> Ask it what's wrong with the code

That's phase 1, ask it to "think deeply" (Claude keyword, only works with the anthropic models) while doing that. Then ask it to make a detailed plan of solving the issue and write that into current-fix.md and ask it to add clearly testable criteria when the issuen is solved.

Now you manually check the criteria wherever they sound plausible, if not - it's analysis failed and its output was worthless.

But if it sounds good, you can then start a new session and ask it to read the-markdown-file and implement the change.

Now you can plausibility check the diff and are likely done

But as the sister comment pointed out, agentic coding really breaks apart with large files like you usually have in brownfield projects.