From the Kansas Supreme Court opinion:
This is a tale of two cities. On March 23, 2006, the cities of Spring Hill and Olathe entered into a written agreement (Agreement) to restrict their future growth by establishing boundaries for annexing land lying adjacent to the two cities. Olathe agreed not to seek annexation of property south of the boundary line, while Spring Hill agreed not to seek to annex property north of the line. Each city reserved the right to annex land within their respective boundary lines…. The Agreement had no fixed expiration term. Instead, it was to “remain in effect until terminated,” and termination could “occur only upon mutual consent of the parties.” … The Agreement is simply a promise not to do something for an indeterminate length of time. It is very different from an agreement to provide “routine maintenance” of services, an administrative function. … It instead relates to “the development, introduction, or improvement of services,” a governmental function. … We therefore conclude that the Agreement is an unenforceable attempt to bind future City Councils to a governmental policy decision.
From the synopsis:
An elected governing body may not use its legislative power to constrain future governing bodies to follow its governmental, or legislative, policy decisions. An elected governing body may use its administrative or proprietary authority to enter into enforceable contracts to pay a specified sum over a specified time. The development, introduction, or improvement of services are, by and large, considered governmental, but the routine maintenance of the resulting services is generally deemed proprietary.
Read the full opinion here.