diff --git a/tests/integration/test_semaphore.py b/tests/integration/test_semaphore.py
new file mode 100644
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+++ b/tests/integration/test_semaphore.py
@@ -0,0 +1,149 @@
+# SPDX-License-Identifier: Apache-2.0
+
+"""Redis-semaphore integration tests against a live Redis.
+
+``RedisSemaphore`` caps concurrent NetBox API requests: ``osism.tasks.netbox``
+builds one per NetBox URL through ``create_netbox_semaphore``. Holders live in a
+sorted set and are admitted by a server-side Lua script (``ZREMRANGEBYSCORE``
+plus ``ZCARD`` plus ``ZADD``).
+
+The script's logic is not what needs a live server:
+``tests/unit/utils/test_init_semaphore.py`` runs the production ``_ACQUIRE_LUA``
+against ``fakeredis``, which executes Lua through ``lupa``. What only the real
+server provides is Redis' own Lua sandbox and ``redis.call`` bindings, the
+numeric coercion of the ``now``, ``maxsize`` and expiry arguments that ``lupa``
+only approximates, and the race the unit suite explicitly defers to here:
+many clients contending for the same free slot at the same moment.
+"""
+
+import concurrent.futures
+import hashlib
+import threading
+import uuid
+
+import pytest
+
+from osism import utils
+
+pytestmark = pytest.mark.integration
+
+
+def semaphore_redis_key(key):
+    """The prefixed key ``RedisSemaphore`` stores its sorted set under."""
+    return f"semaphore:{key}"
+
+
+@pytest.fixture
+def redis_client():
+    """The shared Redis client the semaphore itself uses.
+
+    ``create_netbox_semaphore`` wires this very client into the semaphores it
+    builds, so seeding and inspecting through it observes what the code under
+    test sees. It is a cached module global held for the whole session, so it
+    is not closed here: that would break every later test that touches it.
+    """
+    return utils.redis
+
+
+@pytest.fixture
+def semaphore_key(redis_client):
+    """A key unique to this test, removed from Redis afterwards.
+
+    ``RedisSemaphore`` prefixes what it is handed, so the sorted set lives under
+    ``semaphore:<key>``. Raw seeding and inspection use that prefixed form.
+    """
+    key = f"itest-sem-{uuid.uuid4()}"
+    yield key
+    redis_client.delete(semaphore_redis_key(key))
+
+
+def test_release_frees_slot(redis_client, semaphore_key):
+    """The slot a holder gives up is granted to the caller that was refused.
+
+    The unit suite covers a slot freed by expiry, never one freed by
+    ``release`` and granted to a waiter inside its retry loop.
+    """
+    holder = utils.RedisSemaphore(redis_client, semaphore_key, 1, timeout=5)
+    assert holder.acquire() is True
+
+    waiter = utils.RedisSemaphore(redis_client, semaphore_key, 1, timeout=0.5)
+    assert waiter.acquire() is False
+
+    holder.release()
+
+    assert waiter.acquire() is True
+    assert redis_client.zcard(waiter.key) == 1
+
+    waiter.release()
+
+
+def test_create_netbox_semaphore_key_and_maxsize(redis_client):
+    """The NetBox helper derives its key from the URL and works against Redis."""
+    url = f"https://netbox-{uuid.uuid4()}.example"
+    url_hash = hashlib.md5(url.encode()).hexdigest()[:8]
+    redis_key = f"semaphore:netbox_semaphore_{url_hash}"
+
+    sem = utils.create_netbox_semaphore(url, max_connections=2)
+
+    try:
+        assert isinstance(sem, utils.RedisSemaphore)
+        assert sem.key == redis_key
+        assert sem.maxsize == 2
+
+        assert sem.acquire() is True
+        members = {member.decode() for member in redis_client.zrange(redis_key, 0, -1)}
+        assert members == {sem.identifier}
+
+        sem.release()
+        assert redis_client.zcard(redis_key) == 0
+    finally:
+        redis_client.delete(redis_key)
+
+
+def test_concurrent_acquire_never_exceeds_maxsize(redis_client, semaphore_key):
+    """Under a real race only ``maxsize`` of many contending clients get in.
+
+    The atomic Lua script is what makes this hold: a capacity check and a slot
+    reservation split across two round trips would let several clients observe
+    the same free slot and all take it. This is the many-client race the unit
+    suite defers to a live server; ``fakeredis`` never reproduces it, because a
+    single sequential client cannot over-admit.
+
+    One round only lands the interleaving that exposes an over-admitting
+    implementation about nine times in ten, so the race is run repeatedly. A
+    round costs a full acquire timeout, since the losers each wait theirs out,
+    which is why the timeout here is much shorter than elsewhere in this file.
+
+    Every thread hands its result back through a future, so a worker that dies
+    fails the test. Left unchecked it would only warn, and the race this test
+    exists for would silently shrink to the threads that survived.
+    """
+    maxsize = 3
+    thread_count = 12
+    rounds = 5
+    redis_key = semaphore_redis_key(semaphore_key)
+
+    def contend(barrier):
+        sem = utils.RedisSemaphore(redis_client, semaphore_key, maxsize, timeout=0.1)
+        barrier.wait()
+        return sem if sem.acquire() else None
+
+    for _ in range(rounds):
+        # A timeout on the barrier turns a thread that never arrives into a
+        # failed assertion rather than a hung CI job.
+        barrier = threading.Barrier(thread_count, timeout=30)
+
+        with concurrent.futures.ThreadPoolExecutor(max_workers=thread_count) as pool:
+            futures = [pool.submit(contend, barrier) for _ in range(thread_count)]
+            acquired = [sem for sem in (future.result() for future in futures) if sem]
+
+        # Nothing is released before every future has been collected, so the
+        # holder count is stable here.
+        assert len(acquired) == maxsize
+        assert redis_client.zcard(redis_key) == maxsize
+
+        for sem in acquired:
+            sem.release()
+
+        assert redis_client.zcard(redis_key) == 0
+        redis_client.delete(redis_key)