This is an open-ended agreement that can be canceled at any time.

single open-ended spanner
《a ～》〈英〉＝single open-end wrench

single open-ended wrench
《a ～》〈米〉《工具》片口スパナ

ask an open-ended question
自由回答式の質問をする

give someone open-ended authority
（人）に制限のない権限を与える

This is an open-ended agreement that can be canceled at any time.
これは無期限の協約で、いつでも無効にすることができます。