Is there a good way to auto switch between models for planning/implementation?

Pretty much every coding harness allows you to define skills and agents so yes

In most coding harnesses you can just instruct the agent to do so. When using Fable, I often say things like:

> Save your context. Always use a subagent (Opus 5 or GPT 5.6) for performing the implementation and then review the work yourself. You are the orchestrator and coordinator it’s up to you to ensure a cohesive final result.

I’ve done more or less the same thing with other agents/versions but with Fable consuming usage credits/tokens so quickly I do it more regularly than usual. I know people who will specify Composer (to my chagrin) as the implementing agent.

Addendum: this really goes a long way, and I can use a single chat session for days before I get into the context danger zone and need to compact/summarize.

OpenCode can.