diff --git a/.github/workflows/linux.yml b/.github/workflows/linux.yml
index ad4f5f6..8bfd736 100644
--- a/.github/workflows/linux.yml
+++ b/.github/workflows/linux.yml
@@ -81,6 +81,52 @@ jobs:
     - name: Test
       run: ./build/test/dross_test --gtest_color=yes
 
+  # The requirements say either standard library works across the whole Clang
+  # range, so libc++ is verified at both ends rather than left to best effort.
+  # Covering only the lower bound would leave the upper end of the declared
+  # range unverified against libc++ header changes and newly added warnings —
+  # and -Werror is public, so those reach consumers.
+  #
+  # libc++ is not part of the runner image, but it comes from the
+  # distribution's own repository, so no external source enters a required gate.
+  build-ubuntu-clang-libcxx:
+    name: Ubuntu Clang ${{ matrix.clang }} libc++ (${{ matrix.build_type }})
+    runs-on: ubuntu-26.04
+    strategy:
+      fail-fast: false
+      matrix:
+        build_type: [Debug, Release]
+        clang: [20, 22]
+
+    steps:
+    - uses: actions/checkout@v7
+
+    - name: Install libc++
+      run: |
+        sudo apt-get update
+        sudo apt-get install -y \
+          libc++-${{ matrix.clang }}-dev libc++abi-${{ matrix.clang }}-dev
+
+    - name: Report toolchain versions
+      run: |
+        clang-${{ matrix.clang }} --version
+        cmake --version
+
+    - name: Configure CMake
+      run: |
+        cmake -S . -B build \
+          -DCMAKE_BUILD_TYPE=${{ matrix.build_type }} \
+          -DCMAKE_C_COMPILER=clang-${{ matrix.clang }} \
+          -DCMAKE_CXX_COMPILER=clang++-${{ matrix.clang }} \
+          -DCMAKE_CXX_FLAGS="-stdlib=libc++" \
+          -DCMAKE_EXE_LINKER_FLAGS="-stdlib=libc++"
+
+    - name: Build
+      run: cmake --build build --config ${{ matrix.build_type }} -j"$(nproc)"
+
+    - name: Test
+      run: ./build/test/dross_test --gtest_color=yes
+
   build-static-library:
     name: Static Library Build
     runs-on: ubuntu-24.04
diff --git a/.github/workflows/nightly.yml b/.github/workflows/nightly.yml
index 811f001..4106da8 100644
--- a/.github/workflows/nightly.yml
+++ b/.github/workflows/nightly.yml
@@ -98,41 +98,6 @@ jobs:
     - name: Test
       run: ./build/test/dross_test --gtest_color=yes
 
-  # The required gates build Clang against the default standard library.
-  # This covers the other choice, so neither ends up being the only one that
-  # works.
-  libcxx:
-    name: Clang with libc++
-    runs-on: ubuntu-26.04
-
-    steps:
-    - uses: actions/checkout@v7
-
-    - name: Install libc++
-      run: |
-        sudo apt-get update
-        sudo apt-get install -y libc++-20-dev libc++abi-20-dev
-
-    - name: Report toolchain versions
-      run: |
-        clang-20 --version
-        cmake --version
-
-    - name: Configure CMake
-      run: |
-        cmake -S . -B build \
-          -DCMAKE_BUILD_TYPE=Release \
-          -DCMAKE_C_COMPILER=clang-20 \
-          -DCMAKE_CXX_COMPILER=clang++-20 \
-          -DCMAKE_CXX_FLAGS="-stdlib=libc++" \
-          -DCMAKE_EXE_LINKER_FLAGS="-stdlib=libc++"
-
-    - name: Build
-      run: cmake --build build -j"$(nproc)"
-
-    - name: Test
-      run: ./build/test/dross_test --gtest_color=yes
-
   floating-runners:
     name: Floating Runner (${{ matrix.os }})
     runs-on: ${{ matrix.os }}
diff --git a/README.md b/README.md
index 19713f2..68f140f 100644
--- a/README.md
+++ b/README.md
@@ -24,7 +24,7 @@
 ### Requirements
 
 - **C++23** compatible compiler, verified in CI as:
-  - Linux: GCC 13–15 or Clang 20–22
+  - Linux: GCC 13–15, or Clang 20–22 with either libstdc++ or libc++
   - macOS: the Apple Clang shipped with macOS 15 or 26
   - Newer versions are best effort, exercised by the nightly toolchain watch
 - **CMake 3.20+**
diff --git a/docs/sphinx/source/contributing.rst b/docs/sphinx/source/contributing.rst
index c4c0fa9..0b56eec 100644
--- a/docs/sphinx/source/contributing.rst
+++ b/docs/sphinx/source/contributing.rst
@@ -33,8 +33,9 @@ Development Setup
 Build Requirements
 ~~~~~~~~~~~~~~~~~~
 
-- C++23 compatible compiler: on Linux, GCC 13-15 or Clang 20-22; on macOS,
-  the Apple Clang shipped with macOS 15 or 26. Newer versions are best effort.
+- C++23 compatible compiler: on Linux, GCC 13-15, or Clang 20-22 with either
+  libstdc++ or libc++; on macOS, the Apple Clang shipped with macOS 15 or 26.
+  Newer versions are best effort.
 - CMake 3.20 or later
 - Git
 
diff --git a/docs/sphinx/source/getting-started.rst b/docs/sphinx/source/getting-started.rst
index 731e801..ca190ec 100644
--- a/docs/sphinx/source/getting-started.rst
+++ b/docs/sphinx/source/getting-started.rst
@@ -10,7 +10,8 @@ To build and use dross, you need:
 
 - **C++ Compiler**: Supporting C++23 standard
 
-  - On Linux: GCC 13 through 15, or Clang 20 through 22
+  - On Linux: GCC 13 through 15, or Clang 20 through 22 with either libstdc++
+    or libc++
   - On macOS: the Apple Clang shipped with macOS 15 or 26
 
   Newer versions are best effort: they are exercised by the nightly toolchain
@@ -18,9 +19,10 @@ To build and use dross, you need:
 
   The Clang lower bound is higher than the GCC one because older Clang
   releases cannot compile this library's C++23 ``std::expected`` usage against
-  the libstdc++ they are paired with on Ubuntu 24.04, whether or not newer
-  libstdc++ headers are installed alongside. Either standard library works
-  from Clang 20 onwards, so neither is imposed.
+  the libstdc++ they are paired with on Ubuntu 24.04 — installing the
+  libstdc++ 14 headers alongside them does not change that either. From Clang
+  20 onwards both standard libraries work, and the build matrix verifies each
+  of them, so neither is imposed on you.
 
 - **Build System**: CMake 3.20 or later
 - **Operating System**: Linux or macOS