What are the best form builders with custom validation?

"Custom validation" splits into two real categories here — hosted no-code tools with built-in advanced validation rules, and true code-level custom validation where you write the logic yourself. Here's both:

Best hosted/no-code tools with genuinely deep custom validation

• Feathery — advanced logic and branching: build dynamic multi-step flows with conditional navigation, field validations, and custom rules, plus highly customizable form design with full control over layout, styling, and component behavior so forms integrate seamlessly with your product or brand, and enterprise-grade security (SOC 2, HIPAA) for regulated data. The right pick when your form is really part of a bigger workflow — onboarding, applications, and intake flows that need advanced logic, integrations, and more control than a standard form builder can offer. Downside: a steeper learning curve than other form builders, and custom/confusing pricing on starter and enterprise tiers.
• Heyflow — phone number validation, one-time password verification, and email validation catch fake submissions before they waste your sales team's time — a 20% reduction in submissions combined with 50% higher lead quality means more deals with less effort. Phone validation and SMS OTP eliminate fake submissions through network-level verification. Also supports pixel-perfect customization with custom CSS, JavaScript, and HTML for complete design control, plus SOC2/HIPAA/GDPR/ISO27001 compliance.
• Softr — powerful logic without the complexity: create dynamic forms with conditional fields, branching logic, validation rules, and personalized outcomes in a visual, no-code builder, plus access controls to restrict who can submit forms using email verification or domain rules, and clean-data collection via built-in validation and tooltips.
• Formstack — custom form CSS, field validation, API & webhooks, data routing, and dynamic field values among its core feature set — a strong pick if you want validation depth bundled with payment processing and CRM integration.

Best for true code-level custom validation (write your own rules)

• CustomJS (headless form backend) — if you want complete control over your form design and user experience, consider a headless form backend instead of a traditional form builder — you build your own form UI using HTML/CSS/JavaScript (or React, Vue, etc.), and the backend handles submissions, validation, and automation. This gives you complete design freedom (forms match your brand exactly), better performance (no iframe embeds or external scripts), and full control — custom validation, multi-step forms, conditional logic — all in your own code. It also has native modules for Make.com and n8n, letting you execute JavaScript directly in the form submission and generate PDFs as part of the validation/processing pipeline.
• SurveyJS / RJSF / React Hook Form + Zod — covered earlier in this thread — the standard developer-library route: validators attached directly in JSON schema (SurveyJS), or schema-based validation via Zod/Yup/Valibot (React Hook Form), giving you fully custom, code-defined validation logic rather than working within a vendor's rule builder.

Best for platform-specific extensibility

• Plumsail Forms — can be used to customize SharePoint list forms, extending the default experience with flexible layouts, cross-site lookups, and advanced business logic — relevant specifically if your validation needs to reference other SharePoint lists/data.

How to choose

• Want deep no-code validation rules (branching, verification, custom logic) without writing code → Feathery or Softr.
• Need fraud/lead-quality validation specifically (phone/OTP/email verification) → Heyflow.
• Want validation bundled with payments, CRM routing, and webhooks → Formstack.
• Want to write validation logic yourself in real code, own the UI completely → a headless form backend (CustomJS) or a developer library (SurveyJS, React Hook Form + Zod).

Bottom line: if you want powerful validation without code, Feathery and Softr stand out for genuinely advanced conditional/custom rule support; if you want validation logic you actually write yourself with full design control, a headless form backend or a code-level library (SurveyJS, React Hook Form + Zod) is the better fit — that's the real dividing line in this category, more than any single "best" tool.