diff --git a/tests/test_litellm/proxy/agent_session_endpoints/test_followup_concurrency.py b/tests/test_litellm/proxy/agent_session_endpoints/test_followup_concurrency.py
new file mode 100644
index 00000000000..a6b45bd4b5a
--- /dev/null
+++ b/tests/test_litellm/proxy/agent_session_endpoints/test_followup_concurrency.py
@@ -0,0 +1,130 @@
+"""
+Validation #16 — `/followup` does not bypass the run-busy guard.
+
+A prior version of ``followup`` skipped the ``_has_active_run`` check
+when ``latest_run`` was terminal-or-absent and went straight to
+``litellm_agentrun.create``. Two concurrent ``POST /followup`` requests
+on an idle session both passed the ``latest_run.status in
+RUN_ACTIVE_STATUSES`` check and both inserted runs, breaking the
+"one active run per session" invariant that ``POST /runs`` enforces
+via 409 ``run_busy``.
+
+This test reproduces the race deterministically by setting up a session
+that already has an active run, then sending /followup with a fresh
+request — followup must return 409 ``run_busy`` rather than enqueue a
+duplicate run.
+"""
+
+
+def _bootstrap_session(client):
+    agent = client.post(
+        "/v2/agents",
+        headers={"Authorization": "Bearer k"},
+        json={"name": "concurrent", "model": "gpt-4"},
+    ).json()
+    sess = client.post(
+        "/v2/sessions",
+        headers={"Authorization": "Bearer k"},
+        json={"agent_id": agent["id"], "repos": []},
+    ).json()
+    return sess["id"]
+
+
+def test_followup_returns_409_when_active_run_is_not_the_latest(
+    client, fake_prisma_client, noop_provider
+):
+    """Reproducer for the original bug.
+
+    Scenario: a session has both
+      * an OLDER run still in ``running`` status, and
+      * a NEWER run already in a terminal status (``finished``).
+
+    The buggy code path used ``latest_run`` (newest by ``created_at``) to
+    decide whether to fall through to the "create new run" branch. Since
+    the newest run is terminal, the buggy version skipped ``_has_active_run``
+    and inserted a duplicate run — breaking the one-active-run invariant.
+
+    Two concurrent /followup calls trigger this same race in production
+    even when there's a single run: both observe the same terminal-or-absent
+    ``latest_run`` and both reach the create branch. We can't easily race
+    asyncio in a unit test, so we model the equivalent state directly: an
+    older active run + newer terminal run. With the fix in place, the
+    busy check catches the older active run and returns 409.
+    """
+    from datetime import datetime, timedelta, timezone
+
+    sid = _bootstrap_session(client)
+
+    earlier = datetime.now(timezone.utc) - timedelta(minutes=10)
+    later = datetime.now(timezone.utc)
+
+    # Inject directly into the fake prisma rows so we control created_at
+    # exactly. Bypassing ``.create()`` keeps the ``_now()``-based default
+    # from collapsing the timestamps.
+    from tests.test_litellm.proxy.agent_session_endpoints.conftest import _Row
+
+    fake_prisma_client.db.litellm_agentrun.rows.append(
+        _Row(
+            id="run_old_active",
+            session_id=sid,
+            status="running",
+            prompt={"text": "old"},
+            created_at=earlier,
+            updated_at=earlier,
+        )
+    )
+    fake_prisma_client.db.litellm_agentrun.rows.append(
+        _Row(
+            id="run_newer_terminal",
+            session_id=sid,
+            status="finished",
+            prompt={"text": "newer"},
+            created_at=later,
+            updated_at=later,
+        )
+    )
+
+    res = client.post(
+        f"/v2/sessions/{sid}/followup",
+        headers={"Authorization": "Bearer k"},
+        json={"prompt": {"text": "should be blocked"}},
+    )
+    assert res.status_code == 409
+    assert "run_busy" in res.json()["detail"]
+
+
+def test_followup_normal_path_creates_run_when_no_active_runs(client, noop_provider):
+    """The new busy check must not break the happy path: when there is
+    no active run, /followup still creates a fresh queued run."""
+    sid = _bootstrap_session(client)
+    res = client.post(
+        f"/v2/sessions/{sid}/followup",
+        headers={"Authorization": "Bearer k"},
+        json={"prompt": {"text": "first"}},
+    )
+    assert res.status_code == 200
+    body = res.json()
+    assert body["action"] == "new_run"
+    assert body["run_id"]
+
+
+def test_followup_appends_event_when_run_is_active(client, noop_provider):
+    """Other happy path: latest run IS active — /followup injects a
+    user_message event rather than a new run. Unchanged by the new
+    busy guard (we only fall through to the create branch when the
+    latest run is terminal/absent)."""
+    sid = _bootstrap_session(client)
+    # First followup creates a queued run.
+    first = client.post(
+        f"/v2/sessions/{sid}/followup",
+        headers={"Authorization": "Bearer k"},
+        json={"prompt": {"text": "first"}},
+    ).json()
+    # Second followup should append an event onto the now-active run.
+    second = client.post(
+        f"/v2/sessions/{sid}/followup",
+        headers={"Authorization": "Bearer k"},
+        json={"prompt": {"text": "appended"}},
+    ).json()
+    assert second["action"] == "queued"
+    assert second["run_id"] == first["run_id"]