diff --git a/tests/test_litellm/proxy/agent_session_endpoints/test_idempotency.py b/tests/test_litellm/proxy/agent_session_endpoints/test_idempotency.py
new file mode 100644
index 00000000000..92601004697
--- /dev/null
+++ b/tests/test_litellm/proxy/agent_session_endpoints/test_idempotency.py
@@ -0,0 +1,65 @@
+"""
+Validation #9 — idempotency.
+
+POST /sessions twice with same Idempotency-Key  -> same session_id.
+POST /runs twice with same Idempotency-Key      -> same run_id, count == 1.
+"""
+
+
+def _create_agent(client) -> str:
+    return client.post(
+        "/v2/agents",
+        headers={"Authorization": "Bearer k"},
+        json={"name": "t", "model": "gpt-4"},
+    ).json()["id"]
+
+
+def test_session_idempotency(client, noop_provider, fake_prisma_client):
+    agent_id = _create_agent(client)
+
+    headers = {"Authorization": "Bearer k", "Idempotency-Key": "uuid-A"}
+    a = client.post(
+        "/v2/sessions",
+        headers=headers,
+        json={"agent_id": agent_id, "repos": []},
+    )
+    b = client.post(
+        "/v2/sessions",
+        headers=headers,
+        json={"agent_id": agent_id, "repos": []},
+    )
+    assert a.status_code == 200 and b.status_code == 200
+    assert a.json()["id"] == b.json()["id"]
+    # Only one row in the table.
+    assert len(fake_prisma_client.db.litellm_agentsession.rows) == 1
+
+
+def test_run_idempotency(client, noop_provider, fake_prisma_client):
+    agent_id = _create_agent(client)
+    sess = client.post(
+        "/v2/sessions",
+        headers={"Authorization": "Bearer k"},
+        json={"agent_id": agent_id, "repos": []},
+    ).json()
+    sid = sess["id"]
+    daemon_token = sess["daemon_token"]
+    client.post(
+        f"/v2/sessions/{sid}/internal/register",
+        headers={"Authorization": f"Bearer {daemon_token}"},
+        json={"vm_id": "i-noop"},
+    )
+
+    headers = {"Authorization": "Bearer k", "Idempotency-Key": "run-key-1"}
+    a = client.post(
+        f"/v2/sessions/{sid}/runs",
+        headers=headers,
+        json={"prompt": {"text": "x"}},
+    )
+    b = client.post(
+        f"/v2/sessions/{sid}/runs",
+        headers=headers,
+        json={"prompt": {"text": "x"}},
+    )
+    assert a.status_code == 200 and b.status_code == 200
+    assert a.json()["id"] == b.json()["id"]
+    assert len(fake_prisma_client.db.litellm_agentrun.rows) == 1