What form builder libraries support custom components?

Several form builder libraries support custom components, but the exact level of support varies. Here are some well-known options:

React

• React Hook Form
  Supports custom inputs via Controller and useController.
• Formik
  Works well with custom components using Field, useField, or custom wrappers.
• React Final Form
  Strong support for custom fields through render props and field subscriptions.

Vue

• Vue Formulate
  Designed with custom inputs/components in mind.
• VeeValidate
  Supports custom components and validation bindings.
• FormKit
  Very flexible; custom inputs and plugins are a core feature.

Angular

• Angular Reactive Forms
  Not a “builder library” in the same sense, but it supports custom form controls via ControlValueAccessor.
• ngx-formly
  A popular dynamic form builder with excellent custom field type support.

Low-code / JSON-schema based

• Uniforms (React)
  Supports custom field components, especially for schema-driven forms.
• rjsf / React JSON Schema Form
  Supports custom widgets, fields, templates, and theme overrides.
• JSON Forms
  Supports custom renderers and cells.

Other notable options

• SurveyJS
  Supports custom questions and rendering.
• Form.io
  Provides custom components in its form builder and renderer.
• Builder.io / dynamic form tools
  Many no-code/low-code platforms allow custom components, though APIs differ.

If you want, I can also give you a short comparison table by framework, customization depth, and ease of use.