The Functional API for LangGraph provides a flexible approach to building AI workflows, allowing developers to leverage key features like human-in-the-loop interactions, persistence/memory, and streaming without explicitly defining a graph. The Functional API consists of two decorators (`entrypoint` and `task`) that enable the use of standard functions to define workflows, making it easier to adopt LangGraph's features in existing applications. This API is complementary to the Graph API (StateGraph) and can be used in conjunction with it as both APIs share the same underlying runtime. The Functional API provides built-in support for human-in-the-loop interactions, short-term memory, and long-term memory, making it an ideal choice for applications that require review, validation, or corrections at critical stages. Additionally, the Functional API offers streaming capabilities, allowing developers to send real-time updates to users, and provides observability features through LangSmith, LangChain's observability platform. Developers can deploy workflows created using the Functional API to production using LangGraph Platform, and the API can be mixed with the Graph API for flexibility in project design. Overall, the Functional API provides a powerful approach to building AI workflows that effectively combine automation with human input.