diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 0972e2f..f55465e 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -91,3 +91,27 @@ jobs:
 
       - name: Test
         run: ctest --test-dir build --output-on-failure -C Release
+
+  build-and-test-scalar:
+    name: Linux x64 GCC (scalar)
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Configure
+        env:
+          CC: gcc
+          CXX: g++
+        run: |
+          cmake -S . -B build \
+            -DCMAKE_BUILD_TYPE=Release \
+            -DBITCAL_BUILD_TESTS=ON \
+            -DBITCAL_BUILD_EXAMPLES=ON \
+            -DBITCAL_NATIVE_ARCH=OFF \
+            -DCMAKE_CXX_FLAGS="-mno-avx2"
+
+      - name: Build
+        run: cmake --build build --config Release -j"$(nproc)"
+
+      - name: Test
+        run: ctest --test-dir build --output-on-failure -C Release
diff --git a/AGENTS.md b/AGENTS.md
index cfd7367..5bb1c70 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -59,10 +59,10 @@ BitCal 是一个 C++23、header-only、实验性质的 SIMD 位运算练习库
 
 ### 4.3 工程化
 
-- BitCal 是 header-only 实验库；CI 只保留最小可信路径（format-check + GCC/Clang 两个 build/test job）
+- BitCal 是 header-only 实验库；CI 只保留最小可信路径（format-check + GCC/Clang/scalar 三个 build/test job）
 - 不引入分发库基础设施（install/export/LTO/hardening/cmake config 包）
 - 版本号单一事实源：`include/bitcal/config.hpp`（`CMakeLists.txt` 的 `project()` 版本独立硬编码，仅为 CMake 元数据）
-- scalar 路径与 sanitizer 验证在本地手动执行，不进 CI 矩阵
+- sanitizer 验证在本地手动执行，不进 CI 矩阵（scalar 路径已由 CI scalar job 覆盖）
 - `.clang-format` 的 `Standard` 设为 `Latest`，确保与项目语言基线（C++23）一致
 
 ### 4.4 注释与风格
diff --git a/CHANGELOG.md b/CHANGELOG.md
index bab60c4..769a108 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -31,6 +31,8 @@
 - `benchmarks/CMakeLists.txt` 删除冗余 `cmake_minimum_required`（子目录无需重复声明）
 - 删除 benchmark 文件的 Doxygen `@file`/`@brief` 头（项目无 Doxygen 工具链）
 - 删除 `examples/basic_usage.cpp` 与 `benchmarks/benchmark_compare.cpp` 中静态 span 改造后遗留的未使用 `#include <span>`（`from_words` 改为接收 `std::array` 隐式转换，调用方不再显式使用 span）
+- CI 新增 scalar build/test job（`-mno-avx2` + `BITCAL_NATIVE_ARCH=OFF`），覆盖 `BITCAL_HAS_AVX2 == 0` 分支；此前 CI 两个 job 均走 AVX2 路径，scalar 分支从未被编译和测试
+- `README.md` 与 `AGENTS.md` §4.3 scalar 路径描述同步（"本地手动执行，不进 CI" -> "已进 CI"；sanitizer 验证仍为本地手动）
 
 ### ♻️ 重构
 
@@ -43,6 +45,7 @@
 ### 🧪 测试
 
 - 补 `equals` 的 AVX2 路径回归覆盖：256/512 位确定性用例（块内部分相等、跨 vec 块不等）+ 256 位随机对照参考模型。此前 `equals` 的"不等 -> 提前返回"分支仅在 ≤192 位（走 scalar 尾部）或自比（恒真）下被覆盖，AVX2 路径（≥4 字）无断言保护
+- 补 `shift_left` / `shift_right` 的 256 位随机对照测试：随机 count ∈ [0, Bits] 对照逐 bit 参考模型，覆盖此前手工用例未碰的中间 shift 值
 
 ## [4.1.0] - 2026-07-20
 
diff --git a/README.md b/README.md
index 90b28ab..0dd0a29 100644
--- a/README.md
+++ b/README.md
@@ -104,7 +104,7 @@ cmake --build build --config Release -j"$(nproc)"
 ctest --test-dir build --output-on-failure -C Release
 ```
 
-Scalar 路径（验证 `BITCAL_HAS_AVX2 == 0` 分支，本地手动执行，不进 CI）：
+Scalar 路径（验证 `BITCAL_HAS_AVX2 == 0` 分支，已进 CI；以下本地命令供调试复用）：
 
 ```bash
 cmake -S . -B build-scalar -DCMAKE_BUILD_TYPE=Release -DBITCAL_BUILD_TESTS=ON -DBITCAL_NATIVE_ARCH=OFF -DCMAKE_CXX_FLAGS="-mno-avx2"
diff --git a/tests/support/random_cases.hpp b/tests/support/random_cases.hpp
index aebe8c6..d26b38c 100644
--- a/tests/support/random_cases.hpp
+++ b/tests/support/random_cases.hpp
@@ -67,4 +67,31 @@ std::vector<random_unary_case<Bits>> make_random_unary_cases(const std::uint64_t
     return cases;
 }
 
