From c9d41f9bb9fc0308017a134fdbf76fa9e9927862 Mon Sep 17 00:00:00 2001
From: Viliam Gonda <gonda.v@gjh.sk>
Date: Tue, 3 Nov 2026 16:19:12 +0100
Subject: [PATCH] fix(langchain): fallback to exception class name when error
 message is empty

---
 langfuse/langchain/CallbackHandler.py |  2 +-
 tests/unit/test_langchain.py          | 26 ++++++++++++++++++++++++++
 2 files changed, 27 insertions(+), 1 deletion(-)

diff --git a/langfuse/langchain/CallbackHandler.py b/langfuse/langchain/CallbackHandler.py
index dac76f4fc..bdc5bace6 100644
--- a/langfuse/langchain/CallbackHandler.py
+++ b/langfuse/langchain/CallbackHandler.py
@@ -343,7 +343,7 @@ def _get_error_level_and_status_message(
         if any(isinstance(error, t) for t in CONTROL_FLOW_EXCEPTION_TYPES):
             return "DEFAULT", str(error) or type(error).__name__
 
-        return "ERROR", str(error)
+        return "ERROR", str(error) or type(error).__name__
 
     def _get_observation_type_from_serialized(
         self, serialized: Optional[Dict[str, Any]], callback_type: str, **kwargs: Any
diff --git a/tests/unit/test_langchain.py b/tests/unit/test_langchain.py
index c55eaa3d2..95c81d547 100644
--- a/tests/unit/test_langchain.py
+++ b/tests/unit/test_langchain.py
@@ -487,6 +487,32 @@ class DummyControlFlowError(RuntimeError):
         )
 
 
+def test_error_without_message_uses_exception_type_as_status_message(
+    langfuse_memory_client, get_span
+):
+    handler = CallbackHandler()
+    run_id = uuid4()
+
+    handler.on_tool_start(
+        {"name": "failing_tool"},
+        "{}",
+        run_id=run_id,
+    )
+    handler.on_tool_error(
+        ValueError(),
+        run_id=run_id,
+    )
+
+    handler._langfuse_client.flush()
+
+    span = get_span("failing_tool")
+    assert span.attributes[LangfuseOtelSpanAttributes.OBSERVATION_LEVEL] == "ERROR"
+    assert (
+        span.attributes[LangfuseOtelSpanAttributes.OBSERVATION_STATUS_MESSAGE]
+        == "ValueError"
+    )
+
+
 def test_streamed_llm_error_cleans_only_its_completion_start_time_memo(
     langfuse_memory_client,
 ):