From 4a8986dd725b041b1aa3e8f738b66b3a579a656b Mon Sep 17 00:00:00 2001
From: mrinal <mrinal@berri.ai>
Date: Tue, 15 Sep 2026 20:38:35 +0000
Subject: [PATCH] fix(langsmith): keep events appended during an in-flight
 flush instead of clearing them

Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>
---
 litellm/integrations/langsmith.py             |  2 ++
 .../integrations/test_langsmith_init.py       | 32 +++++++++++++++++++
 2 files changed, 34 insertions(+)

diff --git a/litellm/integrations/langsmith.py b/litellm/integrations/langsmith.py
index 9607eccef52..32664ed75d2 100644
--- a/litellm/integrations/langsmith.py
+++ b/litellm/integrations/langsmith.py
@@ -39,6 +39,8 @@ def is_serializable(value):
 
 
 class LangsmithLogger(CustomBatchLogger):
+    preserve_events_added_during_flush = True
+
     def __init__(
         self,
         langsmith_api_key: str | None = None,
diff --git a/tests/test_litellm/integrations/test_langsmith_init.py b/tests/test_litellm/integrations/test_langsmith_init.py
index 0bc9e279fbf..4b4b94da22d 100644
--- a/tests/test_litellm/integrations/test_langsmith_init.py
+++ b/tests/test_litellm/integrations/test_langsmith_init.py
@@ -531,3 +531,35 @@ class TestLangsmithRootRunIdConsistency:
 
         assert data["trace_id"] == "trace-1"
         assert data["dotted_order"] == dotted
+
+
+@pytest.mark.asyncio
+async def test_events_appended_during_flush_are_not_dropped():
+    logger = LangsmithLogger(langsmith_api_key="test-key", langsmith_project="test-project")
+    sent_batches: list[list[dict]] = []
+    late_event = {"credentials": logger.default_credentials, "data": {"id": "late"}}
+
+    async def fake_post(url, json, headers):
+        if not sent_batches:
+            logger.log_queue.append(late_event)
+        sent_batches.append(json["post"])
+        response = MagicMock()
+        response.status_code = 200
+        response.raise_for_status = MagicMock()
+        return response
+
+    logger.async_httpx_client = MagicMock(post=AsyncMock(side_effect=fake_post))
+    logger.log_queue = [
+        {"credentials": logger.default_credentials, "data": {"id": "a"}},
+        {"credentials": logger.default_credentials, "data": {"id": "b"}},
+    ]
+
+    await logger.flush_queue()
+
+    assert [e["id"] for e in sent_batches[0]] == ["a", "b"]
+    assert logger.log_queue == [late_event]
+
+    await logger.flush_queue()
+
+    assert [e["id"] for e in sent_batches[1]] == ["late"]
+    assert logger.log_queue == []