I finally found a solution that works for me for now and thought to share it. In Zotero if one specifies "Item type" Thesis *and* "Type" `mastersthesis` (mind `mathesis` will not work) then the bibtex export generates `@masterthesis`.

I have not yet found a solution for Bachelor Theses but have currently also no need for those.

OK I understand. But the question is what IS supported. Since there seem to be conflicting information of what gets rendered how.

The alternative is switching to biblatex export but here I have to come back to my very first question: Is there a way to group by year when biblatex only contains the date field and not the year field like bibtex.

So I don't export or edit the bibfiles manually but are dependent what Zotero's export gives me. And that is what I have described so far.

(ma|ba)thesis gets exported as (Ma|Ba)thesis

and type thesis will become @phdthesis in bibtex files.

As we have established in a different thread I don't want bibbase expose my whole Zotero database. Hence I continued to investigate the alternative of hosting a bib file which bibbase renders. Zotero, or rather betterbib(la)tex offers the functionality of auto export. So I've selected the two collections I like to get presented on my webpage and whenever I make a change in Zotero the exported bib files get automatically sychronised. 

That process now seems to have uncovered some problems when it comes to Bibbase's rendering of user-provided public, bibfiles. That is what this thread is basically about. Does this make sense now?

So I updated the test but it will still not follow your diagnosis.

In zotero I have now:

which upon export to betterbibtex (normal bibtex does not work since it does not have the type field at all) becomes:

` type = {Mathesis}` so it get's uppercased.

As for the minimal example, I've changed all titles and years in order to make them unique and made the last item of type 'bathesis'. Still every entry (including the last one) is rendered as Ph.D. thesis:

OK slight misunderstanding here. "*Item* Type" in Zotero I always have "Thesis" (which becomes @phdthesis in bibtex and @thesis in biblatex export).  As for the "Type", this is a simple text field in Zotero, so one can define whatever one likes. That's why I asked what the correct strings are that bibbase needs and gets triggered on. So now from your screenshot I guess I need to put "mathesis" or "phdthesis" in there. Are there other strings that bibbase regognises?

I've run a test and as you can see nothing gets recognised by the bibbase rendering :-( 

The next problem now is that Zotero changes "mathesis" to "Mathesis" in the bibtex export. Not sure if bibbase is parsing case sensitive or not.

Unfortunately not. I just double checked. It could have to do with Zotero exporting "Master's Thesis" in the type field to

type = {Master's {{Thesis}}}

Which then bibbase does not recognise. Which strings is bibase matching? 

The problem is that Zotero exports "thesis" types as @phdthesis by default when selecting bibtex and as @thesis when using biblatex. So if I want the grouping by year I need to choose bibtex format it seems and hence would need that bibbase still checks the type field also for @phdthesis items. Would this be possible?