Resolving issues from an issue tracker on a source-code repository is tedious and expensive when done by hand. Recently, the SWE-bench leaderboard has seen submissions by several LLM-based agents that do this automatically. Unfortunately, these agents rely on closed-source frontier models, making them expensive and raising data-sharing concerns for industrial use. In contrast, we built IBM software agent for code, which works with a variety of open-source models such as Llama, Granite, and Mistral. ISAC uses sub-agents that are specialized for sub-tasks of localization, editing, and testing. Each sub-task is within reach of the capabilities of an open-source model. Furthermore, ISAC uses automated checking and repair of various common mistakes made by models, uses structured formats for data passed between sub-agents, and uses ensembling at multiple levels.