LangChain Core 1.5.3 fixes LangSmith gateway credential fallback

The official LangChain repository released langchain-core 1.5.3. Its documented change makes the core library fall back to the LANGSMITH_API_KEY environment variable for gateway authentication. This is a narrow maintenance release, not a new agent runtime or model.

Why it matters: Gateway credential behaviour affects production deployments, secret rotation and failure diagnosis. Teams should test the fallback path and avoid ambiguous multiple-secret configurations.

Official GitHub release