diff --git a/crates/blockchain/src/lib.rs b/crates/blockchain/src/lib.rs
index 8be7c89f..e8d83dc4 100644
--- a/crates/blockchain/src/lib.rs
+++ b/crates/blockchain/src/lib.rs
@@ -1,4 +1,5 @@
 use std::collections::{HashMap, HashSet, VecDeque};
+use std::fmt;
 use std::time::{Duration, Instant, SystemTime};
 
 use ethlambda_network_api::{BlockChainToP2PRef, InitP2P};
@@ -59,6 +60,44 @@ pub use ethlambda_types::block::MAX_ATTESTATIONS_DATA;
 /// See: leanSpec PR #682.
 pub const GOSSIP_DISPARITY_INTERVALS: u64 = 1;
 
+#[derive(Clone, Copy, Debug, Eq, PartialEq)]
+pub(crate) enum SlotInterval {
+    BlockPublication,
+    AttestationProduction,
+    Aggregation,
+    SafeTargetUpdate,
+    EndOfSlot,
+}
+
+impl SlotInterval {
+    pub(crate) fn from_slot_index(index: u64) -> Self {
+        match index {
+            0 => Self::BlockPublication,
+            1 => Self::AttestationProduction,
+            2 => Self::Aggregation,
+            3 => Self::SafeTargetUpdate,
+            4 => Self::EndOfSlot,
+            _ => unreachable!("slots only have 5 intervals"),
+        }
+    }
+
+    fn as_slot_index(self) -> u64 {
+        match self {
+            Self::BlockPublication => 0,
+            Self::AttestationProduction => 1,
+            Self::Aggregation => 2,
+            Self::SafeTargetUpdate => 3,
+            Self::EndOfSlot => 4,
+        }
+    }
+}
+
+impl fmt::Display for SlotInterval {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.as_slot_index().fmt(f)
+    }
+}
+
 /// Milliseconds until the next interval boundary, measured relative to genesis.
 fn ms_until_next_interval(now_ms: u64, genesis_time_ms: u64) -> u64 {
     // Before genesis: wait until genesis itself.
@@ -192,7 +231,9 @@ impl BlockChainServer {
         // Calculate current slot and interval from milliseconds
         let time_since_genesis_ms = timestamp_ms.saturating_sub(genesis_time_ms);
         let slot = time_since_genesis_ms / MILLISECONDS_PER_SLOT;
-        let interval = (time_since_genesis_ms % MILLISECONDS_PER_SLOT) / MILLISECONDS_PER_INTERVAL;
+        let interval = SlotInterval::from_slot_index(
+            (time_since_genesis_ms % MILLISECONDS_PER_SLOT) / MILLISECONDS_PER_INTERVAL,
+        );
 
         // Idempotency guard
         //
@@ -252,7 +293,7 @@ impl BlockChainServer {
         // needs (those stragglers surface in the `late` section instead). Skip
         // empty snapshots so a missed round keeps the last set we saw. Pure
         // observability.
-        if interval == 4
+        if interval == SlotInterval::EndOfSlot
             && let Some(snapshot) = coverage::snapshot_new_payloads(&self.store)
         {
             self.pre_merge_coverage = Some(snapshot);
@@ -260,7 +301,7 @@ impl BlockChainServer {
 
         // Whether one of our validators proposes this slot. Drives the store's
         // interval-0 attestation acceptance.
-        let is_proposer = (interval == 0 && slot > 0)
+        let is_proposer = (interval == SlotInterval::BlockPublication && slot > 0)
             .then(|| self.get_our_proposer(slot))
             .flatten()
             .is_some();
@@ -280,14 +321,14 @@ impl BlockChainServer {
             // advances the store to this slot's interval 0 before building (see
             // `propose_block`). The real interval-0 tick is then skipped by the
             // idempotency guard above, since the store clock is already here.
-            0 => {}
+            SlotInterval::BlockPublication => {}
 
