diff --git a/cpp/monoprop/detail/evolution/layer_build/Engine.h b/cpp/monoprop/detail/evolution/layer_build/Engine.h
index e507e96d..f19bddf7 100644
--- a/cpp/monoprop/detail/evolution/layer_build/Engine.h
+++ b/cpp/monoprop/detail/evolution/layer_build/Engine.h
@@ -500,7 +500,8 @@ struct LayerBuildEngine {
                 }
                 // kNotFound == kMissingIndex == size_t max, so one bound check covers both.
                 if (found[j] < op_size) {
-                    if (is_leader_pass) {
+                    // Freshly inserted partners (found >= combined_size) skip the mark: combined_size bounds it.
+                    if (is_leader_pass && found[j] < combined_size) {
                         matched.mark(found[j]);
                     }
                     sink.self_hit(srcs[j], found[j], phases[j], v_src);
diff --git a/cpp/tests/evolution_detail_tests.cpp b/cpp/tests/evolution_detail_tests.cpp
index 39555948..733ccfb4 100644
--- a/cpp/tests/evolution_detail_tests.cpp
+++ b/cpp/tests/evolution_detail_tests.cpp
@@ -12,21 +12,49 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-// MatchedEpochSet and CutoffContext driven directly, not through build_layer.
+// MatchedEpochSet, CutoffContext and the self-resolve mark guard driven directly, not through build_layer.
 
 #include <boost/test/unit_test.hpp>
 
+#include <cstddef>
 #include <cstdint>
 #include <limits>
+#include <utility>
+#include <vector>
 
 #include "monoprop/TypeAliases.h"
+#include "monoprop/algebra/Algebra.h"
 #include "monoprop/detail/evolution/CutoffContext.h"
 #include "monoprop/detail/evolution/layer_build/Common.h"
+#include "monoprop/detail/evolution/layer_build/Engine.h"
+#include "monoprop/detail/operator/MPOperator.h"
 
 using namespace monoprop;
 using monoprop::detail::CutoffContext;
 using monoprop::detail::MatchedEpochSet;
 
+namespace {
+
+// resolve_range_ touches only wants_values and self_hit, so the engine drives without the cross-rank sink surface.
+struct RecordingSink {
+    static constexpr bool wants_values = false;
+    std::vector<std::pair<size_t, size_t>> hits; // (src, found)
+    auto self_hit(size_t src, size_t found, int /*phase*/, double /*v_src*/) -> void { hits.emplace_back(src, found); }
+};
+
+// append_term writes a row only; find_batch needs the hash index, which insert_absent_terms populates.
+auto indexed_op(const std::vector<Monomial<8>> &terms) -> detail::MPOperator<8> {
+    detail::MPOperator<8> op;
+    detail::insert_absent_terms<8>(
+        op,
+        terms.size(),
+        [&](size_t k) -> const Monomial<8> & { return terms[k]; },
+        [&](size_t k, size_t base) { assign_row<8>(*op.store, base + k, terms[k]); });
+    return op;
+}
+
+} // namespace
+
 BOOST_AUTO_TEST_CASE(matched_epoch_begin_gate_clears_all) {
     MatchedEpochSet set;
     set.begin_gate(5);
@@ -74,6 +102,44 @@ BOOST_AUTO_TEST_CASE(matched_epoch_u32_wrap_resets) {
     BOOST_TEST(set.is_marked(2));
 }
 
+// A self-resolve hit whose index the store only grew into after construction is a real hit -- it must reach
+// the sink -- but it is outside the matched set, whose array is sized to combined_size.
+BOOST_AUTO_TEST_CASE(self_resolve_mark_bounded_by_combined_size) {
+    std::vector<Monomial<8>> terms;
+    for (size_t i = 0; i < 6; ++i) {
+        terms.push_back(indices_to_bitset<8>({i, i + 8}));
+    }
+    detail::MPOperator<8> op = indexed_op(terms);
+    const size_t combined_size = 4; // rows 4 and 5 stand for terms this layer inserted after construction
+
+    MatchedEpochSet matched;
+    // Pre-grown past combined_size on purpose: an unguarded mark then lands in an observable slot rather
+    // than past the end of epoch_, where it would be silent undefined behaviour.
+    matched.begin_gate(op.size());
+
+    detail::LayerBuildEngine<8, RecordingSink> eng(op,
+                                                   mpi::Comm{},
+                                                   /*R_=*/1,
+                                                   /*my_rank_=*/0,
+                                                   matched,
+                                                   combined_size,
+                                                   RecordingSink{});
+    detail::query_push<8>(eng.queries_r[0], terms[1], 1);
+    detail::query_push<8>(eng.queries_r[0], terms[5], -1);
+    eng.src_idx_r[0] = {0, 2};
+
+    eng.resolve_self_queries(/*is_leader_pass=*/true);
+
+    // Both keys are in the store, so both resolve as hits and neither may be deferred as a miss.
+    BOOST_TEST(eng.deferred_self_misses.empty());
+    BOOST_TEST_REQUIRE(eng.sink.hits.size() == 2U);
+    BOOST_TEST(eng.sink.hits[0].second == 1U);
+    BOOST_TEST(eng.sink.hits[1].second == 5U);
+    BOOST_TEST(matched.is_marked(1));
+    BOOST_TEST(!matched.is_marked(4));
+    BOOST_TEST(!matched.is_marked(5));
+}
+
 BOOST_AUTO_TEST_CASE(cutoff_context_abs_coeff_for) {
     const VecD coeffs{-3.0, 2.0, 0.0};