diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 47a6bb0a08..0ae8ea932e 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -27,6 +27,10 @@ jobs:
         which parallel  # Verify installation
     - name: Test code
       run: test/run
+      env:
+        # Ubuntu runners ship GNU parallel, causing --tap + --jobs count mismatches.
+        # Force single-threaded on Linux; let macOS use parallel (installed above).
+        TEST_JOBS: ${{ runner.os == 'Linux' && '1' || 'detect' }}
 
   build-docs:
     runs-on: ubuntu-latest
diff --git a/test/run b/test/run
index 7846736654..1e88a2af2f 100755
--- a/test/run
+++ b/test/run
@@ -25,7 +25,8 @@ fi
 # AND that it is the GNU version of `parallel`.
 # If that is the case, try to guess the number of CPU cores,
 # so we can run `bats` in parallel processing mode, which is a lot faster.
-if command -v parallel &> /dev/null \
+if [[ "${TEST_JOBS:-detect}" != "1" ]] \
+	&& command -v parallel &> /dev/null \
 	&& parallel -V &> /dev/null \
 	&& { parallel -V 2> /dev/null | grep -q '^GNU\>'; }; then
 	# Expect to run at least on a dual-core CPU; slightly degraded performance