lsp_plugin: add lsps2_buy request and handler

diff --git a/plugins/lsps-plugin/src/client.rs b/plugins/lsps-plugin/src/client.rs
index b861ca54..99bae347 100644
--- a/plugins/lsps-plugin/src/client.rs
+++ b/plugins/lsps-plugin/src/client.rs
@@ -1,17 +1,21 @@
 use anyhow::{anyhow, Context};
 use cln_lsps::jsonrpc::client::JsonRpcClient;
+use cln_lsps::lsps0::primitives::Msat;
 use cln_lsps::lsps0::{
     self,
     transport::{Bolt8Transport, CustomMessageHookManager, WithCustomMessageHookManager},
 };
-use cln_lsps::lsps2::model::{Lsps2GetInfoRequest, Lsps2GetInfoResponse};
+use cln_lsps::lsps2::model::{
+    compute_opening_fee, Lsps2BuyRequest, Lsps2BuyResponse, Lsps2GetInfoRequest,
+    Lsps2GetInfoResponse, OpeningFeeParams,
+};
 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::primitives::{AmountOrAny, PublicKey};
 use cln_rpc::ClnRpc;
-use log::debug;
+use log::{debug, info, warn};
 use serde::{Deserialize, Serialize};
 use std::path::Path;
 use std::str::FromStr as _;
@@ -51,6 +55,11 @@ async fn main() -> Result<(), anyhow::Error> {
             "Low-level command to request the opening fee menu of an LSP",
             on_lsps_lsps2_getinfo,
         )
+        .rpcmethod(
+            "lsps-lsps2-buy",
+            "Low-level command to return the lsps2.buy result from an ",
+            on_lsps_lsps2_buy,
+        )
         .configure()
         .await?
     {
@@ -108,6 +117,103 @@ async fn on_lsps_lsps2_getinfo(
     Ok(serde_json::to_value(info_res)?)
 }
 
+/// Rpc Method handler for `lsps-lsps2-buy`.
+async fn on_lsps_lsps2_buy(
+    p: cln_plugin::Plugin<State>,
+    v: serde_json::Value,
+) -> Result<serde_json::Value, anyhow::Error> {
+    let req: ClnRpcLsps2BuyRequest =
+        serde_json::from_value(v).context("Failed to parse request JSON")?;
+    debug!(
+        "Asking for a channel from lsp {} with opening fee params {:?} and payment size {:?}",
+        req.lsp_id, req.opening_fee_params, req.payment_size_msat
+    );
+
+    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?;
+
+    // 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 Transport and Client
+    let transport = Bolt8Transport::new(
+        &req.lsp_id,
+        rpc_path.clone(), // Clone path for potential reuse
+        p.state().hook_manager.clone(),
+        None, // Use default timeout
+    )
+    .context("Failed to create Bolt8Transport")?;
+    let client = JsonRpcClient::new(transport);
+
+    // Convert from AmountOrAny to Msat.
+    let payment_size_msat = if let Some(payment_size) = req.payment_size_msat {
+        match payment_size {
+            AmountOrAny::Amount(amount) => Some(Msat::from_msat(amount.msat())),
+            AmountOrAny::Any => None,
+        }
+    } else {
+        None
+    };
+
+    let selected_params = req.opening_fee_params;
+
+    if let Some(payment_size) = payment_size_msat {
+        if payment_size < selected_params.min_payment_size_msat {
+            return Err(anyhow!(
+                "Requested payment size {}msat is below minimum {}msat required by LSP",
+                payment_size,
+                selected_params.min_payment_size_msat
+            ));
+        }
+        if payment_size > selected_params.max_payment_size_msat {
+            return Err(anyhow!(
+                "Requested payment size {}msat is above maximum {}msat allowed by LSP",
+                payment_size,
+                selected_params.max_payment_size_msat
+            ));
+        }
+
+        let opening_fee = compute_opening_fee(
+            payment_size.msat(),
+            selected_params.min_fee_msat.msat(),
+            selected_params.proportional.ppm() as u64,
+        )
+        .ok_or_else(|| {
+            warn!(
+                "Opening fee calculation overflowed for payment size {}",
+                payment_size
+            );
+            anyhow!("failed to calculate opening fee")
+        })?;
+
+        info!(
+            "Calculated opening fee: {}msat for payment size {}msat",
+            opening_fee, payment_size
+        );
+    } else {
+        info!("No payment size specified, requesting JIT channel for a variable-amount invoice.");
+        // Check if the selected params allow for variable amount (implicitly they do if max > min)
+        if selected_params.min_payment_size_msat >= selected_params.max_payment_size_msat {
+            // This shouldn't happen if LSP follows spec, but good to check.
+            warn!("Selected fee params seem unsuitable for variable amount: min >= max");
+        }
+    }
+
+    debug!("Calling lsps2.buy for peer {}", req.lsp_id);
+    let buy_req = Lsps2BuyRequest {
+        opening_fee_params: selected_params, // Pass the chosen params back
+        payment_size_msat,
+    };
+    let buy_res: Lsps2BuyResponse = client
+        .call_typed(buy_req)
+        .await
+        .context("lsps2.buy call failed")?;
+
+    Ok(serde_json::to_value(buy_res)?)
+}
+
 async fn on_lsps_listprotocols(
     p: cln_plugin::Plugin<State>,
     v: serde_json::Value,
@@ -188,6 +294,14 @@ async fn ensure_lsp_connected(cln_client: &mut ClnRpc, lsp_id: &str) -> Result<(
     Ok(())
 }
 
