How do I prevent a proofreading tool from introducing errors into my text?

The Pages AI proofreader apparently is designed to make changes where it sees fit, not improvement recommendations. Though it offers the opportunity to revert each change, you have no control over the AI model (beyond the three choices) that introduce document changes. The proofreader will also work on blocks of selected text, allowing you to limit its scope.

Unsupported character sequences (e.g. *nbsp; ) in Pages document content may confuse the AI proofreader into flawed behavior. Because it is AI-based, the models used to train it will never think like us, and injected errors may be an inherent weakness in this proofreading feature.

As I mentioned earlier, the free to use Grammarly for Mac does first flag and recommend, allowing you to subsequently choose its suggested correction, or dismiss it.

With Apple Intelligence enabled on macOS Tahoe, selected text in Pages v15.3.1 offers a contextual menu revealing just proofread, or Show Writing Tools. The latter has its own pop up pane where you can alter the degree of proofreading via Friendly, Professional, or Concise:

Proofread, whether chosen from the separate menu item, or by clicking that Proofread button, will scan the selected text and make automatic changes to issues it has detected. It also presents this floating panel:

that allows you to step through those changes and use Revert to undo any unwanted change. What you cannot do is customize its content replacement a priori. If by copy/paste, you introduce "martians" (e.g. &nsbp;) then this may send the AI Proofreading on a rampage as Pages is not a web-aware application.

An alternative to turning Apple Intelligence loose on your document content may be the proofreading capabilities in Grammarly which does not automatically change content, but allows you to see what it is flagging and decide on its offered replacement.

I can’t tell if that's cut-and-paste errors, or what. The   stuff is an embedded space, and that usage more common in web-formatted text

If you are using Apple Intelligence or any added LLMs, test again after shutting it all off, and see if things work better.

JimOnLatona wrote:

Within Pages, I chose <Proofread> from the writing tools icon. I did not select the level. It worked on the entire document. Yes, the panel to select each change appears, but I don't expect the tool to introduce errors, only to suggest improvements. All the errors were added by the writing tool, not by me.

That’s probably best discussed with Apple directly, as it seems Apple Intelligence has gone badly off the rails.

More so than how LLMs can usually go off the rails, that is.

How to: Contact Apple Support - Apple Support

With Grammarly policies, I wouldn’t upload anything personal or sensitive to them.

You do not state the operating system, the specific application, and both versions where you are attempting proof reading. Let's start with that information…