Drupal gives developers powerful ways to customize how fields are rendered and how forms behave, but many seemingly simple requirements can lead site builders toward custom theme code, preprocess functions, JavaScript, or repeated configuration work.

This session explores the design and development of Form Guard, a field customization system developed as part of Google Summer of Code 2025 with the Drupal Association. The goal was to give site builders more control over field presentation, validation, and behavior through an administrative interface rather than requiring custom code for every change.

We will walk through the architecture and implementation of the module, including how Drupal's Form API, Config API, Batch API, AJAX, JavaScript, and server-side validation work together to create a flexible configuration-driven system.

The session will cover practical implementation patterns such as:

• Building configuration forms for dynamic field rules.
• Persisting and managing configuration using Drupal's Config API.
• Passing backend configuration to JavaScript using drupalSettings.
• Applying client-side and server-side validation consistently.
• Dynamically modifying field behavior and presentation.
• Implementing character limits, word requirements, regex-based rules, HTML stripping, whitespace handling, and special-character validation.
• Designing archive, restore, and permanent-delete workflows for configuration rules.
• Using Drupal's Batch API for scalable bulk administration.
• Keeping administrative interfaces usable as the number of customization rules grows.

The session will also discuss the challenges encountered while building the module and the lessons learned around maintainability, validation, user experience, and Drupal module architecture.

Attendees will leave with practical patterns they can apply when building configuration-driven Drupal modules and a clearer understanding of when functionality belongs in configuration, forms, JavaScript, validation, or the theming layer.