Shipping Custom Fields is Trivial as of 2026

> How to ship one in less than 30 minutes

Marcel Gruber

January 17, 2026

Index

Overview
How Custom Fields Work in SitecoreAI
How I Did It In 30 minutes
Try it Yourself
My Feedback to Sitecore
Conclusion

Overview

I used the following to ship a custom repeater grid field in SitecoreAI in under 30 minutes:

Cursor with Claude Opus 4.5
SitecoreAI Marketplace
https://github.com/Sitecore/marketplace-icon-picker as a starting point
Next.js
Vercel

How Custom Fields Work in SitecoreAI

SitecoreAI has a new field typed called "Plugin," which allows you to store arbitrary JSON within a field the CMS.

New Plugin Field Type

All you need to do is:

Build a UI/app for editing a JSON value
Host it somewhere
Register the marketplace application and point it to the hosted URL
Install the marketplace application on your site
Enter the application ID in Source

I'm mostly going to focus on the process of building the app because there's plenty of information out there on how to do the other steps.

How I Did It In 30 minutes

Clone the https://github.com/Sitecore/marketplace-icon-picker repo, which is Sitecore's go-to example for a custom field in the Marketplace.
Open up the folder in Cursor or your agentic IDE of choice.
Prompt:

Use this sample sitecore marketplace custom field repo and repurpose/update it with best practices to be a repeating, multi-column editor (a “grid/repeater” UI). The user should be able to specify n number of text fields per row and add/remove as many rows as they want.
Use Sitecore's official Blok design system for the UI (https://blok.sitecore.com/).
Crawl https://doc.sitecore.com/mp/en/developers/marketplace/page-builder-custom-fields.html to see how custom fields work in SitecoreAI Marketplace.
Ask me any clarifying questions that you have before getting started.

If you want the agent to read content from URLs at any point in this process, tell the agent to crawl the URLs directly. I found that even when I provide the agent with direct URLs, it tries to initialize a web search instead of reading the URL directly. For some reason, the web search also doesn't seem to take them to the page in question (or the agent experiences some issue accessing it). This will almost certainly be a non-issue in the not too distant future.

Iterate on the UI/app
Launch on Vercel

Try it Yourself

Demo: https://grid-repeater-field.vercel.app/

Source: https://github.com/marcelgruber/grid-repeater-field

My Feedback to Sitecore

Something I would like to see changed in SitecoreAI is how the field itself is rendered. Notice how on the right, it displays the raw JSON and has a button to open the app. For marketers, seeing the raw JSON is not helpful, and it would be nice to have a way to preview or even edit the field without having to click "Open App".

Plugin Field Preview Source: https://doc.sitecore.com/mp/en/developers/marketplace/page-builder-custom-fields.html#user-interaction-with-a-custom-field