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# LUD-14: `balanceCheck`: reusable `withdrawRequest`s

> Display inline code and code blocks

`author: fiatjaf` `discussion: https://t.me/lnurl/9729`

***

## `balanceCheck`

This optional property is meant to allow a `WALLET` to check and display users' balances on a `SERVICE` and also enable automatic delivery of funds from services to wallets. It works as follows:

* If `LN WALLET` supports the `balanceCheck` scheme and sees a `balanceCheck` property from a withdrawal it performs, it may store that URL locally and call it later (for example, at its next startup, or after 24 hours).
* The URL is to be treated exactly as the URL from an LNURL QR code after bech32-decoding step.
* The goal of doing this is to see if there is some balance waiting to be withdrawn on that `LN SERVICE`. If there is any, the wallet may decide to withdraw it automatically.
* After calling the `balanceCheck` URL `LN WALLET` must check the response for the presence of a new `balanceCheck` property and replace the previous with the new one -- just erase the previous if there is not a new one.
* To support this flow `LN SERVICE` only needs to implement a scheme for renewing the `balanceCheck` URL on every call it receives, returning the next URL every time. Then proceed to redeem withdraw requests when they come. `LN SERVICE` may also choose to offer a static LNURL-withdraw endpoint for each user, which can be less safe, but also simplifies implementation. In this case it would just indicate that same endpoint every time as the `balanceCheck` value.

The callback JSON response is modified as such:

```diff theme={null}
 {
   "tag": "withdrawRequest",
   "callback": string,
   "k1": string,
   "defaultDescription": string,
   "minWithdrawable": number,
   "maxWithdrawable": number,
+  "balanceCheck": string
 }
```

The value of `balanceCheck` here is an URL that can be called next time the wallet wants to perform a balance check, the call will be the same as performed in this step and the expected response is the same.

### `currentBalance`

In any `balanceCheck` response, `maxWithdrawable` can be interpreted by `WALLET` as corresponding to the current user's balance in the `SERVICE`. Unless the optional `currentBalance` field is present. In that case `currentBalance` should take priority. It is useful in case `maxWithdrawable` corresponds to less than the total balance due to any reason.