             // ==== interval 1 ====
             //
             // Produce attestations at interval 1 (all validators including
             // proposer). Reuse the same snapshot so self-delivery decisions
             // match the rest of the tick.
-            1 => {
+            SlotInterval::AttestationProduction => {
                 // Emit the post-block coverage report for the previous slot.
                 // Fired at interval 1 (not 0) so the block carrying `slot - 1`'s
                 // votes — proposed at interval 0 of this slot — has typically
@@ -309,7 +350,7 @@ impl BlockChainServer {
             }
 
             // ==== interval 2 ====
-            2 => {
+            SlotInterval::Aggregation => {
                 if is_aggregator {
                     coverage::emit_agg_start_new_coverage(
                         &self.store,
@@ -324,7 +365,7 @@ impl BlockChainServer {
             // ==== interval 3 ====
             //
             // Safe-target update is handled inside `store::on_tick`.
-            3 => {}
+            SlotInterval::SafeTargetUpdate => {}
 
             // ==== interval 4 ====
             //
@@ -335,7 +376,7 @@ impl BlockChainServer {
             // rather than stashing it for the interval-0 tick — keeps it robust:
             // `on_tick` skips the interval-0 tick whenever this build overruns
             // its interval.
-            4 => {
+            SlotInterval::EndOfSlot => {
                 let next_slot = slot + 1;
                 let next_proposer = self
                     .get_our_proposer(next_slot)
@@ -345,8 +386,6 @@ impl BlockChainServer {
                     self.propose_block(next_slot, validator_id).await;
                 }
             }
-
-            _ => {}
         }
 
         // Update safe target slot metric (updated by store.on_tick at interval 3)
diff --git a/crates/blockchain/src/store.rs b/crates/blockchain/src/store.rs
index 20f7cf5c..07e9e7c0 100644
--- a/crates/blockchain/src/store.rs
+++ b/crates/blockchain/src/store.rs
@@ -18,7 +18,7 @@ use tracing::{info, trace, warn};
 
 use crate::{
     GOSSIP_DISPARITY_INTERVALS, INTERVALS_PER_SLOT, MAX_ATTESTATIONS_DATA,
-    MILLISECONDS_PER_INTERVAL, MILLISECONDS_PER_SLOT,
+    MILLISECONDS_PER_INTERVAL, MILLISECONDS_PER_SLOT, SlotInterval,
     block_builder::{PostBlockCheckpoints, ProposerConfig, build_block},
     metrics,
 };
@@ -278,7 +278,7 @@ pub fn on_tick(store: &mut Store, timestamp_ms: u64, has_proposal: bool) {
             .expect("set_time should succeed");
 
         let slot = store.time() / INTERVALS_PER_SLOT;
-        let interval = store.time() % INTERVALS_PER_SLOT;
+        let interval = SlotInterval::from_slot_index(store.time() % INTERVALS_PER_SLOT);
 
         trace!(%slot, %interval, "processing tick");
 
@@ -292,27 +292,26 @@ pub fn on_tick(store: &mut Store, timestamp_ms: u64, has_proposal: bool) {
         // the actor's message loop stays unblocked during the expensive XMSS
         // proofs. See `BlockChainServer::start_aggregation_session` in `lib.rs`.
         match interval {
-            0 => {
+            SlotInterval::BlockPublication => {
                 // Start of slot - process attestations if proposal exists
                 if should_signal_proposal {
                     accept_new_attestations(store, false);
                 }
             }
-            1 => {
+            SlotInterval::AttestationProduction => {
                 // Vote propagation — no action
             }
-            2 => {
+            SlotInterval::Aggregation => {
                 // Aggregation is driven by the actor (off-thread); nothing to do here.
             }
-            3 => {
+            SlotInterval::SafeTargetUpdate => {
                 // Update safe target for validators
                 update_safe_target(store);
             }
-            4 => {
+            SlotInterval::EndOfSlot => {
                 // End of slot - accept accumulated attestations and log tree
                 accept_new_attestations(store, true);
             }
-            _ => unreachable!("slots only have 5 intervals"),
         }
     }
 }