diff --git a/.gitignore b/.gitignore
index 57c1251..7aa631c 100644
--- a/.gitignore
+++ b/.gitignore
@@ -24,6 +24,8 @@ __pycache__/
 .harness/config.json
 .harness/build/
 .harness/build-ninja-msvc-debug/
+.harness/build-ninja-msvc-debug-english/
+.harness/build-ninja-msvc-debug-prefix/
 .harness/build-ninja-msvc-release/
 .harness/performance/
 .harness/doxygen/
diff --git a/docs/calculation-performance/mkl-coverage.md b/docs/calculation-performance/mkl-coverage.md
index f02cccc..57e92ae 100644
--- a/docs/calculation-performance/mkl-coverage.md
+++ b/docs/calculation-performance/mkl-coverage.md
@@ -151,6 +151,26 @@ T1의 실제 Hex8 VTune stack은 `TwoSum → Add → Multiply → hex8_high_low:
   TwoProduct의 least-bit-sum < −1074 거부와 FMA residual·TwoSum 정규화는 그대로다.
 - zero/low-zero shortcut, nonzero multiplication association 변경은 채택하지 않았다.
 
+이후 공통 산술 담당자는 실제 `fpclassify` 비용을 근거로 이 private header의
+finite 분류만 보강했다. `IsFiniteBinary64`는 memcpy로 읽은 binary64 exponent가
+2047인지 확인하며 NaN/Inf를 거부한다. 연산식·검사 위치·정규화·FMA·exact-tail
+기준은 그대로다. [독립 source 검토](../../.harness/performance/shared-classification/source-numerical-review.md)는
+분류와 지원 환경의 값·실패 조건 보존을 확인했다. Signaling NaN의 exception flag나
+trap 관측까지 동등하다고 주장하지 않는다. 원래 분류의 새 경계 검사 9/9와
+변경 후 pair bit oracle·Hex8·paired state/kinematics 62/62가 native 0으로 통과했다.
+[변경 전 기록](../../.harness/performance/shared-classification/before-native.json),
+[변경 후 기록](../../.harness/performance/shared-classification/after-native.json)과
+[검사 로그](../../.harness/performance/shared-classification/after-checks.txt)를 보존한다.
+이 결과는 최종 모델의 시간 목표나 전체 회귀 검사를 대체하지 않는다.
+
+위 검증 중 기존 MSVC/Ninja의 include prefix 인식 오류도 확인했다. 이 PC에는
+한국어 compiler resource만 있어 `VSLANG=1033`만 지정한 fresh build도 dependency가
+0개였다. 실제 출력 접두사 `참고: 포함 파일: `를 별도 로컬 CMake include로 지정한
+fresh build에서는 `hex8_high_low.cpp.obj`의 13개 dependency와 산술 header를 확인했다.
+[실제 dependency](../../.harness/performance/shared-classification/after-dependencies.txt)를 보존한다.
+공용 preset·compiler flags·설치 환경은 바꾸지 않았다. 기존 자료의 직접 cpp 재컴파일과
+header 변경 추적의 한계를 구별하며 최종 검증도 보정한 fresh build에서 수행한다.
+
 [독립 수치 검토](../../.harness/performance/t2/arithmetic-numerical-review.md)는
 위 두 helper proof/source 범위 PASS이며 실제 검사/속도와 전체 MKL diff 검토를
 대체하지 않는다. Frozen original oracle는 pinned `b4fc1bb`의 원본이며 include guard와
diff --git a/src/fesa/math/high_low_arithmetic.h b/src/fesa/math/high_low_arithmetic.h
index 05ccad6..b64c9de 100644
--- a/src/fesa/math/high_low_arithmetic.h
+++ b/src/fesa/math/high_low_arithmetic.h
@@ -38,9 +38,18 @@ struct HighLow {
 inline HighLow InvalidHighLow() {
   return {std::numeric_limits<double>::quiet_NaN(), 0};
 }
+/// @brief Classifies finite binary64 values without floating arithmetic.
+inline bool IsFiniteBinary64(double value) {
+  static_assert(sizeof(double) == sizeof(std::uint64_t) &&
+                std::numeric_limits<double>::is_iec559 &&
+                std::numeric_limits<double>::digits == 53);
+  std::uint64_t bits = 0;
+  std::memcpy(&bits, &value, sizeof(bits));
+  return (bits & 0x7ff0000000000000ULL) != 0x7ff0000000000000ULL;
+}
 /// @brief Checks both physical components, before comparisons or publication.
 inline bool IsFinite(HighLow value) {
-  return std::isfinite(value.high) && std::isfinite(value.low);
+  return IsFiniteBinary64(value.high) && IsFiniteBinary64(value.low);
 }
 /// @brief Recovers the exact error of supported finite binary64 addition.
