I ran into this as well and was also only able to resolve it with help from @jglick in #jenkins.

In my case, I was trying to copy artifacts from the branch of a multipline build. The Pipeline syntax generated only generated syntax that included the projectName as "My Pipeline". The correct syntax I eventually figured out was "/My Pipeline/mybranch".

Well, the syntax offered is going to depend on where you clicked the Pipeline Syntax link; it will offer a path valid from that point. In the case of a multibranch project, you probably want to run it from a sample branch project, so for example from qatest/master you could refer to dev/releases as either /dev/releases or ../../dev/releases. Just like Unix filesystem paths.

It is also important to note that if the Multibranch Pipeline is searching in branch folders with forward slashes in the sub-job names (or any other url-encodable char), it generates sub-jobs with encoded names. Copying the artifacts from these projects require you to specify the folder path, followed by the url-encoded name of the job. This is logical in terms of being able to resolve Folders containing jobs in Jenkins, but may trip you up when writing groovy.

e.g. if I have a Multibranch project called "MyBranches", with base of https://my.repo, and I tell it to search for branches in /branches/jamie/* and it finds /branches/jamie/a and /branches/jamie/b, copying artifacts from these jobs requires me to use /MyBranches/branches%2Fjamie%2Fa.

As a side node on that, unless I am using java.net.URLEncoder wrong, I have to replace the output of encode + with %20 in order for the string to match the job name if it has spaces in the url path.

def job_name = '/MyBranches/' + java.net.URLEncoder.encode('branches/jamie/spaces in path', 'UTF-8').replace('+', '%20')

I also did not know that the Pipeline Syntax link from different pages affected the behaviour of the snippet generator.

I drafted a wiki page based on Jesse Glick's comments: https://wiki.jenkins.io/display/JENKINS/How+to+reference+another+project+by+name

…and a PR: https://github.com/jenkinsci/copyartifact-plugin/pull/107

This was fixed in copyartifact-1.42.1.