Whilst source downloads through the web front end are possible anonymously, it's not possible to access the VSTS back end this way.
This is awkward for Continuous Integration scenarios, because it means using one's own login details in the CI server. If the CI server is shared, this means a disclosure of login details.
A work around (presumably, haven't tried yet) would be to create a specific zero-rights user on a project just for this purpose, but this seams like a bit of a pain, and you'll have a lot of dummy users drifting about in the system.
Either anonymous access, or a generic user with read-only rights to all projects is required.
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