How It Works

Your theme says which version it is in its settings file. Molt checks that claim against its archive of clean releases, because a theme's label can be wrong. One of our test themes calls itself Dawn 15.3.0, and its code matches 15.4.0.

Then it compares every file with that clean copy. Differences group into customizations with plain names, such as a size guide or a free shipping bar. Code changes sit apart from settings and content, because a theme fresh from Shopify already differs from the public release in its colors and layouts.

Molt holds the original release, your version and the new release, so it can tell who changed what. A file only you changed stays yours. A file only Shopify changed takes the new version. When both of you changed a file in different places, the two sets of edits combine line by line.

When both changed the same lines, a model reads all three versions and combines them. It writes an explanation and a confidence score for each file. Under 0.7, the file is marked for your review. Afterward a separate check confirms that each line you added still exists somewhere in the result.

A new release can use a block or section your store has removed. Shopify refuses a theme like that, so Molt brings back the release's copy of the file and tells you it did. Files the release dropped and your code still uses stay in place too.

Shopify's theme checker runs on the merged theme and counts what's new since the release and your original. Molt creates the draft on your store, uploads the files in the order Shopify expects, and checks every file against the store afterward. If Shopify refuses a file, the upgrade stops, Molt removes the unfinished draft, and the error names the file.

Molt opens your home, collection, product, cart, search and contact pages on both themes and compares what it sees: the sections, the headings, script errors and the pixels. It names what the release leaves out and what it adds, in the words of your own store.

When a release drops a section you have today, Keep my current layout puts your page back on the draft and compares it again. If your store has a password page, you enter the password once and Molt keeps it encrypted for these comparisons.

Molt publishes when you click Publish on the review page. The theme that was live stays on your store, and Roll back republishes it. An upgrade you started from a draft stays a draft for you to keep working on in the theme editor.

The backup from before the upgrade stays on the Backups page for as long as your plan keeps it. Restoring adds it as a new unpublished theme, so your live theme stays as it is then too.