# XO0422: unknown wire name option

A field's wire string may carry options after a comma, as in a Go struct tag: `name as "key,omitempty": T`, or `as ",omitempty"` to keep the default key. `omitempty` is the only option: `derive Json` then leaves the field out when its value is empty (None, false, 0, "", or no elements). Go's other options (`string`, `omitzero`) do not exist in Xo. Remove the option. See core 2.2 and decision 0309.

## Example

```xo
struct User {
  id as "id,string": Int
} derive Json
```

## Fix

```xo
struct User {
  id as "id": Int
} derive Json
```

Run `xo explain XO0422` for this text in a terminal, or see
[section 10.6 of the specification](../../spec/core/#106-diagnostic-codes).