intermediatePart 7
Component-level tracing: debugging agent tool calls
· 12 min read
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AI evals & observability
An agent that calls tools does two very different things: it reasons ("I should search the docs") and it acts (actually calls the tool). When something goes wrong, the first question is always which layer failed — did it think wrong, or did the doing break? A flat log can't answer that. A trace can.
In this tutorial you build a small tool-calling agent on WEC Inference, instrument it with Langfuse so every reasoning step and every tool call becomes an inspectable node, then debug two real failures from the trace tree — including the worst kind: a confident, wrong answer that never throws an error. Every command, error, and screenshot below is from a real run.