diff --git a/roles/tox/library/test-constraints.txt b/roles/tox/library/test-constraints.txt
new file mode 100644
index 000000000..d2c6e43fe
--- /dev/null
+++ b/roles/tox/library/test-constraints.txt
@@ -0,0 +1,2 @@
+zuul===3.12.0
+doesnotexistonpypi===0.0.1
diff --git a/roles/tox/library/test_tox_install_sibling_packages.py b/roles/tox/library/test_tox_install_sibling_packages.py
index 6b145c28a..8ab245c8f 100644
--- a/roles/tox/library/test_tox_install_sibling_packages.py
+++ b/roles/tox/library/test_tox_install_sibling_packages.py
@@ -18,10 +18,12 @@
 from __future__ import (absolute_import, division, print_function)
 __metaclass__ = type
 
+import os
 import sys
 import testtools
 
 from .tox_install_sibling_packages import get_installed_packages
+from .tox_install_sibling_packages import write_new_constraints_file
 
 
 class TestToxInstallSiblingPackages(testtools.TestCase):
@@ -35,3 +37,27 @@ class TestToxInstallSiblingPackages(testtools.TestCase):
         #               but this might fail later if we stop adding Zuul
         #               in the unit tests.
         self.assertIn("zuul", pkgs)
+
+    def test_write_new_constraints_file(self):
+        # NOTE(mnaser): Given that we run our tests inside Tox, we can
+        #               leverage the tox virtual environment we use in
+        #               unit tests instead of mocking up everything.
+        pkgs = get_installed_packages(sys.executable)
+
+        # NOTE(mnaser): Zuul should be installed in this virtualenv
+        #               but this might fail later if we stop adding Zuul
+        #               in the unit tests.
+        test_constraints = os.path.join(os.path.dirname(__file__),
+                                        'test-constraints.txt')
+        constraints = write_new_constraints_file(test_constraints, pkgs)
+
+        def cleanup_constraints_file():
+            if os.path.exists(constraints):
+                os.unlink(constraints)
+        self.addCleanup(cleanup_constraints_file)
+
+        self.assertTrue(os.path.exists(constraints))
+        with open(constraints) as f:
+            s = f.read()
+            self.assertNotIn("zuul", s)
+            self.assertIn("doesnotexistonpypi", s)
diff --git a/roles/tox/library/tox_install_sibling_packages.py b/roles/tox/library/tox_install_sibling_packages.py
index abec207e6..915186c1b 100644
--- a/roles/tox/library/tox_install_sibling_packages.py
+++ b/roles/tox/library/tox_install_sibling_packages.py
@@ -121,7 +121,8 @@ def get_installed_packages(tox_python):
 
 
 def write_new_constraints_file(constraints, packages):
-    with tempfile.NamedTemporaryFile(delete=False) as constraints_file:
+    with tempfile.NamedTemporaryFile(mode='w', delete=False) \
+            as constraints_file:
         constraints_lines = open(constraints, 'r').read().split('\n')
         for line in constraints_lines:
             package_name = line.split('===')[0]
diff --git a/test-playbooks/tox-siblings.yaml b/test-playbooks/tox-siblings.yaml
new file mode 100644
index 000000000..3cf5e0686
--- /dev/null
+++ b/test-playbooks/tox-siblings.yaml
@@ -0,0 +1,11 @@
+- hosts: all
+  tasks:
+    - name: Run bindep
+      include_role:
+        name: bindep
+    - name: Run tox with constraints
+      include_role:
+        name: tox
+      vars:
+        tox_envlist: docs
+        tox_constraints_file: '{{ zuul.project.src_dir }}/zuul-tests.d/test-constraints.txt'
diff --git a/zuul-tests.d/python-jobs.yaml b/zuul-tests.d/python-jobs.yaml
index 4b9485532..79f987c26 100644
--- a/zuul-tests.d/python-jobs.yaml
+++ b/zuul-tests.d/python-jobs.yaml
@@ -7,6 +7,16 @@
     vars:
       role_name: ensure-sphinx
 
+- job:
+    name: zuul-jobs-test-tox-siblings
+    description: Test the tox role's sibling functionality
+    files:
+      - roles/tox/.*
+    run: test-playbooks/tox-siblings.yaml
+    required-projects:
+      - zuul/zuul
+      - zuul/nodepool
+
 # -* AUTOGENERATED *-
 #  The following project section is autogenerated by
 #    tools/update-test-platforms.py
@@ -16,5 +26,6 @@
     check:
       jobs: &id001
         - zuul-jobs-test-ensure-sphinx
+        - zuul-jobs-test-tox-siblings
     gate:
       jobs: *id001
diff --git a/zuul-tests.d/test-constraints.txt b/zuul-tests.d/test-constraints.txt
new file mode 100644
index 000000000..8b0f3353f
--- /dev/null
+++ b/zuul-tests.d/test-constraints.txt
@@ -0,0 +1,3 @@
+nodepool===3.10.0
+zuul===3.12.0
+openstacksdk==0.39.0