+#[derive(Debug, Clone, Serialize, Deserialize)]
+struct ClnRpcLsps2BuyRequest {
+    lsp_id: String,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    payment_size_msat: Option<AmountOrAny>,
+    opening_fee_params: OpeningFeeParams,
+}
+
 #[derive(Debug, Clone, Serialize, Deserialize)]
 struct ClnRpcLsps2GetinfoRequest {
     lsp_id: String,
diff --git a/plugins/lsps-plugin/src/lsps2/handler.rs b/plugins/lsps-plugin/src/lsps2/handler.rs
index 2f9162ad..a2858795 100644
--- a/plugins/lsps-plugin/src/lsps2/handler.rs
+++ b/plugins/lsps-plugin/src/lsps2/handler.rs
@@ -1,14 +1,27 @@
 use crate::{
     jsonrpc::{server::RequestHandler, JsonRpcResponse as _, RequestObject, RpcError},
-    lsps2::model::{
-        Lsps2GetInfoRequest, Lsps2GetInfoResponse, Lsps2PolicyGetInfoRequest,
-        Lsps2PolicyGetInfoResponse, OpeningFeeParams, Promise,
+    lsps0::primitives::ShortChannelId,
+    lsps2::{
+        model::{
+            DatastoreEntry, Lsps2BuyRequest, Lsps2BuyResponse, Lsps2GetInfoRequest,
+            Lsps2GetInfoResponse, Lsps2PolicyGetInfoRequest, Lsps2PolicyGetInfoResponse,
+            OpeningFeeParams, Promise,
+        },
+        DS_MAIN_KEY, DS_SUB_KEY,
     },
     util::unwrap_payload_with_peer_id,
 };
 use anyhow::{Context, Result as AnyResult};
 use async_trait::async_trait;
-use cln_rpc::ClnRpc;
+use cln_rpc::{
+    model::{
+        requests::{DatastoreMode, DatastoreRequest, GetinfoRequest},
+        responses::{DatastoreResponse, GetinfoResponse},
+    },
+    ClnRpc,
+};
+use log::warn;
+use rand::{rng, Rng as _};
 use std::path::PathBuf;
 
 #[async_trait]
@@ -17,8 +30,14 @@ pub trait ClnApi: Send + Sync {
         &self,
         params: &Lsps2PolicyGetInfoRequest,
     ) -> AnyResult<Lsps2PolicyGetInfoResponse>;
+
+    async fn cln_getinfo(&self, params: &GetinfoRequest) -> AnyResult<GetinfoResponse>;
+
+    async fn cln_datastore(&self, params: &DatastoreRequest) -> AnyResult<DatastoreResponse>;
 }
 
+const DEFAULT_CLTV_EXPIRY_DELTA: u32 = 144;
+
 #[derive(Clone)]
 pub struct ClnApiRpc {
     rpc_path: PathBuf,
@@ -46,6 +65,22 @@ impl ClnApi for ClnApiRpc {
             .map_err(anyhow::Error::new)
             .with_context(|| "calling dev-lsps2-getpolicy")
     }
+
+    async fn cln_getinfo(&self, params: &GetinfoRequest) -> AnyResult<GetinfoResponse> {
+        let mut rpc = self.create_rpc().await?;
+        rpc.call_typed(params)
+            .await
+            .map_err(anyhow::Error::new)
+            .with_context(|| "calling getinfo")
+    }
+
+    async fn cln_datastore(&self, params: &DatastoreRequest) -> AnyResult<DatastoreResponse> {
+        let mut rpc = self.create_rpc().await?;
+        rpc.call_typed(params)
+            .await
+            .map_err(anyhow::Error::new)
+            .with_context(|| "calling datastore")
+    }
 }
 
