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The pieces

Zend identity. One account per person across Zend and every ZApp. Every app gets the user’s appUserId, username, name and avatar. Email, wallet address and contacts need the user’s consent. ZApp. Your web app, hosted on your own HTTPS origins and registered with a manifest. Bridge. @zendfi/appkit talks to Zend with JSON messages. In the mobile app it uses a WebView bridge; on the web it uses postMessage to the app.usezend.app frame. Zend checks that every message comes from one of your registered origins. Interaction. A stateful object with an ID: a challenge, a round, a booking. Your manifest defines its states and allowed moves; Zend stores it and enforces them. See Interactions. Card. A chat or feed item that Zend draws from a template you fill in. Cards show the live state of their interaction. See Cards.

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Who owns what

Rule of thumb: Zend enforces anything that involves money, identity, or what other people see.

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Lifecycle of a chat ZApp

1

The user starts it

Tunde taps + → Apps → Challenge in a chat with Danny.
2

Zend creates the interaction

Zend creates a challenge interaction with both people and opens your sheet path with ?interaction=int_....
3

Consent, once

The first time, Zend shows “Continue to Accident as Tunde?”.
4

Your page runs

Your page reads zend.launch.interaction, runs the game, and calls zend.interaction.update(...) and zend.postCard(...).
5

The other person joins

Danny sees a card in the chat. Its button opens your app on the same interaction.
6

Your server follows along

Your server receives interaction.updated webhooks and can make moves only it is allowed to make.

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Private and public apps

Anyone with the link app.usezend.app/apps/<slug> can open a private app. See Public apps.

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Custody

Custody decides where money from groups, events and prizes sits before it’s paid out. Set it with custody in the manifest.

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Trust boundaries

Your page is untrusted from Zend’s point of view, and from yours. Anyone can edit it in their browser.
  • Your page can propose money actions. Only the user can approve them, in Zend’s sheet.
  • A resolved promise in the browser is not proof of payment. Confirm with the payment.completed webhook or the server API. See Verifying payments.
  • appSession tokens last 10 minutes. Exchange them on your server and start your own session; don’t use them as your session.
  • Moves that decide outcomes, such as declaring a winner, belong on your server. Mark them serverOnly in the manifest so a page can’t make them.