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Is it possible to change the pruning policy on build definitions?

Accepted answer

No, only what the built in options provide.
You could create automation outside of the tool, if you want. See https://rsjazz.wordpress.com/2015/10/28/build-artifacts-publishing-and-automated-build-output-management-using-the-plain-java-client-libraries/
And https://ryehle.wordpress.com/2012/06/28/custom-build-result-pruner/
You could create automation outside of the tool, if you want. See https://rsjazz.wordpress.com/2015/10/28/build-artifacts-publishing-and-automated-build-output-management-using-the-plain-java-client-libraries/
And https://ryehle.wordpress.com/2012/06/28/custom-build-result-pruner/