 /// Handler for the `lsps2.get_info` method.
@@ -124,6 +159,106 @@ impl<T: ClnApi + 'static> RequestHandler for Lsps2GetInfoHandler<T> {
     }
 }
 
+pub struct Lsps2BuyHandler<A: ClnApi> {
+    pub api: A,
+    pub promise_secret: [u8; 32],
+}
+
+impl<A: ClnApi> Lsps2BuyHandler<A> {
+    pub fn new(api: A, promise_secret: [u8; 32]) -> Self {
+        Self {
+            api,
+            promise_secret,
+        }
+    }
+}
+
+#[async_trait]
+impl<A: ClnApi + 'static> RequestHandler for Lsps2BuyHandler<A> {
+    async fn handle(&self, payload: &[u8]) -> core::result::Result<Vec<u8>, RpcError> {
+        let (payload, peer_id) = unwrap_payload_with_peer_id(payload);
+
+        let req: RequestObject<Lsps2BuyRequest> = serde_json::from_slice(&payload)
+            .map_err(|e| RpcError::parse_error(format!("Failed to parse request: {}", e)))?;
+
+        if req.id.is_none() {
+            // Is a notification we can not reply so we just return
+            return Ok(vec![]);
+        }
+
+        let req_params = req
+            .params
+            .ok_or_else(|| RpcError::invalid_request("Missing params field"))?;
+
+        let fee_params = req_params.opening_fee_params;
+
+        // FIXME: In the future we should replace the \`None\` with a meaningful
+        // value that reflects the inbound capacity for this node from the
+        // public network for a better pre-condition check on the payment_size.
+        fee_params.validate(&self.promise_secret, req_params.payment_size_msat, None)?;
+
+        // Generate a tmp scid to identify jit channel request in htlc.
+        let get_info_req = GetinfoRequest {};
+        let info = self.api.cln_getinfo(&get_info_req).await.map_err(|e| {
+            warn!("Failed to call getinfo via rpc {}", e);
+            RpcError::internal_error("Internal error")
+        })?;
+
+        // FIXME: Future task: Check that we don't conflict with any jit scid we
+        // already handed out -> Check datastore entries.
+        let jit_scid_u64 = generate_jit_scid(info.blockheight);
+        let jit_scid = ShortChannelId::from(jit_scid_u64);
+        let ds_data = DatastoreEntry {
+            peer_id,
+            opening_fee_params: fee_params,
+            expected_payment_size: req_params.payment_size_msat,
+        };
+        let ds_json = serde_json::to_string(&ds_data).map_err(|e| {
+            warn!("Failed to serialize opening fee params to string {}", e);
+            RpcError::internal_error("Internal error")
+        })?;
+
+        let ds_req = DatastoreRequest {
+            generation: None,
+            hex: None,
+            mode: Some(DatastoreMode::MUST_CREATE),
+            string: Some(ds_json),
+            key: vec![
+                DS_MAIN_KEY.to_string(),
+                DS_SUB_KEY.to_string(),
+                jit_scid.to_string(),
+            ],
+        };
+
+        let _ds_res = self.api.cln_datastore(&ds_req).await.map_err(|e| {
+            warn!("Failed to store jit request in ds via rpc {}", e);
+            RpcError::internal_error("Internal error")
+        })?;
+
+        let res = Lsps2BuyResponse {
+            jit_channel_scid: jit_scid,
+            // We can make this configurable if necessary.
+            lsp_cltv_expiry_delta: DEFAULT_CLTV_EXPIRY_DELTA,
+            // We can implement the other mode later on as we might have to do
+            // some additional work on core-lightning to enable this.
+            client_trusts_lsp: false,
+        }
+        .into_response(req.id.unwrap()); // We checked that we got an id before.
+
+        serde_json::to_vec(&res)
+            .map_err(|e| RpcError::internal_error(format!("Failed to serialize response: {}", e)))
+    }
+}
+
+fn generate_jit_scid(best_blockheigt: u32) -> u64 {
+    let mut rng = rng();
+    let block = best_blockheigt + 6; // Approx 1 hour in the future and should avoid collision with confirmed channels
+    let tx_idx: u32 = rng.random_range(0..5000);
+    let output_idx: u16 = rng.random_range(0..10);
+
+    ((block as u64) << 40) | ((tx_idx as u64) << 16) | (output_idx as u64)
+}
+
 #[cfg(test)]
 mod tests {
     use std::sync::{Arc, Mutex};
@@ -136,8 +271,9 @@ mod tests {
         util::wrap_payload_with_peer_id,
     };
     use chrono::{TimeZone, Utc};
-    use cln_rpc::primitives::PublicKey;
+    use cln_rpc::primitives::{Amount, PublicKey};
     use cln_rpc::RpcError as ClnRpcError;
+    use serde::Serialize;
 
