Troubleshooting: Spans showing as 'error' but the chain actually succeeded.

Hey everyone! 👋

Ran into a head-scratcher this week while using LangSmith to monitor a RAG pipeline. The chain was returning perfectly good answers to the user, but in the LangSmith UI, the trace was showing several spans flagged with a red 'error' icon. The overall trace itself wasn't marked as an error, just these internal spans. Took a bit of digging to figure out why a "successful" run looked so problematic.

In my case, the issue was in a custom tool that was part of an agentic workflow. The tool was **raising exceptions for control flow**—like a `ValueError` for invalid parameters—but still returning a valid, user-friendly error message back to the LLM. LangSmith was correctly logging the exception, marking the span as an error, but the agent handled the message and proceeded successfully.

Here's a simplified snippet of what the problematic pattern looked like:

```python
def my_retrieval_tool(query: str):
try:
# ... some logic
if not valid(query):
# This raises an exception, causing an 'error' span
raise ValueError("Query format invalid. Please rephrase.")
except Exception as e:
# We catch it and return a nice message
return f"Tool could not execute: {str(e)}"
```

**The fix?** Instead of raising an exception for non-exceptional cases, I changed the logic to return structured error information. For example:

```python
def my_retrieval_tool(query: str):
if not valid(query):
# Return a dict or a clear string instead of raising
return {"error": "Query format invalid", "suggestion": "Please rephrase."}
# ... normal execution
```

This cleaned up the trace completely. Has anyone else encountered this? I'm curious if there are other common patterns—maybe with async calls or specific LLM provider exceptions—that trigger false error spans.

~CloudOps