It started last month in January when they gave an open-ended question to create an API for one of their projects. I implemented it well and added more features than were asked. Consequently, I was invited for an onsite interview.

They asked me to improve the same project I had worked on before. I performed well and implemented everything they requested, except for one part across two or three rounds of the interview. Now, they have rejected me, stating that I did not cover small details in my project during the first round. I don't understand why they called me onsite if they disliked the project in the first place.

One cannot expect to develop production-ready code in 4-6 hours, especially when it's an open-ended question. Worst interview process.

Create a bunch of APIs for the project in 4-6 hours.