     const PUBKEY: [u8; 33] = [
         0x02, 0x79, 0xbe, 0x66, 0x7e, 0xf9, 0xdc, 0xbb, 0xac, 0x55, 0xa0, 0x62, 0x95, 0xce, 0x87,
@@ -149,15 +285,45 @@ mod tests {
         PublicKey::from_slice(&PUBKEY).expect("Valid pubkey")
     }
 
-    fn create_wrapped_request(request: &RequestObject<Lsps2GetInfoRequest>) -> Vec<u8> {
+    fn create_wrapped_request<T: Serialize>(request: &RequestObject<T>) -> Vec<u8> {
         let payload = serde_json::to_vec(request).expect("Failed to serialize request");
         wrap_payload_with_peer_id(&payload, create_peer_id())
     }
 
+    /// Build a pair: policy params + buy params with a Promise derived from `secret`
+    fn params_with_promise(secret: &[u8; 32]) -> (PolicyOpeningFeeParams, OpeningFeeParams) {
+        let policy = PolicyOpeningFeeParams {
+            min_fee_msat: Msat(2_000),
+            proportional: Ppm(10_000),
+            valid_until: Utc.with_ymd_and_hms(2100, 1, 1, 0, 0, 0).unwrap(),
+            min_lifetime: 1000,
+            max_client_to_self_delay: 42,
+            min_payment_size_msat: Msat(1_000_000),
+            max_payment_size_msat: Msat(100_000_000),
+        };
+        let hex = policy.get_hmac_hex(secret);
+        let promise: Promise = hex.try_into().expect("hex->Promise");
+        let buy = OpeningFeeParams {
+            min_fee_msat: policy.min_fee_msat,
+            proportional: policy.proportional,
+            valid_until: policy.valid_until,
+            min_lifetime: policy.min_lifetime,
+            max_client_to_self_delay: policy.max_client_to_self_delay,
+            min_payment_size_msat: policy.min_payment_size_msat,
+            max_payment_size_msat: policy.max_payment_size_msat,
+            promise,
+        };
+        (policy, buy)
+    }
+
     #[derive(Clone, Default)]
     struct FakeCln {
         lsps2_getpolicy_response: Arc<Mutex<Option<Lsps2PolicyGetInfoResponse>>>,
         lsps2_getpolicy_error: Arc<Mutex<Option<ClnRpcError>>>,
+        cln_getinfo_response: Arc<Mutex<Option<GetinfoResponse>>>,
+        cln_getinfo_error: Arc<Mutex<Option<ClnRpcError>>>,
+        cln_datastore_response: Arc<Mutex<Option<DatastoreResponse>>>,
+        cln_datastore_error: Arc<Mutex<Option<ClnRpcError>>>,
     }
 
