We need to call an external library during the apply phase on a promise Output<string>. Wrapping it inside of

pulumi.interpolate

doesn’t help. Is there any pattern to follow for this kind of functionality?

If there is no existing provider for the API you need to call, I would check if I could model it as a resource backed by a dynamic provider: https://www.pulumi.com/blog/dynamic-providers/

Wrap your continuation with 

if (!pulumi.runtime.isDryRun()) { }

? i.e. inside the 

apply

thanks!