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When you let Pulumi do your build, it examines your binary’s references during plug-in discovery to determine which plugins should be installed, and then looks for them in your nuget cache. When you do the build yourself, plug-in discovery doesn't occur because Pulumi cannot make the assumption that your nuget cache contains those binaries. So in this scenario you must manually call

pulumi plugin install …

to install the plugins explicitly ahead of time.

Ah ok thank you Joshua! Is there a way to query the stack and output the plugin versions used?

I don't think a command like that exists, but if you think about it it would require plugin discovery to function, which currently depends on the Pulumi CLI building your project. So you would be back to square 1. Unfortunately in my Pulumi usage I have needed to keep my plugins and the versions I need aligned in 2 places (both my project and my manual install commands) which is definitely a sore point for me.