     #[async_trait]
@@ -174,6 +340,54 @@ mod tests {
             };
             panic!("No lsps2 response defined");
         }
+
+        async fn cln_getinfo(
+            &self,
+            _params: &GetinfoRequest,
+        ) -> Result<GetinfoResponse, anyhow::Error> {
+            if let Some(err) = self.cln_getinfo_error.lock().unwrap().take() {
+                return Err(anyhow::Error::new(err).context("from fake api"));
+            };
+            if let Some(res) = self.cln_getinfo_response.lock().unwrap().take() {
+                return Ok(res);
+            };
+            panic!("No cln getinfo response defined");
+        }
+
+        async fn cln_datastore(
+            &self,
+            _params: &DatastoreRequest,
+        ) -> Result<DatastoreResponse, anyhow::Error> {
+            if let Some(err) = self.cln_datastore_error.lock().unwrap().take() {
+                return Err(anyhow::Error::new(err).context("from fake api"));
+            };
+            if let Some(res) = self.cln_datastore_response.lock().unwrap().take() {
+                return Ok(res);
+            };
+            panic!("No cln datastore response defined");
+        }
+    }
+
+    fn minimal_getinfo(height: u32) -> GetinfoResponse {
+        GetinfoResponse {
+            lightning_dir: String::default(),
+            alias: None,
+            our_features: None,
+            warning_bitcoind_sync: None,
+            warning_lightningd_sync: None,
+            address: None,
+            binding: None,
+            blockheight: height,
+            color: String::default(),
+            fees_collected_msat: Amount::from_msat(0),
+            id: PublicKey::from_slice(&PUBKEY).expect("pubkey from slice"),
+            network: String::default(),
+            num_active_channels: u32::default(),
+            num_inactive_channels: u32::default(),
+            num_peers: u32::default(),
+            num_pending_channels: u32::default(),
+            version: String::default(),
+        }
     }
 
