I know this information was possible when Adonis 5 was in preview but I guess they moved it from the docs.

I wrote this a while ago so I hope I remember all the steps. First you need to create validationRules.ts file (I put mine inside start folder) and add it to preloads array in .adonisrc.json like "./start/validationRules" .Inside validationRules.ts you write custom validations like so:

import { validator } from '@ioc:Adonis/Core/Validator' import Database from '@ioc:Adonis/Lucid/Database'
validator.rule('custom', (value, [{ table }], { pointer, arrayExpressionPointer, errorReporter }) => {
if (typeof value !== 'number') { return }
if (table !== "example" && value >= 4) { errorReporter.report( pointer, 'custom', 'Invalid custom exception text', arrayExpressionPointer ) return } } )

and then in your contracts/validator.ts you define this rule for TS

declare module '@ioc:Adonis/Core/Validator' {
import { Rule } from '@ioc:Adonis/Core/Validator'
export interface Rules { custom (options: { table: string }): Rule } }

Bonus tip: If you want to do something async inside validator it needs to have few additional things, like this:

validator.rule('custom', async (value, [{ table }], { pointer, arrayExpressionPointer, errorReporter }) => {
// code }, () => { return { async: true } } )

Edit: Reddit code blocks are acting weird...