Prevent Cursor From Deleting Your Entire File When Updating Code

Introduction

It's the "Lazy AI" problem. You ask Cursor to "fix the fetch function," and it rewrites the function perfectly... but replaces the other 500 lines of your file with a comment like: // ... existing code ...

If you hit "Apply," you just deleted 90% of your work.

This guide explains why LLMs do this and the 2 keyboard shortcuts that stop it from happening.

Why Does AI Delete Code?

Why does Cursor replace my code with placeholders?

Cursor replaces your code with placeholders because LLMs (like Claude 3.5 Sonnet) are lazy by design to save output tokens. When you ask for a change, the model often assumes you only want to see the relevant change, not reprint the whole file. If you apply this "diff" blindly, the editor treats the placeholder comment as literal text, deleting the original code.

The Solution: Scoped Edits

1. The "Select First" Rule (Cmd+K)

Never use Cmd+K (Inline Edit) without selecting code first.

• Wrong: Cursor is blinking on line 1. You type "Add a button to the nav."
  • Result: AI might rewrite the whole file.
• Right: Highlight only the <nav> component. Press Cmd+K. Type "Add a button."
  • Result: AI can only touch the highlighted lines. The rest of the file is safe.

2. The Chat "Apply" Trick

When using the Chat sidebar (Cmd+L), you often get code blocks.

• Don't: Click "Apply to File" on a partial snippet.
• Do: Highlight the target code in your editor first. The "Apply" button will often change to "Apply to Selection" or intelligent merge.

3. The "No Lazy" Prompt

If you must rewrite the whole file (e.g., a refactor), explicitly tell the model: "Return the full file. Do not use placeholders or comments like // existing code."

FlowQL: AI Hygiene

At FlowQL, we teach "AI Hygiene." Using tools like Cursor without discipline is like running with scissors. You move fast, but you might cut yourself.

We help teams build standard operating procedures (SOPs) for AI coding so junior devs don't accidentally nuke the codebase.

Conclusion

Your Action Plan:

1. Highlight: Always select the specific block you want to change.
2. Verify: Check the "diff" view (red/green lines) before accepting.
3. Reject: If you see // ... code, hit Reject and re-prompt.

Edit precise, not broad. [Book a session with FlowQL] to refine your AI workflow.

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FAQ

How do I undo a Cursor apply?

You can standard Undo (Cmd+Z or Ctrl+Z) to revert a Cursor change. Cursor edits are treated as a single transaction in the editor's history stack.

Why is 'Apply' greyed out in Cursor?

The 'Apply' button is often greyed out if the AI did not return a valid code block (triple backticks) or if Cursor lost the context of which file you are currently looking at. Try switching tabs to the target file and asking the AI to "output the code again."

Does Cursor have a git history?

No, Cursor itself does not have a separate version control system, but it relies on your project's Git. This is why it is critical to commit your changes (git commit) before starting a heavy AI refactoring session, so you can git checkout . if it destroys the file.