     #[tokio::test]
@@ -236,4 +450,181 @@ mod tests {
         assert_eq!(error.code, 200);
         assert!(error.message.contains("failed to fetch policy"));
     }
+
+    #[tokio::test]
+    async fn buy_ok_fixed_amount() {
+        let secret = [0u8; 32];
+        let fake = FakeCln::default();
+        *fake.cln_getinfo_response.lock().unwrap() = Some(minimal_getinfo(900_000));
+        *fake.cln_datastore_response.lock().unwrap() = Some(DatastoreResponse {
+            generation: Some(0),
+            hex: None,
+            string: None,
+            key: vec![],
+        });
+
+        let handler = Lsps2BuyHandler::new(fake, secret);
+        let (_policy, buy) = params_with_promise(&secret);
+
+        // Set payment_size_msat => "MPP+fixed-invoice" mode.
+        let req = Lsps2BuyRequest {
+            opening_fee_params: buy,
+            payment_size_msat: Some(Msat(2_000_000)),
+        }
+        .into_request(Some("ok-fixed".into()));
+        let payload = create_wrapped_request(&req);
+
+        let out = handler.handle(&payload).await.unwrap();
+        let resp: ResponseObject<Lsps2BuyResponse> = serde_json::from_slice(&out).unwrap();
+        let resp = resp.into_inner().unwrap();
+
+        assert_eq!(resp.lsp_cltv_expiry_delta, DEFAULT_CLTV_EXPIRY_DELTA);
+        assert!(!resp.client_trusts_lsp);
+        assert!(resp.jit_channel_scid.to_u64() > 0);
+    }
+
+    #[tokio::test]
+    async fn buy_ok_variable_amount_no_payment_size() {
+        let secret = [2u8; 32];
+        let fake = FakeCln::default();
+        *fake.cln_getinfo_response.lock().unwrap() = Some(minimal_getinfo(900_100));
+        *fake.cln_datastore_response.lock().unwrap() = Some(DatastoreResponse {
+            generation: Some(0),
+            hex: None,
+            string: None,
+            key: vec![],
+        });
+
+        let handler = Lsps2BuyHandler::new(fake, secret);
+        let (_policy, buy) = params_with_promise(&secret);
+
+        // No payment_size_msat => "no-MPP+var-invoice" mode.
+        let req = Lsps2BuyRequest {
+            opening_fee_params: buy,
+            payment_size_msat: None,
+        }
+        .into_request(Some("ok-var".into()));
+        let payload = create_wrapped_request(&req);
+
+        let out = handler.handle(&payload).await.unwrap();
+        let resp: ResponseObject<Lsps2BuyResponse> = serde_json::from_slice(&out).unwrap();
+        assert!(resp.into_inner().is_ok());
+    }
+
+    #[tokio::test]
+    async fn buy_rejects_invalid_promise_or_past_valid_until_with_201() {
+        let secret = [3u8; 32];
+        let handler = Lsps2BuyHandler::new(FakeCln::default(), secret);
+
+        // Case A: wrong promise (derive with different secret)
+        let (_policy_wrong, mut buy_wrong) = params_with_promise(&[9u8; 32]);
+        buy_wrong.valid_until = Utc.with_ymd_and_hms(2100, 1, 1, 0, 0, 0).unwrap(); // future, so only promise is wrong
+        let req_wrong = Lsps2BuyRequest {
+            opening_fee_params: buy_wrong,
+            payment_size_msat: Some(Msat(2_000_000)),
+        }
+        .into_request(Some("bad-promise".into()));
+        let err1 = handler
+            .handle(&create_wrapped_request(&req_wrong))
+            .await
+            .unwrap_err();
+        assert_eq!(err1.code, 201);
+
+        // Case B: past valid_until
+        let (_policy, mut buy_past) = params_with_promise(&secret);
+        buy_past.valid_until = Utc.with_ymd_and_hms(1970, 1, 1, 0, 0, 0).unwrap(); // past
+        let req_past = Lsps2BuyRequest {
+            opening_fee_params: buy_past,
+            payment_size_msat: Some(Msat(2_000_000)),
+        }
+        .into_request(Some("past-valid".into()));
+        let err2 = handler
+            .handle(&create_wrapped_request(&req_past))
+            .await
+            .unwrap_err();
+        assert_eq!(err2.code, 201);
+    }
+
+    #[tokio::test]
+    async fn buy_rejects_when_opening_fee_ge_payment_size_with_202() {
+        let secret = [4u8; 32];
+        let handler = Lsps2BuyHandler::new(FakeCln::default(), secret);
+
+        // Make min_fee already >= payment_size to trigger 202
+        let policy = PolicyOpeningFeeParams {
+            min_fee_msat: Msat(10_000),
+            proportional: Ppm(0), // no extra percentage
+            valid_until: Utc.with_ymd_and_hms(2100, 1, 1, 0, 0, 0).unwrap(),
+            min_lifetime: 1000,
+            max_client_to_self_delay: 42,
+            min_payment_size_msat: Msat(1),
+            max_payment_size_msat: Msat(u64::MAX / 2),
+        };
+        let hex = policy.get_hmac_hex(&secret);
+        let promise: Promise = hex.try_into().unwrap();
+        let buy = OpeningFeeParams {
+            min_fee_msat: policy.min_fee_msat,
+            proportional: policy.proportional,
+            valid_until: policy.valid_until,
+            min_lifetime: policy.min_lifetime,
+            max_client_to_self_delay: policy.max_client_to_self_delay,
+            min_payment_size_msat: policy.min_payment_size_msat,
+            max_payment_size_msat: policy.max_payment_size_msat,
+            promise,
+        };
+
+        let req = Lsps2BuyRequest {
+            opening_fee_params: buy,
+            payment_size_msat: Some(Msat(9_999)), // strictly less than min_fee => opening_fee >= payment_size
+        }
+        .into_request(Some("too-small".into()));
+
+        let err = handler
+            .handle(&create_wrapped_request(&req))
+            .await
+            .unwrap_err();
+        assert_eq!(err.code, 202);
+    }
+
+    #[tokio::test]
+    async fn buy_rejects_on_fee_overflow_with_203() {
+        let secret = [5u8; 32];
+        let handler = Lsps2BuyHandler::new(FakeCln::default(), secret);
+
+        // Choose values likely to overflow if multiplication isn't checked:
+        // opening_fee = min_fee + payment_size * proportional / 1_000_000
+        let policy = PolicyOpeningFeeParams {
+            min_fee_msat: Msat(u64::MAX / 2),
+            proportional: Ppm(u32::MAX), // 4_294_967_295 ppm
+            valid_until: Utc.with_ymd_and_hms(2100, 1, 1, 0, 0, 0).unwrap(),
+            min_lifetime: 1000,
+            max_client_to_self_delay: 42,
+            min_payment_size_msat: Msat(1),
+            max_payment_size_msat: Msat(u64::MAX),
+        };
+        let hex = policy.get_hmac_hex(&secret);
+        let promise: Promise = hex.try_into().unwrap();
+        let buy = OpeningFeeParams {
+            min_fee_msat: policy.min_fee_msat,
+            proportional: policy.proportional,
+            valid_until: policy.valid_until,
+            min_lifetime: policy.min_lifetime,
+            max_client_to_self_delay: policy.max_client_to_self_delay,
+            min_payment_size_msat: policy.min_payment_size_msat,
+            max_payment_size_msat: policy.max_payment_size_msat,
+            promise,
+        };
+
+        let req = Lsps2BuyRequest {
+            opening_fee_params: buy,
+            payment_size_msat: Some(Msat(u64::MAX / 2)),
+        }
+        .into_request(Some("overflow".into()));
+
+        let err = handler
+            .handle(&create_wrapped_request(&req))
+            .await
+            .unwrap_err();
+        assert_eq!(err.code, 203);
+    }
 }
diff --git a/plugins/lsps-plugin/src/lsps2/mod.rs b/plugins/lsps-plugin/src/lsps2/mod.rs
index b217f987..7ed8e744 100644
--- a/plugins/lsps-plugin/src/lsps2/mod.rs
+++ b/plugins/lsps-plugin/src/lsps2/mod.rs
@@ -13,3 +13,6 @@ pub const OPTION_PROMISE_SECRET: options::StringConfigOption =
         "dev-lsps2-promise-secret",
         "A 64-character hex string that is the secret for promises",
     );
+
+pub const DS_MAIN_KEY: &'static str = "lsps";
+pub const DS_SUB_KEY: &'static str = "lsps2";
diff --git a/plugins/lsps-plugin/src/service.rs b/plugins/lsps-plugin/src/service.rs
index 11905dbf..ca620f9e 100644
--- a/plugins/lsps-plugin/src/service.rs
+++ b/plugins/lsps-plugin/src/service.rs
@@ -6,7 +6,7 @@ use cln_lsps::jsonrpc::{server::JsonRpcServer, JsonRpcRequest};
 use cln_lsps::lsps0::handler::Lsps0ListProtocolsHandler;
 use cln_lsps::lsps0::model::Lsps0listProtocolsRequest;
 use cln_lsps::lsps0::transport::{self, CustomMsg};
-use cln_lsps::lsps2::model::Lsps2GetInfoRequest;
+use cln_lsps::lsps2::model::{Lsps2BuyRequest, Lsps2GetInfoRequest};
 use cln_lsps::util::wrap_payload_with_peer_id;
 use cln_lsps::{lsps0, lsps2, util, LSP_FEATURE_BIT};
 use cln_plugin::options::ConfigOption;
@@ -94,11 +94,15 @@ async fn main() -> Result<(), anyhow::Error> {
                 };
 
