diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index 8c5b4e87..917be21b 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -6,16 +6,28 @@ on:
     - cron: '3 20 * * SUN'
 permissions: {}
 jobs:
+  # The -m32 variants build and test as 32-bit. ssize_t and long are 32 bits
+  # wide on i686, which exposes signedness and range problems that the 64-bit
+  # builds cannot see. The flag goes in CC rather than CFLAGS so that the
+  # libtap build, which has its own Makefile, and the test scripts that
+  # compile code themselves pick it up too. CFLAGS must stay out of the job
+  # environment: when it is set, even empty, make exports the configured
+  # CFLAGS to libtap, and libtap does not build on macOS with _POSIX_C_SOURCE.
   test-autoconf:
     strategy:
       matrix:
         os: [ubuntu-latest, macos-latest]
         cc: [gcc, clang]
         posix: ['', -D_POSIX_C_SOURCE=200112L]
-    name: Autotools build on ${{matrix.os}} using ${{matrix.cc}} ${{matrix.posix}}
+        arch: ['', -m32]
+        exclude:
+          - os: macos-latest
+            arch: -m32
+    name: Autotools build on ${{matrix.os}} using ${{matrix.cc}} ${{matrix.posix}} ${{matrix.arch}}
     runs-on: ${{ matrix.os }}
     env:
-      CC: ${{ matrix.cc }}
+      CC: ${{ matrix.cc }} ${{ matrix.arch }}
+      CXXFLAGS: ${{ matrix.arch }}
       VERBOSE: 1
     steps:
       - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
@@ -24,6 +36,8 @@ jobs:
           persist-credentials: false
       - run: sudo apt install libipc-run3-perl
         if: ${{ matrix.os == 'ubuntu-latest' }}
+      - run: sudo apt-get update && sudo apt-get install -y gcc-multilib g++-multilib
+        if: ${{ matrix.os == 'ubuntu-latest' && matrix.arch == '-m32' }}
       - run: brew install autoconf automake libtool
         if: ${{ matrix.os == 'macos-latest' }}
       - run: ./bootstrap
diff --git a/t/decoder_limits_t.pl b/t/decoder_limits_t.pl
index 3284565e..0d9c05f5 100755
--- a/t/decoder_limits_t.pl
+++ b/t/decoder_limits_t.pl
@@ -21,14 +21,16 @@
 my $root        = abs_path("$Bin/..");
 my $include_dir = "$root/include";
 my $src_dir     = "$root/src";
-my $cc          = $ENV{CC} || 'cc';
+
+# CC may carry flags, such as CC="gcc -m32".
+my @cc = split ' ', $ENV{CC} || 'cc';
 
 # The checks below rebuild the library with -Werror. Only gcc and clang are
 # known to compile it cleanly with the flags used here, so skip elsewhere
 # instead of failing on a missing compiler or an unrelated warning.
 my ( $cc_version, $cc_stderr ) = ( q{}, q{} );
 my $cc_status = eval {
-    run3( [ $cc, '--version' ], \undef, \$cc_version, \$cc_stderr );
+    run3( [ @cc, '--version' ], \undef, \$cc_version, \$cc_stderr );
     $?;
 };
 $cc_version .= $cc_stderr;
@@ -44,7 +46,7 @@
     map { split ' ' } grep { defined } @ENV{ 'CFLAGS', 'LDFLAGS' };
 
 my @base = (
-    $cc,
+    @cc,
     @instrumentation,
     '-std=c99',
     '-Wall',