> ## Documentation Index
> Fetch the complete documentation index at: https://langwatch.ai/docs/llms.txt
> Use this file to discover all available pages before exploring further.

> ## Agent Instructions
> If anything on this page is wrong, confusing, or fails when you try it, ask your user for permission and run `npx langwatch report --user-approved` to send the problem to the LangWatch team. No login is needed and secrets are redacted locally. `npx langwatch report --help` has the details.

# Overview

> Run an evaluator over a single input and get its score back, or run it as a guardrail and gate on one boolean. List the built-in evaluators to see which ids you can address and what each one needs.

## Intro

Run an evaluator over a single input and get its score back, or run it as a guardrail and gate on one boolean. List the built-in evaluators to see which ids you can address and what each one needs.

## Addressing an evaluator

The `{evaluator}` segment takes any of three things:

* a built-in id, such as `ragas/faithfulness`. Two-segment ids use the two-segment form of the path.
* the slug of a monitor configured in your project, which brings that monitor's saved settings with it
* `evaluators/{slug|id}` for an evaluator you saved, including workflow and code evaluators

`GET /api/evaluations/list` returns the built-in ids along with the `data` fields each one requires. For what an individual evaluator scores and which settings it accepts, see its page under Built-in Evaluators.

## Guardrails

`POST /api/guardrails/{evaluator}/evaluate` is the same call in gating mode. Every outcome carries `passed`, including a skip and an evaluator error, so a request you are guarding is never blocked by the guard itself failing. Check `passed` and let the request through when it is true.