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

# CrossmintExternalWalletSignCallback

> Flutter Type Alias

**Type Alias**

Callback signature for user-owned external-wallet signers.

```dart theme={null}
typedef CrossmintExternalWalletSignCallback = Future<String> Function(String payload)
```

Receives the payload to sign (hex-encoded for EVM, base58-encoded for
Solana, base64-encoded transaction blob for Stellar) and returns the
user-produced signature in the same encoding. Implementations typically
delegate to a wallet SDK like WalletConnect, MetaMask Flask, or a
platform-native wallet adapter.