fix: load paginated run history messages (#3305)

backend/tests/test_runs_api_endpoints.py

@@ -5,6 +5,7 @@ from __future__ import annotations
 from unittest.mock import AsyncMock, MagicMock
 
 from _router_auth_helpers import make_authed_test_app
+from _run_message_pagination_helpers import assert_run_message_page
 from fastapi.testclient import TestClient
 
 from app.gateway.routers import runs
@@ -97,6 +98,51 @@ def test_run_messages_has_more_true_when_extra_row_returned():
     body = response.json()
     assert body["has_more"] is True
     assert len(body["data"]) == 50  # trimmed to limit
+    assert [m["seq"] for m in body["data"]] == list(range(2, 52))
+
+
+def test_run_messages_default_page_keeps_newest_messages_when_extra_row_returned():
+    """Default latest-page trimming drops the older sentinel row, not the newest message."""
+    rows = [_make_message(i) for i in range(16, 67)]
+    run_record = {"run_id": "run-2", "thread_id": "thread-2"}
+    app = _make_app(
+        run_store=_make_run_store(run_record),
+        event_store=_make_event_store(rows),
+    )
+    with TestClient(app) as client:
+        assert_run_message_page(client, "/api/runs/run-2/messages", expected_seq=list(range(17, 67)))
+
+
+def test_run_messages_before_seq_page_keeps_newest_side_when_extra_row_returned():
+    """Backward pagination trims the older sentinel so adjacent pages do not miss the boundary message."""
+    rows = [_make_message(i) for i in range(1, 18)]
+    run_record = {"run_id": "run-2", "thread_id": "thread-2"}
+    app = _make_app(
+        run_store=_make_run_store(run_record),
+        event_store=_make_event_store(rows),
+    )
+    with TestClient(app) as client:
+        assert_run_message_page(
+            client,
+            "/api/runs/run-2/messages?before_seq=18&limit=16",
+            expected_seq=list(range(2, 18)),
+        )
+
+
+def test_run_messages_after_seq_page_keeps_oldest_side_when_extra_row_returned():
+    """Forward pagination still trims the newer sentinel row."""
+    rows = [_make_message(i) for i in range(11, 62)]
+    run_record = {"run_id": "run-2", "thread_id": "thread-2"}
+    app = _make_app(
+        run_store=_make_run_store(run_record),
+        event_store=_make_event_store(rows),
+    )
+    with TestClient(app) as client:
+        assert_run_message_page(
+            client,
+            "/api/runs/run-2/messages?after_seq=10",
+            expected_seq=list(range(11, 61)),
+        )
 
 
 def test_run_messages_passes_after_seq_to_event_store():