A countdown through the last few days before my apidays Australia workshop - a repo that had to go public once I checked its history, a fix that was reported done but silently wasn't live for six days, a blog post that got its own facts wrong about the app it was describing, and a Slack message to colleagues that found four real bugs in fifteen minutes. Then the workshop itself.
Part nine of the series on building this demo.
Every other client in this Auth0 demo suite keeps a server somewhere to hold tokens - a Vercel Function, a Cloudflare Worker, a Deno Deploy isolate.
This one has none. A public SPA client deployed as static files to Firebase Hosting, tokens in browser memory only.
sub stays the citizen the whole way through. act.sub is what actually names the AI agent calling the API on their behalf - and the two are not the same claim.
An explainer on how Auth0 FGA, on-behalf-of token exchange, and Agent as Principal fit together in a real demo app - sign up and try it yourself, no login flow to imagine, it's live:
Upgraded a sample HMAC Custom Token Exchange demo to a real Firebase Authentication ID token, verified against Google's published JWKS instead of a shared secret, with email_verified gating to close an account-takeover path Firebase's default signup leaves open.
Also covers a "critical" adversarial-review finding that didn't hold up once checked against the live Auth0 tenant.