Thanks @Eduard Lebedyuk ! I'll give it a try!

Will it work for POST requests too?

And is it for Postman only? E.g. if I use swagger-ui how could I get this page displayed?

Another one-liner suggested by @Robert Cemper recently:
 

k ^SPOOL s %io=$I O 2 u 2 ZW %anyvariable c 2 u %io

E.g. if you want to expose the content of %request during REST API method debugging, to write all the contents of %request to ^SPOOL global:

k ^SPOOL s %io=$I O 2 u 2 ZW %request c 2 u %io

and then ZW ^SPOOL in the terminal.

It is a global, again, though... But a very interesting and useful way to convert any write to a device into a global. 

Deserves a one liner input! 

Thank you, @Robert Cemper !

This works like a charm! 

So, in my case, I also wanted to see what's in a stream object that comes into the method (you may ask me how I don't know this, as it is method I coded? ) I don't, as it is a generated one via %^REST):

ClassMethod submitForm(formData As %Stream.Object) As %Stream.Object

{

set formDataObj= {}.%FromJSON(formData.Read())

k ^SPOOL s %io=$I O 2 u 2 do formDataObj.%ToJSON() c 2 u %io

return $$$OK

And then I do a REST API call and can see data in the terminal with zw ^SPOOL global:

USER>zw ^SPOOL
^SPOOL(1,1)="{""amount"":0,""name"":""John Doe"",""taxid"":""AB123456C"",""nationality"":""british"",""email"":""john.doe@example.com""}"
^SPOOL(1,2147483647)="{67504,40535{2{"

Simple and easy! Fantastic, @Robert Cemper !

Thanks, @Robert Cemper !

But here I'm more interested in debugging the REST API implementation method, so it is not very useful in this case, right?

Wow, thanks @David Hockenbroch ! 

Is it as simple as that? So when we receive a web call, it is all about the %request with the Method and content filled?

This could also be the way to unit-test REST-API methods

Hi Ed @Eduard Lebedyuk !

Where do I see this displayed data by:

do ##class(%CSP.Utils).DisplayAllObjects()
?

This works like a charm, @David Hockenbroch !

I'd also add to a VSCode a convenient extra link to see all the errors of such like here:

"links": {
        "UnitTest Portal": "${serverUrl}/csp/sys/%25UnitTest.Portal.Home.cls?$NAMESPACE=IRISAPP",
        "Error Log": "${serverUrl}/csp/sys/op/UtilSysAppErrorNamespaces.csp"
      }

Thank you, @David Hockenbroch ! 
I'll give it a try!

Hi @Brett Saviano !

Tried it!

Here is the feedback:

1 - can we add the link to the VSCode extension to be installed? I spent some time before I understood that, besides InterSystems ObjectScript, I need to install a special ObjectScript Extension pack .

2 - to make a REST API call I need to fill all the fields manually. Even I have an Open API spec class. Is it possible to read from it and provide all the fields ready to for tests? Filed a task and an idea.

3. Wasn't able to start debugging: faced the following error:

Thanks Ed! I’ll take a look!

Thanks @Robert Cemper ! As always - many options! Will try all

Thanks @Brett Saviano ! I'll give it a try

Thanks @David Hockenbroch ! Is it like @Robert Cemper suggests with Do LOG^%ETN()?