Android XML Is there a way to use the tool namespace with custom attributes?

My opinion is that you can't.

There's an official, though very old, page here that quotes:

Designtime attributes can only be used for framework resources, not custom attributes at this point.

I was looking for the same thing for a while now. Decided to make a library that allows one to do it. https://github.com/giljulio/sneakpeek

I've written a tiny library that allows you define design-time attributes for your views: https://github.com/paulmanta/app-tools-attrs

Firstly, you have to declare separate attributes that you want to as at design-time:

<declare-styleable name="MyWidget">    <attr name="title" format="string"/>    <attr name="tools_title" format="string"/></declare-styleable>

Finally, when you initialize your custom view use the AppToolsAttrs class instead of accessing attributes directly from a TypedArray:

AppToolsAttrs.getString(a, isInEditMode(), R.styleable.MyWidget_title, R.styleable.MyWidget_tools_title)

You can then use your design-time attributes like this:

<com.myapplication.MyWidget    android:layout_width="match_parent"    android:layout_height="wrap_content"    app:tools_title="Lorem ipsum"    app:title="@string/real_title"/>

Gil already posted an answer with a different library that allows you do this. His implementation is really smart and it allows you to use the actual tools: namespace without having to duplicate attributes. Unfortunately, it's incompatible with Android Studio's auto-complete feature.