My wallet says invalid descriptor checksum: did I copy it wrong?

When you paste a descriptor and the wallet says 'invalid checksum', the fear is that your backup is broken; actually, that error is telling you that you copied something wrong.

What a descriptor checksum is

A descriptor is a text string that describes how to derive addresses from a key. The checksum is a short sequence at the end that lets you verify you copied the descriptor correctly. The source is BIP-380, checksum section.

That section says verbatim: 'Any 1 symbol error is always detected'.

Why you see 'invalid checksum'

That message appears when the descriptor you pasted doesn't match its checksum. It almost always means there was a typo or copy error, not that your backup is broken.

The checksum catches any single-symbol error, always. It also catches two or three errors in descriptors up to 49154 characters. A random error has 1 chance in 2 raised to the 40 of passing unnoticed.

What to do with that error

Check the descriptor character by character against the original source. Pay attention to similar characters, like uppercase and lowercase, or symbols that get mixed up when copying from a screen.

If you still see the error, look for the descriptor in another format or from another trusted source. Don't try to guess the checksum or modify the descriptor to make it pass.

It doesn't tell you if it's the right descriptor

Source: BIP-380, seccion del checksum