                 let cln_api_rpc = lsps2::handler::ClnApiRpc::new(rpc_path);
-                let getinfo_handler = lsps2::handler::Lsps2GetInfoHandler::new(cln_api_rpc, secret);
-                lsps_builder = lsps_builder.with_handler(
-                    Lsps2GetInfoRequest::METHOD.to_string(),
-                    Arc::new(getinfo_handler),
-                );
+                let getinfo_handler =
+                    lsps2::handler::Lsps2GetInfoHandler::new(cln_api_rpc.clone(), secret);
+                let buy_handler = lsps2::handler::Lsps2BuyHandler::new(cln_api_rpc, secret);
+                lsps_builder = lsps_builder
+                    .with_handler(
+                        Lsps2GetInfoRequest::METHOD.to_string(),
+                        Arc::new(getinfo_handler),
+                    )
+                    .with_handler(Lsps2BuyRequest::METHOD.to_string(), Arc::new(buy_handler));
             }
         }
 
diff --git a/tests/test_cln_lsps.py b/tests/test_cln_lsps.py
index 53dcf386..1d1a1886 100644
--- a/tests/test_cln_lsps.py
+++ b/tests/test_cln_lsps.py
@@ -64,3 +64,27 @@ def test_lsps2_getinfo(node_factory):
 
     res = l1.rpc.lsps_lsps2_getinfo(lsp_id=l2.info['id'])
     assert res["opening_fee_params_menu"]
+
+
+def test_lsps2_buy(node_factory):
+    # We need a policy service to fetch from.
+    plugin = os.path.join(os.path.dirname(__file__), 'plugins/lsps2_policy.py')
+
+    l1, l2 = node_factory.get_nodes(2, opts=[
+        {"dev-lsps-client-enabled": None},
+        {
+            "dev-lsps-service-enabled": None,
+            "dev-lsps2-service-enabled": None,
+            "dev-lsps2-promise-secret": "0" * 64,
+            "plugin": plugin
+            }
+        ])
+
+    # We don't need a channel to query for lsps services
+    node_factory.join_nodes([l1, l2], fundchannel=False)
+
+    res = l1.rpc.lsps_lsps2_getinfo(lsp_id=l2.info['id'])
+    params = res["opening_fee_params_menu"][0]
+
+    res = l1.rpc.lsps_lsps2_buy(lsp_id=l2.info['id'], payment_size_msat=None, opening_fee_params=params)
+    assert res