Rein

Status: proof of concept. Private.

Rein is a language for agent flows. Other tools manage the harness or a graph that the agent runs in. Rein does not. It gives the agent instructions for what to do. Each function is one file. It returns the next step as text, as a reference to another function, or as a template call. When it needs a fact, it asks the agent. When it needs a step done, it sends the agent a request and waits for the answer.

Rein mimics TypeScript syntax and types, except @ references. There are no imports, no else, and no default. Every function returns on every path. A function takes one parameter: a scalar or a struct, and a shared struct is a <name>.struct.rein file.

Sample

examples/rein/merge-pr/merge_pr.rein:

// merge_pr.rein

interface Input {
  /** Does the PR change only documentation files? */
  docs_only: boolean;
  /** What is the CI status of the PR? */
  ci: "green" | "red" | "pending";
}

/**
 * Decide whether a pull request can be merged.
 */
export default function mergePr({ docs_only, ci }: Input) {
  if (docs_only) {
    return `approved`;
  }
  switch (ci) {
    case "green":
      return @./merge_pr/review_code;
    case "red":
      return @./merge_pr/fix;
    case "pending":
      return `wait for CI, then run merge_pr again`;
  }
}

How an agent uses it

The agent steps the flow with rein step. The runtime keeps no state, so each call takes every answer so far. Each call returns one of four kinds: question, request, done, or error. The walk below runs examples/rein/support-ticket. Each line after $ is one call. # exit is its exit code:

$ cd examples/rein/support-ticket
$ rein step --json
{"kind":"question","version":1,"key":"main.vip","function":"main","name":"vip","type":"boolean","options":null,"description":"Is the customer on the VIP plan?","answers":{},"trace":["main"]}
# exit 2

$ rein step --json --answer main.vip=no
{"kind":"question","version":1,"key":"main.category","function":"main","name":"category","type":"union","options":["billing","account","how_to"],"description":"What category best fits the ticket?","answers":{"main.vip":"no"},"trace":["main"]}
# exit 2

$ rein step --json --answer main.vip=no --answer main.category=how_to
{"kind":"question","version":1,"key":"faq.article_found","function":"faq","name":"article_found","type":"boolean","options":null,"description":"Did you find a help article that answers the question?","answers":{"main.category":"how_to","main.vip":"no"},"trace":["main","faq"]}
# exit 2

$ rein step --json --answer main.vip=no --answer main.category=how_to --answer faq.article_found=yes
{"kind":"request","version":1,"key":"faq.solved","function":"faq","instruction":"Reply with the help article link and ask the customer if it solved the problem.","response":{"name":"solved","type":"boolean","options":null,"description":"Did the customer confirm the article solved the problem?"},"answers":{"faq.article_found":"yes","main.category":"how_to","main.vip":"no"},"trace":["main","faq"]}
# exit 3

$ rein step --json --answer main.vip=no --answer main.category=how_to --answer faq.article_found=yes --answer faq.solved=yes
{"kind":"done","version":1,"instruction":"Close the ticket as solved.","answers":{"faq.article_found":"yes","faq.solved":"yes","main.category":"how_to","main.vip":"no"},"trace":["main","faq"]}
# exit 0

A call follows each reference in the same call. The trace lists the functions it visited. For a question or a request, the agent asks the user or does the step, adds the key to its answers, and steps again. Without --json, rein step prints plain text. rein eval evaluates one function, for debugging. The protocol and exit codes are in spec.md section 11.

Quickstart

cargo install --path crates/rein        # puts `rein` on PATH
cd examples
rein list                               # the programs visible from here
rein run support-ticket                 # run one program by name, answer on stdin
rein graph support-ticket               # its decision flow as Mermaid (--format dot|json, -o graph.svg)
rein serve --mcp                        # the MCP host for agents (see https://github.com/rein-lang/rein-lang.com/blob/main/language/interface.md)
rein watch                              # a page of the agents that use rein now

To install, update, or remove a program, see Install a program.

Check an example:

cargo run -p rein -- check examples/rein/merge-pr

Examples: merge-pr, incident-triage, support-ticket, pr-handling, priority-work.

The VS Code extension lives in rein-lang/vscode-rein. Its tests run there with pnpm test.

The docs site and the landing page live in rein-lang/rein-lang.com and deploy to https://rein-lang.com.

Repository layout

Path	Contents
crates/rein-core	Lexer, parser, checker, evaluator, and the std library (std/).
crates/rein	CLI and language server.
rein-lang/vscode-rein	VS Code extension, in its own repo. A thin JS client for the language server, a Rein grammar, snippets, and tests.
examples	Example projects.
rein-lang/rein-lang.com	Language reference, decisions and the docs site, in its own repo.

Docs

• Language reference: syntax, rules, evaluation, CLI.
• Decisions: owner facts, PoC choices, open questions.
• Agent interface: programs as named apps, the MCP host, and rein run.

Enable the repo hooks once with git config core.hooksPath .githooks. The pre-push hook needs linecheck and runs it. It builds rein from the branch and runs rein check on every installed program (scripts/check-installed-programs.sh). A checker change that breaks an installed program blocks the push. The site's CLI and example pages are generated from this repo in rein-lang/rein-lang.com. Its CI fails when they drift from the pinned rein commit.

License

MIT. See LICENSE.