A multichip package that includes a programmable interposer is provided. Multiple integrated circuits may be mounted on the interposer. Active circuitry may also be embedded in the interposer device to facilitate protocol-based communications, debugging, and other desired circuit operations. The interposer device may include programmable interconnect routing circuitry that serves primarily to provide routing for the different circuits within the multichip package. A design tool may be used to design the interposer device. The design tool may include a standard die footprint library from which standard interface templates can be selected when designing an interposer device that has to communicate various on-interposer integrated circuits. The use of standard die footprints may simplify the design of interposers by enabling a family of devices to interface with a given interposer.