# Conditional & Cross-Field Validation

> Source: https://agilitycms.com/docs/developers/conditional-and-cross-field-validation

Agility content models support built-in field validation — required, length, and format constraints — plus **regular-expression validation** on a single field via the model's Advanced tab for precise format rules (emails, phone numbers, postal/zip codes, custom patterns). For logic that depends on **another field's value** — for example, "if Status is *Published*, then a Publish Date is required," or "End Date must be after Start Date" — validation is implemented with a **custom field app**: a developer-built field extension that has access to the item's values and enforces the conditional/cross-field rule. Agility does not use server-side "validation scripts"; per-field formatting is handled by regex, and anything beyond a single field is handled by a custom field app.

## Built-in, per-field validation

Standard model field settings cover the common cases:

- **Required** — the field must have a value.
- **Length** — minimum/maximum length constraints.
- **Format via regex** — on the field's **Advanced** tab, add a regular-expression pattern plus a custom validation message; the field then only accepts values matching the pattern. Useful for emails, phone numbers, URLs, postal codes, SKUs, and other structured formats.

These rules evaluate a single field in isolation and do not reference other fields.

## Conditional and cross-field validation (depends on another field)

When a rule must consider more than one field, it is implemented with a **custom field app** — a custom field/UI extension you build and register on the content model. Because the app renders a field within the editor and has access to the item's other field values, it can enforce conditional and cross-field rules and surface a validation message to the editor.

Typical cross-field / conditional patterns a custom field app can enforce:

- **Conditional requirement:** Field B is required only when Field A has a certain value (e.g. require *Redirect URL* only when *Type = External*).
- **Comparison across fields:** *End Date* must be after *Start Date*; *Discount Price* must be less than *Price*.
- **Mutually exclusive / dependent fields:** exactly one of two fields must be filled; or Field C is forbidden when Field D is set.
- **Aggregate/conditional constraints:** total of several numeric fields must not exceed a cap.

A worked custom-field-app example (e.g. End Date > Start Date) isn't published here; [contact Agility](https://agilitycms.com/contact-agility-cms) for current sample code and guidance.

## Choosing the right tool

| Need | Use |
| --- | --- |
| Single field required / length | Built-in field settings |
| Single field must match a format | Regex validation (Advanced tab) |
| Rule depends on another field's value | Custom field app |
| Rule depends on external data/service | Custom field app (calling your service from the app) |

## Shared responsibilities

| Area | Agility | Implementation partner / customer |
| --- | --- | --- |
| Built-in + regex validation | ✅ Provided in the content model UI | ✅ Configures patterns and messages |
| Cross-field / conditional validation | ✅ Provides the custom field app framework | ✅ Builds, tests, and maintains the custom field app |
| Editor experience of errors | ✅ Surfaces validation failures in the editor | ✅ Writes clear, actionable validation messages |
| Governance | — | ✅ Decides which rules are enforced and documents them for editors |