diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index e09540b3e8..0dc0a82ceb 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -360,9 +360,10 @@ jobs:
             container_suffix: "-bookworm"
           - clang: 18
             std: 20
+            cxx_flags: "-Werror -Wall -Wextra -Wwrite-strings -Wunreachable-code -Wpointer-arith -Wredundant-decls"
             container_suffix: "-bookworm"
 
-    name: "🐍 3 • Clang ${{ matrix.clang }} • C++${{ matrix.std }} • x64"
+    name: "🐍 3 • Clang ${{ matrix.clang }} • C++${{ matrix.std }} • x64${{ matrix.cxx_flags && ' • cxx_flags' || '' }}"
     container: "silkeh/clang:${{ matrix.clang }}${{ matrix.container_suffix }}"
 
     steps:
@@ -378,6 +379,7 @@ jobs:
         -DPYBIND11_WERROR=ON
         -DDOWNLOAD_CATCH=ON
         -DCMAKE_CXX_STANDARD=${{ matrix.std }}
+        -DCMAKE_CXX_FLAGS="${{ matrix.cxx_flags }}"
         -DPYTHON_EXECUTABLE=$(python3 -c "import sys; print(sys.executable)")
 
     - name: Build
diff --git a/include/pybind11/detail/internals.h b/include/pybind11/detail/internals.h
index 5fcaf9b9c1..13f82d0307 100644
--- a/include/pybind11/detail/internals.h
+++ b/include/pybind11/detail/internals.h
@@ -37,22 +37,14 @@
 /// further ABI-incompatible changes may be made before the ABI is officially
 /// changed to the new version.
 #ifndef PYBIND11_INTERNALS_VERSION
-#    if PY_VERSION_HEX >= 0x030C0000 || defined(_MSC_VER)
-// Version bump for Python 3.12+, before first 3.12 beta release.
-// Version bump for MSVC piggy-backed on PR #4779. See comments there.
-#        ifdef Py_GIL_DISABLED
-#            define PYBIND11_INTERNALS_VERSION 6
-#        else
-#            define PYBIND11_INTERNALS_VERSION 5
-#        endif
-#    else
-#        define PYBIND11_INTERNALS_VERSION 4
-#    endif
+#    define PYBIND11_INTERNALS_VERSION 6
 #endif
 
 // This requirement is mainly to reduce the support burden (see PR #4570).
 static_assert(PY_VERSION_HEX < 0x030C0000 || PYBIND11_INTERNALS_VERSION >= 5,
               "pybind11 ABI version 5 is the minimum for Python 3.12+");
+static_assert(PYBIND11_INTERNALS_VERSION >= 4,
+              "pybind11 ABI version 4 is the minimum for all platforms.");
 
 PYBIND11_NAMESPACE_BEGIN(PYBIND11_NAMESPACE)
 
diff --git a/include/pybind11/eigen/matrix.h b/include/pybind11/eigen/matrix.h
index 5cf1f0a2a0..d2999d61be 100644
--- a/include/pybind11/eigen/matrix.h
+++ b/include/pybind11/eigen/matrix.h
@@ -316,8 +316,11 @@ struct type_caster<Type, enable_if_t<is_eigen_dense_plain<Type>::value>> {
             return false;
         }
 
+        PYBIND11_WARNING_PUSH
+        PYBIND11_WARNING_DISABLE_GCC("-Wmaybe-uninitialized") // See PR #5516
         // Allocate the new type, then build a numpy reference into it
         value = Type(fits.rows, fits.cols);
+        PYBIND11_WARNING_POP
         auto ref = reinterpret_steal<array>(eigen_ref_array<props>(value));
         if (dims == 1) {
             ref = ref.squeeze();