Link between sales and projects

For me, it is a wrong assumption. You can start working on a project even if it is no won. For example you can prepare the quotation.

Idem the project may have some progress and still be lost.

Yes, you can start working on a project but in order to prepare the project and start some qualification.
For me this will involve employees time sheets but no real progress on the project.

Once the project is accepted then you start progressing on the project so you can set 0% if none of the work is used for the project or an higher percentage if some of the previous work has some benefit for the project.

I do not agree. There will be a task “Quotation” and it will probably go up to finished before knowing if the project is won or not.

As I said previously, a project workflow is and should be very flexible. And so there are very few assumption that can be done.

Your right, haven’t though about this case :man_facepalming:

Then I think the unique option is to explicity mark on each state if the opportunity is “Won” or “Lost” by using an optional Selection field.

That does not sound right neither. You can not put on each task such value because only the root of a project concern the opportunity.
I think the best option is to manually convert an opportunity linked to a project as won or lost.

I’ve created Link between opportunities and projects (#7986) · Issues · Tryton / Tryton · GitLab which implements it.

Which does not require to have total freedom over the project states