diff --git a/client/beefy/src/lib.rs b/client/beefy/src/lib.rs
index 9b2bf383df8ef..4dceccafa290a 100644
--- a/client/beefy/src/lib.rs
+++ b/client/beefy/src/lib.rs
@@ -27,7 +27,7 @@ use sc_network_gossip::{GossipEngine, Network as GossipNetwork};
 use sp_api::ProvideRuntimeApi;
 use sp_blockchain::HeaderBackend;
 use sp_keystore::SyncCryptoStorePtr;
-use sp_runtime::traits::Block;
+use sp_runtime::traits::{Block, NumberFor};
 
 use beefy_primitives::BeefyApi;
 
@@ -111,7 +111,7 @@ where
 	B: Block,
 	BE: Backend<B>,
 	C: Client<B, BE>,
-	C::Api: BeefyApi<B>,
+	C::Api: BeefyApi<B, NumberFor<B>>,
 	N: GossipNetwork<B> + Clone + Send + 'static,
 {
 	/// BEEFY client
@@ -142,7 +142,7 @@ where
 	B: Block,
 	BE: Backend<B>,
 	C: Client<B, BE>,
-	C::Api: BeefyApi<B>,
+	C::Api: BeefyApi<B, NumberFor<B>>,
 	N: GossipNetwork<B> + Clone + Send + 'static,
 {
 	let BeefyParams {
diff --git a/client/beefy/src/worker.rs b/client/beefy/src/worker.rs
index 0c7d8d4ffdc9c..1e38ae0fcff50 100644
--- a/client/beefy/src/worker.rs
+++ b/client/beefy/src/worker.rs
@@ -100,7 +100,7 @@ where
 	B: Block + Codec,
 	BE: Backend<B>,
 	C: Client<B, BE>,
-	C::Api: BeefyApi<B>,
+	C::Api: BeefyApi<B, NumberFor<B>>,
 {
 	/// Return a new BEEFY worker instance.
 	///
@@ -146,7 +146,7 @@ where
 	B: Block,
 	BE: Backend<B>,
 	C: Client<B, BE>,
-	C::Api: BeefyApi<B>,
+	C::Api: BeefyApi<B, NumberFor<B>>,
 {
 	/// Return `true`, if we should vote on block `number`
 	fn should_vote_on(&self, number: NumberFor<B>) -> bool {
@@ -247,14 +247,19 @@ where
 
 				debug!(target: "beefy", "🥩 New Rounds for id: {:?}", id);
 
-				self.best_beefy_block = Some(*notification.header.number());
+				let at = BlockId::hash(notification.header.hash());
+				let session_boundary = self.client.runtime_api().get_session_boundary(&at).ok();
+				self.best_beefy_block = session_boundary.clone();
 				self.beefy_best_block_sender
 					.notify(|| Ok::<_, ()>(notification.hash.clone()))
 					.expect("forwards closure result; the closure always returns Ok; qed.");
 
-				// this metric is kind of 'fake'. Best BEEFY block should only be updated once we
-				// have a signed commitment for the block. Remove once the above TODO is done.
-				metric_set!(self, beefy_best_block, *notification.header.number());
+				// this metric is kind of 'fake'. Best BEEFY block should only be updated once
+				// we have a signed commitment for the block. Remove once the above TODO is
+				// done.
+				if let Some(session_boundary) = session_boundary {
+					metric_set!(self, beefy_best_block, session_boundary);
+				}
 			}
 		}
 
diff --git a/frame/session/src/lib.rs b/frame/session/src/lib.rs
index 4cf793a9b4739..81509301a8e7c 100644
--- a/frame/session/src/lib.rs
+++ b/frame/session/src/lib.rs
@@ -494,6 +494,7 @@ pub mod pallet {
 
 			Validators::<T>::put(initial_validators_0);
 			<QueuedKeys<T>>::put(queued_keys);
+			SessionStart::<T>::put(T::BlockNumber::default());
 
 			T::SessionManager::start_session(0);
 		}
@@ -509,6 +510,11 @@ pub mod pallet {
 	#[pallet::getter(fn current_index)]
 	pub type CurrentIndex<T> = StorageValue<_, SessionIndex, ValueQuery>;
 
+	/// Block number at which the current session began
+	#[pallet::storage]
+	#[pallet::getter(fn session_start)]
+	pub type SessionStart<T: Config> = StorageValue<_, T::BlockNumber, ValueQuery>;
+
 	/// True if the underlying economic identities or weighting behind the validators
 	/// has changed in the queued validator set.
 	#[pallet::storage]
@@ -573,6 +579,7 @@ pub mod pallet {
 		fn on_initialize(n: T::BlockNumber) -> Weight {
 			if T::ShouldEndSession::should_end_session(n) {
 				Self::rotate_session();
+				SessionStart::<T>::put(n);
 				T::BlockWeights::get().max_block
 			} else {
 				// NOTE: the non-database part of the weight for `should_end_session(n)` is
@@ -899,6 +906,11 @@ impl<T: Config> Pallet<T> {
 	fn clear_key_owner(id: KeyTypeId, key_data: &[u8]) {
 		<KeyOwner<T>>::remove((id, key_data));
 	}
+
+	/// Returns the block number at which the session began
+	pub fn get_session_boundary() -> T::BlockNumber {
+		Pallet::<T>::session_start()
+	}
 }
 
 impl<T: Config> ValidatorRegistration<T::ValidatorId> for Pallet<T> {
diff --git a/primitives/beefy/src/lib.rs b/primitives/beefy/src/lib.rs
index 8dbdd66f3559b..7ce07b75997e8 100644
--- a/primitives/beefy/src/lib.rs
+++ b/primitives/beefy/src/lib.rs
@@ -156,10 +156,12 @@ pub struct VoteMessage<Number, Id, Signature> {
 
 sp_api::decl_runtime_apis! {
 	/// API necessary for BEEFY voters.
-	pub trait BeefyApi
+	pub trait BeefyApi<BlockNumber: codec::Codec>
 	{
 		/// Return the current active BEEFY validator set
 		fn validator_set() -> Option<ValidatorSet<crypto::AuthorityId>>;
+		/// Returns the block number at which the current session began
+		fn get_session_boundary() -> BlockNumber;
 	}
 }