Asking for a letter of recommendation after only some days of collaboration

Question

I plan to apply for a Master's degree at a new university, and I require three letters of recommendation to do so. At my current university, there are two professors I did seminars/my bachelor thesis with, and both have agreed to write one - so there is one left. However, among the rest of the academic staff, there is none who knows me well (meaning I had one or two lectures with them, but not much personal contact). So I am wondering, if the following alternative is viable:

After finishing my thesis, my professor has offered me to continue research, which has now developed into some mathematical software. Our idea was to integrate it in an open source project, so I participated in an hackathon-style event of said project, where I have met another professor. The event was only 2 days, but he was interested in my project and supported me a lot. Therefore, I consider asking him for the last letter of recommendation, as he probably has a better idea of my work than most lecturers at my university.

Is this reasonable?

Answer 1

The outcome of such things is unpredictable, but you might do well with this. But you can have a conversation first with this professor to see what they think of the idea.

Letters of recommendation are best when they make a positive prediction about your future success. It may be that this professor has already formed such an opinion and would be willing to express it formally.

You can ask, of course, but have a conversation first, getting their opinion on your future and how you can optimize it. A letter can flow from such a conversation.

Note that this is a US perspective and a personal one.