+template <std::size_t Bits>
+struct random_shift_case {
+    bitcal::bit_block<Bits> block;
+    std::array<std::uint64_t, Bits / 64> words;
+    std::size_t count;
+};
+
+template <std::size_t Bits>
+std::vector<random_shift_case<Bits>> make_random_shift_cases(const std::uint64_t seed, const std::size_t count) {
+    std::mt19937_64 rng(seed);
+    std::uniform_int_distribution<std::size_t> count_dist(0, Bits);
+    std::vector<random_shift_case<Bits>> cases;
+    cases.reserve(count);
+
+    for (std::size_t i = 0; i < count; ++i) {
+        random_shift_case<Bits> tc{};
+        for (std::size_t word = 0; word < Bits / 64; ++word) {
+            tc.words[word] = rng();
+        }
+        tc.block = bitcal::bit_block<Bits>::from_words(tc.words);
+        tc.count = count_dist(rng);
+        cases.push_back(tc);
+    }
+
+    return cases;
+}
+
 }  // namespace bitcal::test
diff --git a/tests/support/reference_model.hpp b/tests/support/reference_model.hpp
index f4f3d39..78b119c 100644
--- a/tests/support/reference_model.hpp
+++ b/tests/support/reference_model.hpp
@@ -87,4 +87,31 @@ bool reference_equals(const word_array<Bits>& lhs, const word_array<Bits>& rhs)
     return true;
 }
 
+// Bit-by-bit reference shifts: obviously correct, O(Bits) per call.
+template <std::size_t Bits>
+word_array<Bits> reference_shift_left(const word_array<Bits>& words, std::size_t count) {
+    word_array<Bits> out{};
+    if (count >= Bits)
+        return out;
+    for (std::size_t i = 0; i + count < Bits; ++i) {
+        if ((words[i / 64] >> (i % 64)) & 1ULL) {
+            out[(i + count) / 64] |= 1ULL << ((i + count) % 64);
+        }
+    }
+    return out;
+}
+
+template <std::size_t Bits>
+word_array<Bits> reference_shift_right(const word_array<Bits>& words, std::size_t count) {
+    word_array<Bits> out{};
+    if (count >= Bits)
+        return out;
+    for (std::size_t i = count; i < Bits; ++i) {
+        if ((words[i / 64] >> (i % 64)) & 1ULL) {
+            out[(i - count) / 64] |= 1ULL << ((i - count) % 64);
+        }
+    }
+    return out;
+}
+
 }  // namespace bitcal::test
diff --git a/tests/test_bitcal.cpp b/tests/test_bitcal.cpp
index b45237b..fd99020 100644
--- a/tests/test_bitcal.cpp
+++ b/tests/test_bitcal.cpp
@@ -778,6 +778,32 @@ bool test_ctad_bit_block_overload_runtime() {
     return true;
 }
 
+bool test_random_shift_left_matches_reference_model_256() {
+    for (const auto& tc : bitcal::test::make_random_shift_cases<256>(0x51F7ULL, 64)) {
+        const auto actual = bitcal::shift_left<256>(tc.block.view(), tc.count);
+        const auto expected = bitcal::test::reference_shift_left<256>(tc.words, tc.count);
+
+        for (std::size_t i = 0; i < bitcal::bit_block<256>::word_count; ++i) {
+            BITCAL_ASSERT_EQ(actual.word(i), expected[i]);
+        }
+    }
+
+    return true;
+}
+
+bool test_random_shift_right_matches_reference_model_256() {
+    for (const auto& tc : bitcal::test::make_random_shift_cases<256>(0x5B17ULL, 64)) {
+        const auto actual = bitcal::shift_right<256>(tc.block.view(), tc.count);
+        const auto expected = bitcal::test::reference_shift_right<256>(tc.words, tc.count);
+
+        for (std::size_t i = 0; i < bitcal::bit_block<256>::word_count; ++i) {
+            BITCAL_ASSERT_EQ(actual.word(i), expected[i]);
+        }
+    }
+
+    return true;
+}
+
 int main() {
     std::cout << "=== BitCal test suite ===" << std::endl;
 
@@ -856,6 +882,10 @@ int main() {
     bitcal::test::run_case(g_counters, "test_ctad_bit_block_overload_runtime", test_ctad_bit_block_overload_runtime);
     bitcal::test::run_case(g_counters, "test_ctad_bit_block_overload_matches_view_form",
                            test_ctad_bit_block_overload_matches_view_form);
+    bitcal::test::run_case(g_counters, "test_random_shift_left_matches_reference_model_256",
+                           test_random_shift_left_matches_reference_model_256);
+    bitcal::test::run_case(g_counters, "test_random_shift_right_matches_reference_model_256",
+                           test_random_shift_right_matches_reference_model_256);
 
     std::cout << std::endl;
     std::cout << "Passed: " << g_counters.pass << std::endl;