CAD environments enable designers to construct, evaluate, and revise models of engineering systems. GAIA is a CAD-like environment for designing game-playing agents. Unlike engineering systems, intelligent agents may learn from experience. Thus, in GAIA, the human designer and the intelligent agent cooperate to redesign the agent. In this article, the authors describe three elements of this vision: the interactive environment GAIA, an agent modeling language called TMKL2, and a GAIA module called REM that performs meta-reasoning for self-adaptation in game-playing agents. They illustrate these concepts for designing software agents that play variants of Freeciv, a turn-based strategy game.