# XO1501: --stamp names no stampable constant

`xo build --stamp NAME=VALUE` (also `xo run`) replaces the value of the constant `NAME` of the main module for that build, so a program reports a version, commit, or build date set by the build script. The constant must exist in a module named on the command line and be declared `const NAME: Str = "..."` with a plain string literal (no interpolation, no expression): the literal is what the build replaces. Other constants may be built from it.

## Example

```xo
// xo build --stamp VERSION=1.4.0
const VERSION = 14
```

## Fix

```xo
// xo build --stamp VERSION=1.4.0
const VERSION: Str = "dev"
```

Run `xo explain XO1501` for this text in a terminal, or see
[section 10.6 of the specification](../../spec/core/#106-diagnostic-codes).