diff --git a/lean_client/fork_choice/src/handlers.rs b/lean_client/fork_choice/src/handlers.rs
index 778c38d4..59c0bdab 100644
--- a/lean_client/fork_choice/src/handlers.rs
+++ b/lean_client/fork_choice/src/handlers.rs
@@ -409,10 +409,13 @@ pub fn on_attestation(
 /// Verifies the aggregated XMSS proof against participant public keys and stores
 /// it in `latest_new_aggregated_payloads`. At interval 3, these are merged with
 /// `latest_known_aggregated_payloads` (from blocks) to compute safe target.
+///
+/// `verify_proof` gates only the XMSS SNARK check.
 #[inline]
 pub fn on_aggregated_attestation(
     store: &mut Store,
     signed_aggregated_attestation: SignedAggregatedAttestation,
+    verify_proof: bool,
 ) -> Result<()> {
     // Structure: { data: AttestationData, proof: AggregatedSignatureProof }
     let attestation_data = signed_aggregated_attestation.data.clone();
@@ -472,9 +475,11 @@ pub fn on_aggregated_attestation(
         })
         .collect::<Result<Vec<_>>>()?;
 
-    proof
-        .verify(public_keys, data_root, attestation_data.slot.0 as u32)
-        .context("aggregated attestation proof verification failed")?;
+    if verify_proof {
+        proof
+            .verify(public_keys, data_root, attestation_data.slot.0 as u32)
+            .context("aggregated attestation proof verification failed")?;
+    }
 
     let attestation_slot = attestation_data.slot;
     for vid in &validator_ids {
@@ -796,7 +801,7 @@ pub fn apply_verified_block(
         .remove(&block_root)
         .unwrap_or_default();
     for signed_agg in pending_agg {
-        if let Err(err) = on_aggregated_attestation(store, signed_agg) {
+        if let Err(err) = on_aggregated_attestation(store, signed_agg, true) {
             warn!(%err, "Pending aggregated attestation retry failed after block arrival");
         }
     }
diff --git a/lean_client/http_api/src/test_driver.rs b/lean_client/http_api/src/test_driver.rs
index e2f17a87..ed79e1ed 100644
--- a/lean_client/http_api/src/test_driver.rs
+++ b/lean_client/http_api/src/test_driver.rs
@@ -34,6 +34,8 @@ use spec_test_fixtures::{
 use ssz::SszHash;
 use xmss::{AggregatedSignature, Signature};
 
+const MOCK_AGGREGATION_PROOF_MARKER: &[u8] = b"MOCKED-AGGREGATION-PROOF";
+
 /// Shared state for test-driver routes. Carries a writable handle to the
 /// fork-choice store plus the `BlockCache` that `on_block` requires.
 ///
@@ -412,8 +414,8 @@ fn apply_step(
                 // `Checks` steps still see a snapshot.
                 return Ok(());
             };
-            let signed = build_signed_aggregated_attestation(step)?;
-            on_aggregated_attestation(store, signed).map_err(|err| err.to_string())
+            let (signed, verify_proof) = build_signed_aggregated_attestation(step)?;
+            on_aggregated_attestation(store, signed, verify_proof).map_err(|err| err.to_string())
         }
         ForkChoiceStep::Checks { .. } => {
             // Pure-assertion step. The simulator validates against the
@@ -468,20 +470,29 @@ fn hex_root(root: &ssz::H256) -> String {
 /// string (the harness cannot re-aggregate without the signers' private
 /// keys), so we decode it directly into an [`AggregatedSignature`] and
 /// wrap with the participants bitfield from the same payload.
+///
+/// Returns the attestation and whether its proof should be XMSS-verified —
+/// `false` when the proof is the mocked sentinel.
 fn build_signed_aggregated_attestation(
     step: GossipAggregatedAttestationStep,
-) -> Result<SignedAggregatedAttestation, String> {
+) -> Result<(SignedAggregatedAttestation, bool), String> {
     let proof_hex = step.proof.proof.data.trim_start_matches("0x");
     let proof_bytes = hex::decode(proof_hex)
         .map_err(|err| format!("invalid hex in aggregate proof_data: {err}"))?;
+    let verify_proof = !proof_bytes
+        .windows(MOCK_AGGREGATION_PROOF_MARKER.len())
+        .any(|window| window == MOCK_AGGREGATION_PROOF_MARKER);
     let proof_data = AggregatedSignature::new(&proof_bytes)
         .map_err(|err| format!("failed to construct aggregated signature: {err}"))?;
 
-    Ok(SignedAggregatedAttestation {
-        data: step.data.into(),
-        proof: AggregatedSignatureProof {
-            participants: step.proof.participants.into(),
-            proof_data,
+    Ok((
+        SignedAggregatedAttestation {
+            data: step.data.into(),
+            proof: AggregatedSignatureProof {
+                participants: step.proof.participants.into(),
+                proof_data,
+            },
         },
-    })
+        verify_proof,
+    ))
 }