> ## Documentation Index
> Fetch the complete documentation index at: https://docs.opaque.cash/llms.txt
> Use this file to discover all available pages before exploring further.

# Cross-chain announcements

> Universal Announcement Bus (UAB) over Wormhole for multichain stealth inboxes.

The [UAB](https://github.com/opaquecash/spec/blob/main/UAB.md) (Universal Announcement Bus) relays stealth announcements across chains via Wormhole. A payment announced on Solana can appear in an Ethereum user's inbox (and vice versa) without a central indexer.

## How it works

1. Sender calls `announceWithRelay` on **UABSender** (local announce + Wormhole publish)
2. Wormhole guardians attest the 96-byte payload
3. Relayer delivers the VAA to **UABReceiver** on the destination chain
4. Receiver re-emits a standard `Announce` event
5. Recipient scans with the same WASM filter as native announcements

```ts theme={"dark"}
// Build cross-chain relay (Ethereum)
const send = client.prepareStealthSend(metaAddressHex);
const relay = await client.buildAnnounceWithRelayRequest(send);
await walletClient.sendTransaction({
  to: relay.to,
  data: relay.data,
  value: relay.value,  // Wormhole message fee
});

// Or bundled in sendStealthPayment
await client.sendStealthPayment({
  chain: "ethereum",
  recipient,
  amount: parseEther("0.01"),
  relay: true,
});
```

## Scanning cross-chain outputs

```ts theme={"dark"}
// UAB-only scan
const crossOwned = await client.scanCrossChain();

// Unified inbox (native + UAB when configured)
const inbox = await client.scan({
  chains: ["ethereum"],
  includeCrossChain: true,  // default when UAB is deployed
});
// Each output has source: "native" | "uab"
```

## Relayer

Off-chain VAA delivery is handled by [`opaquecash/relayer`](https://github.com/opaquecash/relayer). The SDK builds and reads announcements; the relayer watches Wormhole and submits VAAs to `UABReceiver`.

## Wormhole chain IDs

| Chain    | Wormhole ID |
| -------- | ----------- |
| Solana   | 1           |
| Ethereum | 2           |

Exported as `WORMHOLE_CHAIN_SOLANA` and `WORMHOLE_CHAIN_ETHEREUM` from `@opaquecash/opaque`.

## Starknet

Wormhole has no Starknet support, so Starknet participates in the unified inbox
as a **native scan chain** rather than over the UAB relay: add `"starknet"` to
`scan({ chains })` and its announcements merge alongside Ethereum and Solana. See
[Starknet](/concepts/starknet). Trust-minimised registry/ONS mirroring to
Starknet (native L1→L2 messaging) is on the roadmap.