diff --git a/test/common/CMakeLists.txt b/test/common/CMakeLists.txt
index 0abff0dd1e..b5192c4205 100644
--- a/test/common/CMakeLists.txt
+++ b/test/common/CMakeLists.txt
@@ -15,4 +15,5 @@ add_lbug_test(types_test
         vfs_test.cpp
         boolean_filter_test.cpp
         simd_filter_test.cpp
+        antlr_interval_set_test.cpp
 )
diff --git a/test/common/antlr_interval_set_test.cpp b/test/common/antlr_interval_set_test.cpp
new file mode 100644
index 0000000000..88c45e42f0
--- /dev/null
+++ b/test/common/antlr_interval_set_test.cpp
@@ -0,0 +1,94 @@
+#include <algorithm>
+#include <bitset>
+#include <limits>
+#include <random>
+#include <vector>
+
+#include "gtest/gtest.h"
+#include "misc/IntervalSet.h"
+
+using antlr4::misc::Interval;
+using antlr4::misc::IntervalSet;
+
+namespace {
+
+constexpr ssize_t RANGE = 512;
+
+void checkMatches(const std::vector<Interval>& added) {
+    IntervalSet set;
+    std::bitset<RANGE> expected;
+    for (auto& interval : added) {
+        set.add(interval);
+        for (auto v = interval.a; v <= interval.b; v++) {
+            expected.set(v);
+        }
+    }
+    for (ssize_t v = 0; v < RANGE; v++) {
+        ASSERT_EQ(set.contains(v), expected.test(v)) << v;
+    }
+    // Canonical form: sorted, and neither overlapping nor touching.
+    auto& intervals = set.getIntervals();
+    for (auto i = 1u; i < intervals.size(); i++) {
+        ASSERT_GT(intervals[i].a, intervals[i - 1].b + 1);
+    }
+}
+
+} // namespace
+
+TEST(AntlrIntervalSetTest, AddKeepsCanonicalForm) {
+    std::mt19937 rng(42);
+    std::uniform_int_distribution<ssize_t> start(0, RANGE - 20), length(0, 6);
+    for (auto round = 0; round < 200; round++) {
+        std::vector<Interval> added;
+        for (auto i = 0; i < 40; i++) {
+            auto a = start(rng);
+            added.emplace_back(a, a + length(rng));
+        }
+        checkMatches(added);
+        std::sort(added.begin(), added.end(),
+            [](const Interval& l, const Interval& r) { return l.a < r.a; });
+        checkMatches(added);
+    }
+    // Sorted input that touches or overlaps the last interval: [1,3] and [4,6] merge, [8,10] is
+    // appended, and [10,12] merges with it.
+    std::vector<Interval> touching;
+    for (ssize_t a : {1, 4, 8, 10}) {
+        touching.emplace_back(a, a + 2);
+    }
+    checkMatches(touching);
+}
+
+// The deserializer adds EOF (-1) first: a range starting at 0 must merge with it, one starting
+// at 1 must not.
+TEST(AntlrIntervalSetTest, EofMergesOnlyWithTouchingRange) {
+    IntervalSet merged;
+    merged.add(-1);
+    merged.add(0, 2);
+    ASSERT_EQ(merged.getIntervals().size(), 1u);
+    EXPECT_EQ(merged.getIntervals()[0],
+        Interval(static_cast<ssize_t>(-1), static_cast<ssize_t>(2)));
+    IntervalSet split;
+    split.add(-1);
+    split.add(1, 2);
+    ASSERT_EQ(split.getIntervals().size(), 2u);
+    EXPECT_EQ(split.getIntervals()[0],
+        Interval(static_cast<ssize_t>(-1), static_cast<ssize_t>(-1)));
+    EXPECT_EQ(split.getIntervals()[1], Interval(static_cast<ssize_t>(1), static_cast<ssize_t>(2)));
+}
+
+// An empty interval is ignored, and the fast path's comparisons hold at the top of the range.
+// Each step keeps ANTLR's adjacent() checks below SSIZE_MAX, where they would overflow.
+TEST(AntlrIntervalSetTest, EmptyIntervalAndRangeLimits) {
+    constexpr auto MAX = std::numeric_limits<ssize_t>::max();
+    IntervalSet set;
+    set.add(5, 3);
+    EXPECT_TRUE(set.getIntervals().empty());
+    set.add(MAX - 3);
+    set.add(MAX - 1);
+    ASSERT_EQ(set.getIntervals().size(), 2u);
+    set.add(MAX - 2);
+    ASSERT_EQ(set.getIntervals().size(), 1u);
+    set.add(MAX);
+    ASSERT_EQ(set.getIntervals().size(), 1u);
+    EXPECT_EQ(set.getIntervals()[0], Interval(MAX - 3, MAX));
+}
diff --git a/third_party/antlr4_runtime/src/misc/IntervalSet.cpp b/third_party/antlr4_runtime/src/misc/IntervalSet.cpp
index 058d98703e..be5ad8099b 100755
--- a/third_party/antlr4_runtime/src/misc/IntervalSet.cpp
+++ b/third_party/antlr4_runtime/src/misc/IntervalSet.cpp
@@ -66,6 +66,15 @@ void IntervalSet::add(const Interval &addition) {
     return;
   }
 
+  // Ladybug local patch on ANTLR 4.13.1, which has no such fast path; keep it on a runtime
+  // upgrade. Deserialized sets arrive sorted; appending them one by one would otherwise scan the
+  // whole list per interval.
+  if (_intervals.empty() ||
+      (_intervals.back().b < addition.a && !addition.adjacent(_intervals.back()))) {
+    _intervals.push_back(addition);
+    return;
+  }
+
   // find position in list
   for (auto iterator = _intervals.begin(); iterator != _intervals.end(); ++iterator) {
     Interval r = *iterator;