diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 3bede386..2d23a380 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -52,9 +52,9 @@ jobs:
         id: lean-spec
         run: echo "commit=$(sed -n 's/^LEAN_SPEC_COMMIT_HASH:= *//p' Makefile)" >> $GITHUB_OUTPUT
 
-      - name: Cache test fixtures
+      - name: Restore test fixtures cache
         id: cache-fixtures
-        uses: actions/cache@v4
+        uses: actions/cache/restore@v5
         with:
           path: leanSpec/fixtures
           key: leanspec-fixtures-${{ steps.lean-spec.outputs.commit }}
@@ -90,10 +90,10 @@ jobs:
           HASH=$(echo -n "$URL" | sha256sum | awk '{print $1}')
           echo "hash=$HASH" >> $GITHUB_OUTPUT
 
-      - name: Cache production keys
+      - name: Restore production keys cache
         if: steps.cache-fixtures.outputs.cache-hit != 'true'
         id: cache-prod-keys
-        uses: actions/cache@v4
+        uses: actions/cache/restore@v5
         with:
           path: leanSpec/packages/testing/src/consensus_testing/test_keys/prod_scheme
           key: prod-keys-${{ steps.prod-keys-url.outputs.hash }}
@@ -103,11 +103,34 @@ jobs:
         working-directory: leanSpec
         run: uv run python -m consensus_testing.keys --download --scheme prod
 
+      # Save production keys even if a later step fails, so a re-run does
+      # not have to re-download. See: https://github.com/actions/cache/tree/main/save#always-save-cache
+      #
+      # `cache-hit == 'false'` (rather than `!= 'true'`) only matches when
+      # the restore step actually ran and missed: when fixtures were already
+      # cached, the restore was skipped and `cache-hit` is empty, so save
+      # is skipped too.
+      - name: Save production keys cache
+        if: always() && steps.cache-prod-keys.outputs.cache-hit == 'false'
+        uses: actions/cache/save@v5
+        with:
+          path: leanSpec/packages/testing/src/consensus_testing/test_keys/prod_scheme
+          key: ${{ steps.cache-prod-keys.outputs.cache-primary-key }}
+
       - name: Generate test fixtures
         if: steps.cache-fixtures.outputs.cache-hit != 'true'
         working-directory: leanSpec
         run: uv run fill --fork=Devnet --scheme prod -o fixtures -n 2
 
+      # Save fixtures even if a later step fails, so a re-run does not
+      # have to regenerate them. See: https://github.com/actions/cache/tree/main/save#always-save-cache
+      - name: Save test fixtures cache
+        if: always() && steps.cache-fixtures.outputs.cache-hit != 'true'
+        uses: actions/cache/save@v5
+        with:
+          path: leanSpec/fixtures
+          key: ${{ steps.cache-fixtures.outputs.cache-primary-key }}
+
       # Ensure make sees fixtures as up-to-date (its timestamp must be
       # newer than leanSpec/, which intermediate steps may have modified).
       - name: Mark fixtures as up-to-date
diff --git a/Makefile b/Makefile
index c9c1c17a..013b1573 100644
--- a/Makefile
+++ b/Makefile
@@ -24,8 +24,8 @@ docker-build: ## 🐳 Build the Docker image
 		-t ghcr.io/lambdaclass/ethlambda:$(DOCKER_TAG) .
 	@echo
 
-# 2026-04-20
-LEAN_SPEC_COMMIT_HASH:=bc17f7ae8d16caec276f4d304e04fd3c65e6de3c
+# 2026-04-28: bump for leanSpec PR #682 (validate_attestation future-slot bound).
+LEAN_SPEC_COMMIT_HASH:=62eff6e7e6041a283877a546a07cb3b83f4f7d5b
 
 leanSpec:
 	git clone https://github.com/leanEthereum/leanSpec.git --single-branch
diff --git a/crates/blockchain/src/lib.rs b/crates/blockchain/src/lib.rs
index 77c98826..5e47f2ed 100644
--- a/crates/blockchain/src/lib.rs
+++ b/crates/blockchain/src/lib.rs
@@ -46,6 +46,14 @@ pub const MILLISECONDS_PER_SLOT: u64 = MILLISECONDS_PER_INTERVAL * INTERVALS_PER
 ///
 /// See: leanSpec commit 0c9528a (PR #536).
 pub const MAX_ATTESTATIONS_DATA: usize = 16;
+/// Future-slot tolerance for gossip attestations, expressed in intervals.
+///
+/// Bounds the clock skew the time check is willing to absorb when admitting a
+/// vote whose slot has not yet started locally. One interval is roughly 800 ms,
+/// the lean analogue of mainnet's `MAXIMUM_GOSSIP_CLOCK_DISPARITY`.
+///
+/// See: leanSpec PR #682.
+pub const GOSSIP_DISPARITY_INTERVALS: u64 = 1;
 
