issue: a dropdown prompt variable marked required can be saved with no option selected

Before Submitting

• I searched open and closed issues and discussions for an existing report.
• I checked whether this is already fixed on the dev branch or latest source.
• I understand that maintainers want a well-written issue before any code pull request.
• I am using the latest available version of Open WebUI for my install method.
• This is not a security vulnerability.

Installation Method

Git Clone

Open WebUI Version

dev at a096961a31499be23890b98a040ecf2917405568

Operating System

Ubuntu 26.04.1 LTS

Browser

Firefox 155.0

Ollama Version

0.34.0

Summary

A dropdown input variable in a prompt can be marked required, and the form shows it as required, but the form still saves when no option is chosen. The dropdown's placeholder is then removed from the prompt as if the field were optional. Every other required input type blocks saving until it has a value.

Expected Behavior

When a dropdown is marked required and no option is chosen, clicking Save is blocked and the dropdown is flagged, the same way an empty required text or date field is.

Actual Behavior

The form shows "* required" next to the dropdown, but clicking Save with Select an option still showing closes the form. The dropdown's placeholder is replaced with nothing, so the chat input receives the prompt without a value for that field.

Steps to Reproduce

1. Go to Workspace, Prompts, and create a prompt whose content is the line below. It is the dropdown example from the prompts documentation.

{{priority | select:options=["High","Medium","Low"]:required}}

1. In a chat, type / and run that prompt.
2. The form opens with the dropdown showing Select an option and "* required" beside it.
3. Click Save without choosing an option.
4. The form closes and the chat input is left empty, because the dropdown's placeholder was removed.

Logs, Screenshots, and Config

The form with the required dropdown and no option chosen:

The chat input after clicking Save:

I did not capture browser console or server logs.

Additional Information

Anchored to a096961a31499be23890b98a040ecf2917405568.

Every input in the form except the dropdown receives a required attribute when the variable is marked required, for example the date input. The dropdown has never had one, since it was added in 9f87a0cf21f05f6306fd09969198fdc729a67536 (2025-07-05). https://github.com/open-webui/open-webui/blob/a096961a31499be23890b98a040ecf2917405568/src/lib/components/chat/MessageInput/InputVariablesModal.svelte#L123-L137 https://github.com/open-webui/open-webui/blob/a096961a31499be23890b98a040ecf2917405568/src/lib/components/chat/MessageInput/InputVariablesModal.svelte#L188-L198

The save handler does no validation of its own, so browser form validation is the only check, and nothing tells it the dropdown is required. https://github.com/open-webui/open-webui/blob/a096961a31499be23890b98a040ecf2917405568/src/lib/components/chat/MessageInput/InputVariablesModal.svelte#L28-L41

The prompts documentation uses a required dropdown as its example for this input type.