Data interchange in the form of a standard exchange format (SEF) is only a first step rewards 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. The paper argues that such an API is a logical next step that builds on the current drive towards an SEF. It presents high-level descriptions of three approaches to tool-to-tool APIs and illustrates how requirements for the SEF also apply to the API.