I’m still an actions noob, but intuitively I feel like this action should work without me needing to provide input, yet it prompts me when I run it.
Hi, welcome to the forum, @dannflor, and thanks for checking out the beta!
The short answer is: No, it wouldn’t work, because the Get Browser Tabs action asks for a browser, not a window – but your first block returns a Browser Window, not a Browser reference.
This is how you could do it:
The slightly longer answer is that Shortcuts (the app) isn’t picky when it comes to letting you pass variables and references into parameter fields (like you did with “Dev Window”) but it will complain once the workflow is executed. 
Browser Action is passing references around, e.g. Browser Window or Browser Tab. These references contain the info which browser they belong to, and so BA’s actions will never ask for both a tab/window reference and a browser, because that’d be confusing and redundant to boot.
Does that answer your question?
PS: I’ve just increased the max. amount of images for new posters from 1 to 3.
Interesting. Could you remove the last block and just see what the Get Browser Tabs action returns, please?
Huh. Let’s start at the beginning, just to cover all bases:
- You’ve done the setup, including enabling the AppleScript/Javascript switch?
- Does the action output anything in another browser?
- Which macOS version, which Safari version? (Specifically, any super-early betas?)
Turns out what I needed to do was restart safari. I thought since I had opened the browser window since I ran the setup it would have the new settings enabled, but I guess not. Now it works superbly! (You might want to include a direction to restart your browser in the setup copy)
Good catch! Thank you, I will add that to the onboarding (and the FAQs)!