Building my own agentic harness VS using already existing agentic harnesses (like Claude Code)

Something has been bugging me about the current generation of agentic coding tools, and I want to know if I'm missing something obvious. In a single session I discuss, plan, review, implement, and push. All of it runs through the same model. If any one of those phases needs the expensive model, I'm effectively locked into paying top-tier rates for every phase - including the ones where a cheaper model would do fine. On top of that, the vendor injects a large system prompt on every call, so the token overhead is baked in whether I need it or not. So the question: if I built my own harness, could I split the workflow by phase and route each one to the model it actually needs? Cheap model for scaffolding and boilerplate, expensive one for architecture and review. That seems like it should meaningfully cut cost without cutting quality. I've found tools that cover part of this - Pi Coding Agent and BMAD-METHOD, for example - but nothing that covers the whole loop, and nothing with a UI worth using. That's what surprises me most: plenty of CLI frameworks, almost nothing with an interface. So: What are you actually using - the established tools, or something you rolled yourself? Is there a real reason the established ones are better? Something I'd only discover after building my own? If per-phase model routing is such an obvious cost lever, why isn't anyone shipping it with a decent interface? Genuinely asking. If there's a reason this is a bad idea, I'd rather hear it now. Or, if there already are such tools, I might've lived with my head in my bum - if that's the case, let me know, as I'd be glad to get my head out of that place.