Data interchange in the form of a standard exchange format(SEF) is only a first step towards tool interoperability. Inter-tool communication using files is slow and cumbersome; a better approach would be an application program interface, or API, that allowed tools to communicate with each other directly. This paper argues such an AP is a logical next step that builds on the current drive towards an SEF. It presents high-level descriptions of three approaches to tool-totool APIs and illustrates how requirements for the SEF also apply to the API. Keywords standard exchange format, API, tool interoperability, library, protocol, framework