // Bit 14 (Byte 1, bit 6)
+        assert_eq!(is_feature_bit_set(bitmap, 15), false); // Bit 15 (Byte 1, bit 7)
+    }
+
+    #[test]
+    fn test_out_of_bounds() {
+        let bitmap: &[u8] = &[0b11111111, 0b00000000]; // 16 bits total
+
+        assert_eq!(is_feature_bit_set(bitmap, 15), false); // Last valid bit (is 0)
+        assert_eq!(is_feature_bit_set(bitmap, 16), false); // Out of bounds
+        assert_eq!(is_feature_bit_set(bitmap, 100), false); // Way out of bounds
+    }
+
+    #[test]
+    fn test_empty_bitmap() {
+        let bitmap: &[u8] = &[];
+        assert_eq!(is_feature_bit_set(bitmap, 0), false);
+        assert_eq!(is_feature_bit_set(bitmap, 8), false);
+    }
+
+    #[test]
+    fn test_feature_to_hex_bit_0_be() {
+        // Bit 0 is in Byte 0 (LE index). num_bytes=1. BE index = 1-1-0=0.
+        // Expected map: [0x01]
+        let feature_hex = feature_bit_to_hex(0);
+        assert_eq!(feature_hex, "01");
+        assert!(is_feature_bit_set(&hex::decode(feature_hex).unwrap(), 0));
+    }
+
+    #[test]
+    fn test_feature_to_hex_bit_8_be() {
+        // Bit 8 is in Byte 1 (LE index). num_bytes=2. BE index = 2-1-1=0.
+        // Mask is 0x01 for bit 0 within its byte.
+        // Expected map: [0x01, 0x00] (Byte for 8-15 first, then 0-7)
+        let feature_hex = feature_bit_to_hex(8);
+        let mut decoded = hex::decode(&feature_hex).unwrap();
+        decoded.reverse();
+        assert_eq!(feature_hex, "0100");
+        assert!(is_feature_bit_set(&decoded, 8));
+    }
+
+    #[test]
+    fn test_feature_to_hex_bit_27_be() {
+        // Bit 27 is in Byte 3 (LE index). num_bytes=4. BE index = 4-1-3=0.
+        // Mask is 0x08 for bit 3 within its byte.
+        // Expected map: [0x08, 0x00, 0x00, 0x00] (Byte for 24-31 first)
+        let feature_hex = feature_bit_to_hex(27);
+        let mut decoded = hex::decode(&feature_hex).unwrap();
+        decoded.reverse();
+        assert_eq!(feature_hex, "08000000");
+        assert!(is_feature_bit_set(&decoded, 27));
+    }
 }