How to Add Decision Gates to LangChain Agents Using Jev, and Where a Tutorial Went Wrong

A developer named Nokka published a technical article on September 19, 2026, explaining how to insert decision-making checkpoints into LangChain agent loops using the Jev library from TypeSafe. The piece was prompted by a SitePoint tutorial from the previous day that proposed placing two Jev gates — a Request Router and a Tool Gate — to route queries by complexity and block risky tool calls before execution. While Nokka found the core concept sound, hands-on testing revealed that the SitePoint code references a non-existent class, Decision, which does not appear anywhere in the actual Jev package's exported API. The real package instead relies on a decorator pattern where a function's docstring serves as a Jinja2 template and its return type must be a Pydantic BaseModel subclass, with field types automatically determining the kind of structured question sent to Jev. Nokka's article preserves the original architectural ideas while correcting the code to match the library's actual API.