This would solve...

Unidici throws an error with the message "Body is unusable" when the body of a response is non-null and the stream is either locked or disturbed. In most cases, this situation occurs because someone has attempted to ready the body twice, such as:

const response = await fetch(url);
console.log(await response.text());
const body = await response.json();  // throws "Body is unusable"

The message "Body is unusable" is opaque and requires searching to understand what it means and what the problem is. A more descriptive message would save developers a lot of time.

The implementation should look like...

Changing the string on this line:

Suggestion: "Body is unusable: Body is either null or the stream has already been read."

The spec only specifies that a TypeError must be thrown, but it does not specify the text contained within that error, so it seems like changing the message would still be in-line with the spec.

I have also considered...

Not filing this issue. 😄

Additional context