FR: Configure `/error` mapping in Spring Boot to see root causes of problems in dev web server

[vorburger]

My application using ADK has some bug, and currently when I'm launching the Dev server and access http://localhost:7070 it HTTP 304 redirects to http://localhost:7070/dev-ui which show:

Whitelabel Error Page
This application has no explicit mapping for /error, so you are seeing this as a fallback.
Fri Jul 25 19:50:17 CEST 2025
There was an unexpected error (type=Not Found, status=404).

Perhaps it would be useful if we made it show the root cause of the error on a developer friendly error page by default?

Does anyone reading this know how to configure an `/error/ mapping with Spring?

PS: #181 from @TomorJM and @xinhuagu is a bit related to this, but that issue is much more broad generic Spring Boot related, whereas this is about a specific configuration.

[vorburger]

I've learnt that adding the following to a subclass of com.google.adk.web.AdkWebServer like this one makes it show root causes:

@Bean
@Primary
// Nota bene: @Primary takes precedence over the default ServerPropertiesAutoConfiguration,
// and might therefore disable some of its functionality.
public ServerProperties serverProperties() {
        var serverProperties = new ServerProperties();
        var errorProperties = serverProperties.getError();
        errorProperties.setIncludeStacktrace(ErrorProperties.IncludeAttribute.ALWAYS);
        errorProperties.setIncludeMessage(ErrorProperties.IncludeAttribute.ALWAYS);
        errorProperties.setIncludeException(true);
        return serverProperties;
}

[vorburger]

Perhaps it would be useful if we made it show the root cause of the error on a developer friendly error page by default?

On thinking this through further, it's actually probably wiser not to configure this by default.

As there is a relatively simple way to enable this in one's application, I'm closing this.