@redocly/openapi-core/src/rules/oas3/no-undefined-server-variable.ts

See https://github.com/Redocly/openapi-cli

import { Oas3Rule } from '../../visitors';

export const NoUndefinedServerVariable: Oas3Rule = () => {
  return {
    Server(server, { report, location }) {
      if (!server.url) return;
      const urlVariables = server.url.match(/{[^}]+}/g)?.map((e) => e.slice(1, e.length - 1)) || [];
      const definedVariables = (server?.variables && Object.keys(server.variables)) || [];

      for (const serverVar of urlVariables) {
        if (!definedVariables.includes(serverVar)) {
          report({
            message: `The \`${serverVar}\` variable is not defined in the \`variables\` objects.`,
            location: location.child(['url']),
          });
        }
      }

      for (const definedServerVar of definedVariables) {
        if (!urlVariables.includes(definedServerVar)) {
          report({
            message: `The \`${definedServerVar}\` variable is not used in the server's \`url\` field.`,
            location: location.child(['variables', definedServerVar]).key(),
            from: location.child('url'),
          });
        }
      }
    },
  };
};