diff --git a/backend/app/gateway/routers/thread_runs.py b/backend/app/gateway/routers/thread_runs.py
index 111f9610da..ade649cd33 100644
--- a/backend/app/gateway/routers/thread_runs.py
+++ b/backend/app/gateway/routers/thread_runs.py
@@ -90,6 +90,7 @@ _artifact_archive_slots = asyncio.Semaphore(4)
 _MISSING_REGENERATE_BASE_DETAIL = "Could not find an addressable checkpoint before the target user message"
 _UNSAFE_REGENERATE_LINEAGE_DETAIL = "Could not safely resolve the checkpoint before the target user message"
 THREAD_MESSAGE_LEGACY_SCAN_BATCH = 201
+MAX_EVENT_TYPE_FILTERS = 100
 _LEGACY_IDEMPOTENCY_REQUEST_KEY = "idempotency_request"
 
 
@@ -1850,7 +1851,15 @@ async def list_run_events(
     await _require_run_visible_to_scope(run_id, thread_id, request)
     user_id = await _run_scope_user_id(request, thread_id)
     event_store = get_run_event_store(request)
-    types = event_types.split(",") if event_types else None
+    if event_types:
+        types = event_types.split(",")
+        if len(types) > MAX_EVENT_TYPE_FILTERS:
+            raise HTTPException(
+                status_code=422,
+                detail=f"event_types accepts at most {MAX_EVENT_TYPE_FILTERS} values",
+            )
+    else:
+        types = None
     events = await event_store.list_events(
         thread_id,
         run_id,
diff --git a/backend/tests/test_run_events_endpoint.py b/backend/tests/test_run_events_endpoint.py
index 3afa4e32e1..416f09cedd 100644
--- a/backend/tests/test_run_events_endpoint.py
+++ b/backend/tests/test_run_events_endpoint.py
@@ -196,3 +196,60 @@ async def test_effective_memory_flows_from_injection_to_the_existing_debug_api()
         "project_context_revision": None,
         "project_shelf_revision": None,
     }
+
+
+@pytest.mark.parametrize("count", [100, 101])
+@pytest.mark.anyio
+async def test_list_run_events_bounds_event_type_filter_cardinality(count: int) -> None:
+    from fastapi import HTTPException
+
+    from app.gateway.routers.thread_runs import MAX_EVENT_TYPE_FILTERS, list_run_events
+
+    calls: dict = {}
+
+    class FakeStore:
+        async def list_events(
+            self,
+            thread_id,
+            run_id,
+            *,
+            event_types=None,
+            task_id=None,
+            limit=500,
+            after_seq=None,
+            user_id=None,
+        ):
+            calls["event_types"] = event_types
+            return []
+
+    class FakeState:
+        run_event_store = FakeStore()
+
+    class FakeApp:
+        state = FakeState()
+
+    class FakeRequest:
+        app = FakeApp()
+        _deerflow_test_bypass_auth = True
+        cookies: dict = {}
+
+    event_types = ",".join(f"type-{i}" for i in range(count))
+    kwargs = {
+        "thread_id": "t1",
+        "run_id": "r1",
+        "request": FakeRequest(),
+        "event_types": event_types,
+        "task_id": None,
+        "limit": 500,
+        "after_seq": None,
+    }
+
+    if count > MAX_EVENT_TYPE_FILTERS:
+        with pytest.raises(HTTPException) as exc_info:
+            await list_run_events(**kwargs)
+        assert exc_info.value.status_code == 422
+        assert str(MAX_EVENT_TYPE_FILTERS) in exc_info.value.detail
+        assert calls == {}
+    else:
+        assert await list_run_events(**kwargs) == []
+        assert len(calls["event_types"]) == MAX_EVENT_TYPE_FILTERS