fix(bb): stop reading past end_index in sumcheck extend_edges and partially_evaluate

barretenberg/cpp/src/barretenberg/sumcheck/sumcheck.hpp

@@ -675,11 +675,19 @@ template <typename Flavor> class SumcheckProver {
         parallel_for(source_view.size(), [&](size_t j) {
             BB_BENCH_TRACY_NAME("Sumcheck::partially_evaluate");
             const auto& poly = source_view[j];
-            size_t limit = poly.end_index();
-            for (size_t i = 0; i < limit; i += 2) {
-                dest_view[j].at(i >> 1) = poly[i] + round_challenge * (poly[i + 1] - poly[i]);
+            const size_t limit = poly.end_index();
+            const size_t num_full_pairs = limit / 2;
+            for (size_t i = 0; i < num_full_pairs; ++i) {
+                const size_t idx = i << 1;
+                dest_view[j].at(i) = poly[idx] + round_challenge * (poly[idx + 1] - poly[idx]);
             }
-            dest_view[j].shrink_end_index((limit / 2) + (limit % 2));
+            // If the active region has an odd length, fold the lone trailing coefficient against an
+            // implicit zero partner instead of reading poly[limit] (past end_index, would trip the
+            // virtual-zeroes assertion for polynomials with virtual_size == end_index).
+            if ((limit & 1) != 0) {
+                dest_view[j].at(num_full_pairs) = poly[limit - 1] * (FF(1) - round_challenge);
+            }
+            dest_view[j].shrink_end_index(num_full_pairs + (limit & 1));
         });
     };
 

barretenberg/cpp/src/barretenberg/sumcheck/sumcheck_round.hpp

@@ -160,15 +160,23 @@ template <typename Flavor> class SumcheckProverRound {
                       const size_t edge_idx)
     {
         for (auto [extended_edge, multivariate] : zip_view(extended_edges.get_all(), multivariates.get_all())) {
+            const size_t end = multivariate.end_index();
             if constexpr (Flavor::USE_SHORT_MONOMIALS) {
-                extended_edge = bb::Univariate<FF, 2>({ multivariate[edge_idx], multivariate[edge_idx + 1] });
+                // The main loop iterates up to effective_round_size, which rounds max_end_index up to even.
+                // For odd max_end_index, the second read of the final edge lands at index == end_index, which
+                // would trip the virtual-zeroes assertion for polynomials with virtual_size == end_index
+                // (e.g. gemini_masking_poly). Treat reads past end_index as zero directly.
+                const FF v0 = (edge_idx < end) ? FF(multivariate[edge_idx]) : FF::zero();
+                const FF v1 = (edge_idx + 1 < end) ? FF(multivariate[edge_idx + 1]) : FF::zero();
+                extended_edge = bb::Univariate<FF, 2>({ v0, v1 });
             } else {
-                if (multivariate.end_index() < edge_idx) {
+                if (end <= edge_idx) {
                     static const auto zero_univariate = bb::Univariate<FF, MAX_PARTIAL_RELATION_LENGTH>::zero();
                     extended_edge = zero_univariate;
                 } else {
-                    extended_edge = bb::Univariate<FF, 2>({ multivariate[edge_idx], multivariate[edge_idx + 1] })
-                                        .template extend_to<MAX_PARTIAL_RELATION_LENGTH>();
+                    const FF v0 = multivariate[edge_idx];
+                    const FF v1 = (edge_idx + 1 < end) ? FF(multivariate[edge_idx + 1]) : FF::zero();
+                    extended_edge = bb::Univariate<FF, 2>({ v0, v1 }).template extend_to<MAX_PARTIAL_RELATION_LENGTH>();
                 }
             }
         }