Concepts

Rein describes an agent flow as a set of small functions. Each function is one file. The functions reference each other by path. Together they tell the agent what to do next, one step at a time.

The agent drives the flow

Rein does not run the agent. The agent starts a program with the rein_start MCP tool, or with rein run, and answers it one step at a time. The program holds its state while it runs. The agent does each step it is given. See Evaluation for what a run returns.

Functions are files

A project holds one function per file, and the file path gives the function its name. The entry point is always main. A function describes itself in a doc comment. Every input carries a question of its own, which the agent reads before it answers. See Declarations.

Functions hand off by reference

A function ends in text for the agent, a reference to another function, or a call to a template that builds text. A reference is a relative path from the current file. Rein has no imports. See References and Templates.

Inputs are typed questions

An input is a boolean or a choice of strings. Rein asks for an input only when the path it is on needs that input. Inputs look like TypeScript parameters, so they are easy to read. See Parameters and Conditions.

Requests wait for the agent

Some steps ask the agent to act first. The agent does the step, gives the answer, and runs again. Rein keeps no state between runs. See Async requests.

Strict by design

Rein favors explicit cases over fallbacks. The checker reports a missing case, or a path with no result, before a run starts. The rules live in the checker section. The reasons for these choices live in Decisions.