I've been running a custom harness in production for months (code gen on cloud boxes), it's quite simple (<500 LOC). The model can use sed or other nice bash tricks to efficiently read. U really don't need any tool besides bash plus a good system prompt. Subagents are the same as the main agent (end with a summary). U can just remove tool results (oldest first) to save context, the model can read again if it needs to. High quality memory is the only difficult part. But that can also be solved with high quality documentation