diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index f7ee3f7..62a6f90 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -14,9 +14,14 @@ jobs:
         include:
           - version: '1.10'
             os: ubuntu-latest
-            arch: x64
+            arch: default
           - version: '1'
             os: ubuntu-latest
+            arch: default
+          # PENSDP only ships an Intel macOS binary, so pin to an Intel runner
+          # (macos-latest is arm64; macos-13 is the last native x86_64 image).
+          - version: '1'
+            os: macos-13
             arch: x64
     steps:
       - uses: actions/checkout@v6
@@ -24,6 +29,7 @@ jobs:
         with:
           version: ${{ matrix.version }}
           arch: ${{ matrix.arch }}
+          force-arch: ${{ matrix['force-arch'] || false }}
       - uses: julia-actions/cache@v3
       - uses: julia-actions/julia-buildpkg@v1
       - uses: julia-actions/julia-runtest@v1
diff --git a/deps/build.jl b/deps/build.jl
index dc406ad..6b4a788 100644
--- a/deps/build.jl
+++ b/deps/build.jl
@@ -70,6 +70,18 @@ function _download_and_extract()
     return extract_dir
 end
 
+function _find_macos_gfortran()
+    for name in ("gfortran", "gfortran-14", "gfortran-13", "gfortran-12", "gfortran-11")
+        p = Sys.which(name)
+        p === nothing || return p
+    end
+    error(
+        "No `gfortran` found on PATH. The macOS build needs Homebrew's " *
+        "gfortran for libquadmath and the libgcc soft-float helpers used " *
+        "by `libgfortran_mac.a`. Install it with `brew install gcc`.",
+    )
+end
+
 # Build a shared library that exports the `pensdp` symbol so that it can be
 # loaded by Julia via `Libdl.dlopen` and called with `ccall`.
 function _build_shared_library(extract_dir)
@@ -94,9 +106,16 @@ function _build_shared_library(extract_dir)
     elseif os == :apple
         output = joinpath(LIB_DIR, "libpensdp.dylib")
         libgfortran_mac = joinpath(lib_src, "libgfortran_mac.a")
-        cmd = `gcc -dynamiclib -o $(output)
+        # `libgfortran_mac.a` references libquadmath (`_strtoflt128`,
+        # `_quadmath_snprintf`, ...) and the libgcc soft-float helpers
+        # (`___multf3`, `___subtf3`, ...) used for `long double` arithmetic.
+        # Apple's clang ships neither, so link via Homebrew `gfortran`, which
+        # adds its own libgcc/libquadmath search paths.
+        gfortran = _find_macos_gfortran()
+        cmd = `$(gfortran) -shared -dynamiclib -o $(output)
                    -Wl,-force_load,$(libpensdp_a)
                    $(libgfortran_mac)
+                   -lquadmath
                    -framework Accelerate
                    -lpthread`
         @info "Linking shared library" cmd