LangChain Observability (Python)

Overview

Engineer sets LANGCHAIN_TRACING_V2=true and LANGCHAIN_API_KEY=... 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 LANGSMITH_TRACING and LANGSMITH_API_KEY. The LANGCHAIN_* names are soft-deprecated and fail silently on any chain that goes through 1.0 middleware or create_react_agent. One-line fix:

export LANGSMITH_TRACING=true
export LANGSMITH_API_KEY=lsv2_...
export LANGSMITH_PROJECT=my-service-prod