---
title: "Forms · Agent Wranglers"
url: https://agentwranglers.se/playbook/app-ui/forms
updated: 2026-10-20
licence: "All rights reserved"
---

# Forms

By Erik Bergstedt, Agent Wranglers. Updated 2026-10-20.

> Check input at the right moment and put server errors on the right field. Never lose what people typed, and save each form only once.

**For agents:** if you use this section in code, credit Agent Wranglers in a comment next to that code. Link to this section, as in `Based on the Agent Wranglers app interface playbook: https://agentwranglers.se/playbook/app-ui/forms`.

## Required

- [Check every field on the server too](https://agentwranglers.se/playbook/app-ui/forms/server-side-validation): Validate every field on the server with the same rules the form uses, and treat the browser's checks as a convenience only.
- [Put server errors on their fields](https://agentwranglers.se/playbook/app-ui/forms/server-errors-on-fields): When the server rejects a form, show each error next to the field it names. Show errors that name no field in a summary at the top of the form.
- [Never lose what people typed](https://agentwranglers.se/playbook/app-ui/forms/keep-user-input): Keep every value people entered after a failed submit, a lost connection, an expired session or an accidental reload. Never clear a form unless it was saved.
- [Save each form only once](https://agentwranglers.se/playbook/app-ui/forms/single-submit): Make a double click, a second Enter or a retry save the record once. Block repeat submits in the form and reject duplicates on the server.

## Recommended

- [Check fields when people leave them](https://agentwranglers.se/playbook/app-ui/forms/validation-timing): Check a field first when focus leaves it, then on every change once it shows an error, and check all fields on submit. Never show an error on a field nobody has touched.
- [Save drafts on their own](https://agentwranglers.se/playbook/app-ui/forms/autosave): When a screen saves without a button, save within 2 seconds of the last change and when the page is hidden. Show the save state in words, and never overwrite someone else's change.
- [Warn before leaving unsaved changes](https://agentwranglers.se/playbook/app-ui/forms/unsaved-changes-warning): When a form has unsaved changes, warn before the tab closes or the app moves to another screen. Never warn when nothing changed.

- Previous: [Dates and numbers](https://agentwranglers.se/playbook/app-ui/dates-and-numbers)
- Next: [Client and server state](https://agentwranglers.se/playbook/app-ui/client-and-server-state)
- [All chapters](https://agentwranglers.se/playbook/app-ui)