Stop the warning when changing a file extension on Mac

You rename notes.txt to notes.md and Finder stops you: "Are you sure you want to change the extension from '.txt' to '.md'?" When you do this a dozen times a day it stops being a safeguard and starts being a speed bump. One checkbox — or one Terminal line — turns it off, and it is fully reversible.

Turn the warning off

1. Click any Finder window or the desktop, then open Finder → Settings (Cmd+,).
2. Select the Advanced tab.
3. Uncheck Show warning before changing an extension.

From now on, renaming .txt to .md — or .markdown, or anything else — happens silently. The setting lives in the same spot on macOS Ventura, Sonoma, and Sequoia, and re-ticking the box brings the warning straight back if you ever want it.

Why .txt to .md is safe to do quietly

Finder's warning exists because renaming an extension does not convert a file — and for most formats that matters. But .txt and .md are the special case where it genuinely does not: both are plain text, the exact same bytes on disk. A Markdown file is just a text file that editors and previewers agree to render with headings and links. Changing the extension only tells macOS which app to hand it to; it never rewrites the contents. That is precisely why the confirmation feels pointless for this workflow, and why switching it off costs you nothing here.

Where the warning earns its keep is a rename like photo.heic to photo.jpg — that produces a HEIC file wearing a JPEG's name, which some apps will refuse. If you only ever shuffle text formats around, you will not miss the dialog.

The Terminal one-liner

The checkbox sets a single Finder preference you can write directly — handy in a dotfiles or setup script:

# stop the extension-change warning
defaults write com.apple.finder FXEnableExtensionChangeWarning -bool false
killall Finder

# undo: bring the warning back
defaults write com.apple.finder FXEnableExtensionChangeWarning -bool true
killall Finder

killall Finder relaunches Finder so it reads the new value; your windows reopen on their own. Check the current state any time with defaults read com.apple.finder FXEnableExtensionChangeWarning — 0 means the warning is off.

Make sure you can see the extension

If .md never appears when you rename, Finder is hiding extensions — and you cannot change what you cannot see. Turn on Show all filename extensions in the same Advanced tab so your text files clearly read .txt or .md as you edit them.