fix(langchain): avoid duplicating structured tool inputs in metadata (#1722)

Author: milanagm

langfuse/langchain/CallbackHandler.py

@@ -962,6 +962,9 @@ def on_tool_start(
                 "on_tool_start", run_id, parent_run_id, input_str=input_str
             )
 
+            structured_input = kwargs.get("inputs")
+            tool_input = structured_input if structured_input is not None else input_str
+
             meta = self._get_langchain_observation_metadata(
                 parent_run_id=parent_run_id,
                 tags=tags,
@@ -972,15 +975,16 @@ def on_tool_start(
                 meta = {}
 
             meta.update(
-                {key: value for key, value in kwargs.items() if value is not None}
+                {
+                    key: value
+                    for key, value in kwargs.items()
+                    if value is not None and key != "inputs"
+                }
             )
 
             observation_type = self._get_observation_type_from_serialized(
                 serialized, "tool", **kwargs
             )
-            tool_input = kwargs.get("inputs")
-            if tool_input is None:
-                tool_input = input_str
 
             parent_observation = self._get_parent_observation(parent_run_id)
             if isinstance(parent_observation, Langfuse):

tests/unit/test_langchain.py

@@ -818,6 +818,34 @@ def test_tool_start_prefers_structured_inputs_when_available(
     )
 
 
+def test_tool_when_structured_inputs_only_store_in_inputs_attribute_not_metadata(
+    langfuse_memory_client, get_span, json_attr
+):
+    handler = CallbackHandler()
+    run_id = uuid4()
+    structured_inputs = {
+        "path": "/tmp/example.md",
+        "content": "this should be in inputs, not metadata",
+    }
+
+    handler.on_tool_start(
+        {"name": "write_document"},
+        str(structured_inputs),
+        run_id=run_id,
+        metadata={"custom_key": "custom_value"},
+        inputs=structured_inputs,
+    )
+    handler.on_tool_end("ok", run_id=run_id)
+
+    langfuse_memory_client.flush()
+    span = get_span("write_document")
+
+    metadata_prefix = LangfuseOtelSpanAttributes.OBSERVATION_METADATA
+
+    assert span.attributes[f"{metadata_prefix}.custom_key"] == "custom_value"
+    assert f"{metadata_prefix}.inputs" not in span.attributes
+
+
 def test_handled_tool_error_marks_observation_error(
     langfuse_memory_client, get_span, json_attr
 ):