Metlo supports writing custom templates using Typescript. You can write and test your templates locally and push them to Metlo using the CLI (Setup Instructions).

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Setting Up Your Environment

The first step is to setup a node project to build your templates in. Be sure to have both Node and NPM installed before following these steps! We highly recommend using nvm.
Bash
mkdir metlo-test-templates
cd metlo-test-templates
npm init
After setting up your project install @metlo/testing for the testing utility functions.
Bash
npm install @metlo/testing

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Creating a New Template

Although you can structure your templates however you want, since templates can have different versions we recommend structuring your template repository like this (with folders for templates and typescript files for each version): 
metlo-test-templates/
├── node_modules
├── templates/
│   ├── <TEMPLATE_NAME>/
│   │   ├── v1.ts
│   │   └── v2.ts
└── package.json
First create a folder called templates in your project: 
mkdir templates
After that create a folder inside it for your template, lets make a template that verifies if an endpoint always responds with CSP Headers called VERIFY_CSP_HEADERS. 
cd templates
mkdir VERIFY_CSP_HEADERS
cd VERIFY_CSP_HEADERS
touch v1.ts
Now you can open v1.ts in the editor of your choice and paste the following:
TypeScript
import {
  AssertionType,
  GenTestEndpoint,
  TestBuilder,
  TestStepBuilder,
} from "@metlo/testing";

export default {
  name: "VERIFY_CSP_HEADERS",
  version: 1,
  builder: (endpoint: GenTestEndpoint) => {
    return new TestBuilder()
      .setMeta({
        name: `${endpoint.path} Has CSP Headers`,
        severity: "MEDIUM",
        tags: ["CSP_HEADERS"],
      })
      .addTestStep(
        TestStepBuilder.sampleRequest(endpoint).assert({
          type: AssertionType.JS,
          value: "resp.headers['content-security-policy']",
        })
      );
  },
};
You can see a deeper overview of templates here.

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Generating a Test

You can generate a test with this new template using the Metlo CLI.
Bash
metlo test generate \
 -t templates/VERIFY_CSP_HEADERS/v1.ts \
 -h <ENDPOINT_HOST> \
 -e <ENDPOINT_PATH> \
 -p output.yaml

# By Endpoint UUID

metlo test generate \
 -t templates/VERIFY_CSP_HEADERS/v1.ts \
 -e <ENDPOINT_UUID> \
 -p output.yaml

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Running Your Generated Test

First fill in any values you need in output.yaml. After that, you can run the generated test by running the following:
Bash
metlo test run output.yaml

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Pushing Your Templates to Metlo

After you’ve verified that the template works, you can push your template to Metlo by running:
Bash
metlo template push templates
This template should now be available in the Metlo UI.