 impl BlockChain {
     pub fn spawn(
diff --git a/crates/blockchain/src/store.rs b/crates/blockchain/src/store.rs
index 45b3b220..66baf2c9 100644
--- a/crates/blockchain/src/store.rs
+++ b/crates/blockchain/src/store.rs
@@ -20,8 +20,8 @@ use ethlambda_types::{
 use tracing::{info, trace, warn};
 
 use crate::{
-    INTERVALS_PER_SLOT, MAX_ATTESTATIONS_DATA, MILLISECONDS_PER_INTERVAL, MILLISECONDS_PER_SLOT,
-    metrics,
+    GOSSIP_DISPARITY_INTERVALS, INTERVALS_PER_SLOT, MAX_ATTESTATIONS_DATA,
+    MILLISECONDS_PER_INTERVAL, MILLISECONDS_PER_SLOT, metrics,
 };
 
 const JUSTIFICATION_LOOKBACK_SLOTS: u64 = 3;
@@ -135,7 +135,7 @@ fn update_safe_target(store: &mut Store) {
 ///     2. A vote cannot span backwards in time (source > target).
 ///     3. The head must be at least as recent as source and target.
 ///     4. Checkpoint slots must match the actual block slots.
-///     5. A vote cannot be for a future slot.
+///     5. The vote's slot must have started locally (a small disparity margin is allowed).
 fn validate_attestation_data(store: &Store, data: &AttestationData) -> Result<(), StoreError> {
     let _timing = metrics::time_attestation_validation();
 
@@ -182,13 +182,16 @@ fn validate_attestation_data(store: &Store, data: &AttestationData) -> Result<()
         });
     }
 
-    // Time Check - Validate attestation is not too far in the future.
-    // We allow a small margin for clock disparity (1 slot), but no further.
-    let current_slot = store.time() / INTERVALS_PER_SLOT;
-    if data.slot > current_slot + 1 {
+    // Time Check - Honest validators emit votes only after their slot has begun.
+    // Allow a small disparity margin for clock skew between peers.
+    //
+    // The bound is in intervals, not slots: a whole-slot margin would let an
+    // adversary pre-publish next-slot aggregates ahead of any honest validator.
+    let attestation_start_interval = data.slot.saturating_mul(INTERVALS_PER_SLOT);
+    if attestation_start_interval > store.time() + GOSSIP_DISPARITY_INTERVALS {
         return Err(StoreError::AttestationTooFarInFuture {
             attestation_slot: data.slot,
-            current_slot,
+            store_time: store.time(),
         });
     }
 
@@ -802,11 +805,11 @@ pub enum StoreError {
     },
 
     #[error(
-        "Attestation slot {attestation_slot} is too far in future (current slot: {current_slot})"
+        "Attestation slot {attestation_slot} is too far in future (store time: {store_time} intervals)"
     )]
     AttestationTooFarInFuture {
         attestation_slot: u64,
-        current_slot: u64,
+        store_time: u64,
     },
 
     #[error(
diff --git a/crates/blockchain/state_transition/tests/stf_spectests.rs b/crates/blockchain/state_transition/tests/stf_spectests.rs
index 5c25c8a1..669ea835 100644
--- a/crates/blockchain/state_transition/tests/stf_spectests.rs
+++ b/crates/blockchain/state_transition/tests/stf_spectests.rs
@@ -26,6 +26,14 @@ fn run(path: &Path) -> datatest_stable::Result<()> {
         }
         println!("Running test: {}", name);
 
+        // Fixtures with no blocks come from spec filler runs that raised
+        // before any block was constructed (e.g. negative tests where
+        // `state.process_slots(spec.slot)` aborts pre-build). With nothing
+        // for ethlambda to replay, the spec framework's verdict stands.
+        if test.blocks.is_empty() {
+            continue;
+        }
+
         let mut pre_state: State = test.pre.into();
         let mut result = Ok(());