diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index bdc74da446b..e99774e64bc 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -355,7 +355,7 @@ jobs:
           node scripts/lint.js --setup
           node scripts/lint.js --actionlint
           node scripts/lint.js --yamllint
-          node --test ${{ env.HELPER_TESTS }}
+          node --test --test-concurrency=1 ${{ env.HELPER_TESTS }}
 
       # Avoid setup-node downloads on ECS, where nodejs.org may be unreachable
       # through the egress proxy; reuse the machine's Node instead.
@@ -496,7 +496,7 @@ jobs:
       # regression tests. Linux-only (they're platform-independent).
       - name: 'Run .github/scripts helper tests'
         if: "${{ needs.classify_pr.outputs.skip_ci != 'true' && steps.ci_profile.outputs.ci_profile == 'full' }}"
-        run: 'node --test ${{ env.HELPER_TESTS }}'
+        run: 'node --test --test-concurrency=1 ${{ env.HELPER_TESTS }}'
 
       # The install-script packaging suite needs zip/unzip, and throws on a
       # CI host that ships neither, so a silent skip there is impossible.
diff --git a/scripts/tests/ci-platform-lanes.test.js b/scripts/tests/ci-platform-lanes.test.js
index 6be3996d7e6..2a1cd9c7ad1 100644
--- a/scripts/tests/ci-platform-lanes.test.js
+++ b/scripts/tests/ci-platform-lanes.test.js
@@ -197,6 +197,18 @@ describe('platform lanes — the retired sensitivity classifier', () => {
   });
 });
 
+describe('GitHub helper tests', () => {
+  it('runs every invocation serially', () => {
+    const helperSteps = Object.values(ci.jobs)
+      .flatMap((job) => job.steps ?? [])
+      .filter((step) => String(step.run ?? '').includes('env.HELPER_TESTS'));
+    expect(helperSteps).not.toHaveLength(0);
+    for (const step of helperSteps) {
+      expect(String(step.run), step.name).toContain('--test-concurrency=1');
+    }
+  });
+});
+
 describe('platform lanes — a failing nightly is visible', () => {
   it('files an issue when the scheduled CI run fails on main', () => {
     // A nightly nobody is told about is the same silence the merge-queue gate