Quickstart — Express

Last updated on Oct 17, 2026

Zero to a billed charge in 5 steps.

1. Install and scaffold

bash

npm i @mythos-work/sdk express
npx @mythos-work/sdk init

2. Environment

bash

# .env
MYTHOS_LISTING_ID=your-listing-id
# openssl rand -base64 32  (≥ 32 chars)
MYTHOS_SESSION_SECRET=
# Mythos dashboard → Settings → Mythos key
MYTHOS_KEY=

Node does not read .env on its own — step 5 starts the server with --env-file. In production, set these in your host's environment instead.

3. Server

Missing MYTHOS_KEY is soft in 0.5.0; static listing launches remain silent, while dynamic registration, handshake and callbacks warn once per process. It will be required in 0.6.0. Update deployment env after key regeneration. See the binding matrix.

Troubleshooting code	Fix
PRODUCER_MISMATCH	The MYTHOS_KEY in this deployment belongs to a different Producer, or was regenerated. Copy the current key from the dashboard and redeploy.

typescript

// server.ts
import express from 'express';
import { createMythos, MythosError } from '@mythos-work/sdk';
import { mythosExpress } from '@mythos-work/sdk/express';

const app = express();
const mythos = createMythos();
app.use(express.json());
app.use(mythosExpress(mythos));
app.post('/api/calculate', async (req, res, next) => {
  try {
    await mythos.charge(req, { credits: 1, reason: 'calculate' });
    res.json({ success: true, data: { result: req.body.a + req.body.b } });
  } catch (err) {
    if (err instanceof MythosError) return res.status(err.httpStatus).json({ success: false, error: err.message, code: err.code });
    next(err);
  }
});
app.listen(3000);

4. Page

html

<button onclick="calculate()">Calculate (1 credit)</button>
<script src="https://cdn.jsdelivr.net/npm/@mythos-work/sdk@0.5.0/dist/mythos-client.global.js"></script>
<script>
  const m = Mythos.initMythos();
  m.ready.then((s) => { document.body.dataset.mythos = s.status; });
  async function calculate() {
    const { approved } = await m.confirmCharge({ credits: 1, reason: '1 + 2' });
    if (approved) await m.fetch('/api/calculate', { method: 'POST', headers: { 'Content-Type': 'application/json' }, body: JSON.stringify({ a: 1, b: 2 }) });
  }
</script>

5. Run and check it

bash

node --env-file=.env server.js   # Node ≥ 20.6 (compile server.ts first, or use tsx --env-file=.env server.ts)
npx @mythos-work/sdk doctor

Already using dotenv? import 'dotenv/config' at the top of server.ts works too. Then launch your listing from the Mythos dashboard.