Best options for open source harnesses?

A Claude Code user, who also uses Codex and Spark, is seeking recommendations for open-source harnesses. Their tasks are diverse, ranging from highly technical research and planning to basic questions and coding. They currently use Notion boards as a custom memory system alongside Claude's existing storage. The user notes significant recent progress in open-source models, which are now nearly as effective as frontier models but at a much lower cost, based on API usage and benchmarks.