 inline HighLow TwoSum(double left, double right) {
@@ -48,8 +57,8 @@ inline HighLow TwoSum(double left, double right) {
   const double bridge = sum - left;
   const double restored_left = sum - bridge;
   const double error = (left - restored_left) + (right - bridge);
-  if (!std::isfinite(sum) || !std::isfinite(bridge) ||
-      !std::isfinite(restored_left) || !std::isfinite(error))
+  if (!IsFiniteBinary64(sum) || !IsFiniteBinary64(bridge) ||
+      !IsFiniteBinary64(restored_left) || !IsFiniteBinary64(error))
     return InvalidHighLow();
   return sum == 0 && error == 0 ? HighLow{} : HighLow{sum, error};
 }
@@ -76,7 +85,7 @@ inline HighLow Add(HighLow left, HighLow right) {
   const auto joined = TwoSum(head.high, middle.high);
   const double remainder = (joined.low + middle.low) + tail.low;
   if (!IsFinite(head) || !IsFinite(tail) || !IsFinite(middle) ||
-      !IsFinite(joined) || !std::isfinite(remainder))
+      !IsFinite(joined) || !IsFiniteBinary64(remainder))
     return InvalidHighLow();
   return TwoSum(joined.high, remainder);
 }
@@ -119,14 +128,14 @@ inline int LowestBitExponent(double value) {
 /// @brief Gives a product and its FMA residual when the dyadic tail fits.
 /// @note Rejects underflow-risk exact tails, even if the rounded high is finite.
 inline HighLow TwoProduct(double left, double right) {
-  if (!std::isfinite(left) || !std::isfinite(right)) return InvalidHighLow();
+  if (!IsFiniteBinary64(left) || !IsFiniteBinary64(right)) return InvalidHighLow();
   if (left == 0 || right == 0) return {};
   if (LowestBitExponent(left) + LowestBitExponent(right) < -1074)
     return InvalidHighLow();
   const double product = left * right;
-  if (!std::isfinite(product)) return InvalidHighLow();
+  if (!IsFiniteBinary64(product)) return InvalidHighLow();
   const double error = std::fma(left, right, -product);
-  if (!std::isfinite(error)) return InvalidHighLow();
+  if (!IsFiniteBinary64(error)) return InvalidHighLow();
   return TwoSum(product, error);
 }
 /// @brief Retains all four component products in deterministic order.
@@ -159,13 +168,13 @@ inline HighLow Divide(HighLow numerator, HighLow denominator) {
     return InvalidHighLow();
   if (Compare(numerator, HighLow{}) == 0) return {};
   const double first = numerator.high / denominator.high;
-  if (!std::isfinite(first) || first == 0) return InvalidHighLow();
+  if (!IsFiniteBinary64(first) || first == 0) return InvalidHighLow();
   auto quotient = HighLow{first};
   for (int iteration = 0; iteration < 2; ++iteration) {
     const auto residual = Subtract(numerator, Multiply(denominator, quotient));
     if (!IsNormalized(residual)) return InvalidHighLow();
     const double correction = residual.high / denominator.high;
-    if (!std::isfinite(correction) || (residual.high != 0 && correction == 0))
+    if (!IsFiniteBinary64(correction) || (residual.high != 0 && correction == 0))
       return InvalidHighLow();
     quotient = Add(quotient, HighLow{correction});
   }
diff --git a/tests/unit/math/high_low_arithmetic_test.cpp b/tests/unit/math/high_low_arithmetic_test.cpp
index 03a94ec..fc41fad 100644
--- a/tests/unit/math/high_low_arithmetic_test.cpp
+++ b/tests/unit/math/high_low_arithmetic_test.cpp
@@ -144,6 +144,23 @@ double FromBits(const std::uint64_t bits) {
   return result;
 }
 
+TEST(HighLowArithmetic, FiniteClassificationMatchesBinary64Boundaries) {
+  const auto check = [](const double value) {
+    const bool expected = std::isfinite(value);
+    EXPECT_EQ(IsFinite(HighLow{value, 0.0}), expected);
+    EXPECT_EQ(IsFinite(HighLow{1.0, value}), expected);
+  };
+  for (std::uint64_t exponent = 0; exponent < 2048U; ++exponent) {
+    for (const auto fraction : {0ULL, 1ULL, 1ULL << 51U,
+                                (1ULL << 52U) - 1ULL}) {
+      for (const auto sign : {0ULL, 1ULL << 63U})
+        check(FromBits(sign | (exponent << 52U) | fraction));
+    }
+  }
+  std::mt19937_64 random{0x5800f17U};
+  for (int sample = 0; sample < 10000; ++sample) check(FromBits(random()));
+}
+
 void ExpectSamePair(const HighLow actual,
                     const fesa::high_low_original::HighLow expected) {
   EXPECT_EQ(Bits(actual.high), Bits(expected.high));