I am a phd student in an engineering discipline at a US school and am graduating this year. I recently finished a paper and want to submit it. I sent the manuscript, which only has me listed as an author, to my advisor, letting him know I will submit it soon. His reply: "Thanks Anon, that's great. I will proofread it and let you know my comments so we can submit it asap. Please use [X] as my author affiliation" clearly indicates that he was expecting to be a coauthor on the paper.
I have gifted him authorship in the past, more than once, on papers which were 100% my contribution (idea, work, code, writing, proofreading, everything). He has never had any comments on my work, and I already know he will not have any suggestions for the paper.
I should add that during my phd, I have worked completely independently. I have tried multiple times to work with other professors, but no one has ever wanted to supervise me in any meaningful sense. The reasons given have been that they either don't believe they could supervise me, or are too busy to supervise me. It's also relevant to note that in my program you are assigned an advisor upon admission and you are expected to stay with them (or possibly add a co-advisor later). My advisor has provided funding to me (roughly 25% of my funding comes directly from him) and he has paid for some conference/travel fees. There is no need for labs, experiments, etc. in my discipline. Although he has no intellectual contribution to my research he has helped me with other things like explaining how to find a good journal to submit to, how to respond to reviewers, etc. and is also helping me with my application package.
Now for the question. I want my current paper to be a solo publication. How can I approach this in as diplomatic a way as possible?
I thought the best way to deal with this question is over email. My basic idea for my response is as follows (I have a draft response but don't want to post the quote of it).
"If you want to be an author, I will add you to the paper. One question I've been thinking about is whether it would help my future career to have a solo publication. I'm worried that other students may appear more productive because they work in large groups. I thought if I have a solo paper people will get the idea that I'm very independent and have a lot of unique ideas."
Obviously I need a good letter from my advisor, especially seeing as I don't have anyone who can write me good letters. Giving another gift authorship, when I have already done it several times, is not the hill I want to die on. At the same time, I do believe a solo paper would help me quite a bit. This paper in particular I am quite proud of and it will probably be the highest impact paper I have written yet. The cross-disciplinary flavor the paper has also appears especially impressive given the solo authorship. This paper is also totally outside of my advisor's area, whereas previous papers I gifted authorship on have been in the same general area.
I should lastly add that I do not know how he will react if he is not an author on the paper. The question has never come up before, and as far as I know all of his past students have always included him as a coauthor. I think it's possible that he will actually be supportive of the idea. Still, it's a sensitive issue and not one I want to blunder.