index.js

const core = require('@actions/core');
const github = require('@actions/github');

async function run() {
    try {
        //Get input
        const tag = process.env.TAG || process.env.INPUT_TAG || '';
        const repoInput = core.getInput('repo') || process.env.GITHUB_REPOSITORY;

        console.log(`Searching for tag: ${tag} in ${repoInput}`);

        if (!repoInput.includes('/')) {
            throw new Error(`${repoInput} is not a valid repo`);  
        }
            
        // Get owner and repo from context of payload that triggered the action
        const [ owner, ...repository ] = repoInput.split('/');
        const repo = repository.join('/');
        
        const octokit = github.getOctokit(process.env.GITHUB_TOKEN || core.getInput('github_token'));
        var exists = 'false';

        try {
            const getRefResponse = await octokit.rest.git.getRef({
                owner,
                repo,
                ref: `tags/${tag}`
            });

            if (getRefResponse.status === 200) {
                console.log("Tag was found");
                exists = 'true';
            } else {
                core.setFailed("Unknown status was returned: " + getRefResponse.status);
            }

        } catch(error) {
            if (error.status === 404) {
              console.log("Tag was not found");
            } else {
              core.setFailed("Unknown status was returned: " + error.status);
              console.error(error);
            }
        }
        core.setOutput('exists', exists);
    } catch (error) {
        core.setFailed(error.message);
        console.error(error);
    }
}

run();