diff --git a/.github/buildcheck/tests/test_smokelib.py b/.github/buildcheck/tests/test_smokelib.py
new file mode 100644
index 000000000..d523e48d4
--- /dev/null
+++ b/.github/buildcheck/tests/test_smokelib.py
@@ -0,0 +1,74 @@
+# SPDX-FileCopyrightText: 2026 Bernard Ladenthin <bernard.ladenthin@gmail.com>
+#
+# SPDX-License-Identifier: MIT OR Apache-2.0
+
+import os
+import shutil
+import subprocess
+import tempfile
+import unittest
+import zipfile
+
+from buildcheck.tests.helpers import REPO
+
+BASH = shutil.which("bash")
+UNZIP = shutil.which("unzip")
+SMOKE_LIB = os.path.join(REPO, ".github", "smoke-lib.sh")
+
+# Two rows of natives.csv: the Linux x86-64 library jar and one of its GPU module jars.
+LIBRARY = ("cpu-linux-x86-64", "Linux/x86_64/cpu", "libjllama.so")
+MODULE = ("vulkan-linux-x86-64", "Linux/x86_64/vulkan", "libggml-vulkan.so")
+
+
+def write_jar(path, directory, files):
+    with zipfile.ZipFile(path, "w") as jar:
+        for name, text in files.items():
+            jar.writestr("net/ladenthin/llama/%s/%s" % (directory, name), text)
+
+
+@unittest.skipUnless(BASH and UNZIP, "needs bash and unzip")
+class NativesDirectoryTest(unittest.TestCase):
+    """smoke_natives_directory: the natives jars of a smoke set unpacked into one directory."""
+
+    def setUp(self):
+        self.work = tempfile.mkdtemp()
+        self.addCleanup(shutil.rmtree, self.work)
+        smoke = os.path.join(self.work, "smoke")
+        os.mkdir(smoke)
+        write_jar(os.path.join(smoke, "llama-1.0-%s.jar" % LIBRARY[0]), LIBRARY[1],
+                  {LIBRARY[2]: "library", "jllama-files.txt": "libggml.so\n"})
+        write_jar(os.path.join(smoke, "llama-1.0-%s.jar" % MODULE[0]), MODULE[1],
+                  {MODULE[2]: "module", "jllama-files.txt": MODULE[2] + "\n",
+                   "jllama-build.txt": "llama.cpp=b1\n"})
+
+    def unpack(self, set_dir):
+        out = os.path.join(self.work, "out")
+        script = 'source "$1" && smoke_natives_directory "$2" "$3"'
+        return subprocess.run([BASH, "-c", script, "bash", SMOKE_LIB, set_dir, out],
+                              cwd=self.work, capture_output=True, text=True), out
+
+    def assert_unpacked(self, result, out):
+        self.assertEqual(result.returncode, 0, result.stderr)
+        lib_dir = os.path.join(out, "library", "net", "ladenthin", "llama", LIBRARY[1])
+        self.assertEqual(result.stdout.strip(), lib_dir)
+        self.assertTrue(os.path.isfile(os.path.join(lib_dir, LIBRARY[2])))
+        self.assertTrue(os.path.isfile(os.path.join(lib_dir, MODULE[2])))
+        self.assertFalse(os.path.exists(os.path.join(lib_dir, "jllama-build.txt")))
+        with open(os.path.join(lib_dir, "jllama-files.txt")) as files:
+            self.assertEqual(files.read().split(), ["libggml.so", MODULE[2]])
+
+    def test_relative_set_directory(self):
+        # publish.yml passes the set as `smoke`, relative to the job's directory; the jars are
+        # unpacked after a cd into the output directory (red in run 39799320257).
+        self.assert_unpacked(*self.unpack("smoke"))
+
+    def test_absolute_set_directory(self):
+        self.assert_unpacked(*self.unpack(os.path.join(self.work, "smoke")))
+
+    def test_missing_set_directory_fails(self):
+        result, _ = self.unpack("no-such-set")
+        self.assertNotEqual(result.returncode, 0)
+
+
+if __name__ == "__main__":
+    unittest.main()
diff --git a/.github/smoke-lib.sh b/.github/smoke-lib.sh
index e4d77fc58..cd15b52e3 100644
--- a/.github/smoke-lib.sh
+++ b/.github/smoke-lib.sh
@@ -85,7 +85,10 @@ smoke_classpath_without_natives() {
 # jllama-files.txt lines appended likewise, its own jllama-build.txt and jllama-sbom.json dropped
 # (same tag by contract, which the loader checked when the jars were built). Prints the directory.
 smoke_natives_directory() {
-    local set_dir="$1" out="$2"
+    # Absolute, because the jars are unpacked after a cd into the output directory and the
+    # workflows pass the set relative to the job's directory (`smoke`).
+    local set_dir out="$2"
+    set_dir="$(cd "$1" && pwd)" || { echo "::error::smoke set directory '$1' does not exist" >&2; return 1; }
     rm -rf "$out" && mkdir -p "$out"
     local rows
     rows="$(grep -v -e '^#' -e '^classifier,' -e '^$' "$SMOKE_LIST" | tr -d '\r')"