LangChain Observability (Python) Overview Engineer sets and from the 0.2 docs, restarts the service, and sees zero traces in LangSmith — no errors, no warnings. That is P26: in LangChain 1.0 the canonical env vars are and . The names are soft-deprecated and fail silently on any chain that goes through 1.0 middleware or . One-line fix: Next failure mode: a custom attached via fires on the parent but is silent on LangGraph subgraphs and tool calls — token counts under-report by 30-70% vs the provider dashboard. That is P28: LangGraph creates a child runtime per subgraph, and bound callbacks do…