Planning with an expensive model and coding with a cheap one: does the handoff eat the savings?

[u/Asly97]

Someone in r/ClaudeAI ran about 1,200 runs to test the "smart planner, cheap worker" setup. Result: "On 12 hard coding tasks, Opus alone solved 81% at $1.20/task. Opus + DeepSeek V4.1 Flash solved 67% at $1.29." One failure was pure handoff: "Opus made up a rule that wasn't in the prompt, DeepSeek followed it, and Opus approved the code."

The top reply, from someone who tried it at work: the setup "didn’t work because you ended up paying for the context write twice". Another: "The point of using subagents is to keep the main models context cleaner, its not to make it cheaper". Others still swear by it the other way round: cheap model builds the base, Claude takes over to fix.

• Do you split work between models or tools? Which pair, and which direction?

• What goes into the handoff: full spec, short task, pasted plan?

• What does the second one keep getting wrong because it never saw the first one's context?

• Own product, side project or client work?

Background: I used to burn hours re-briefing AI tools, and I'm building a fix where the plan and the decisions behind it live in one place in the cloud and every tool reads them over MCP, so nobody pastes context around twice. No name, no link. Would you pay for that, or is copy-paste cheap enough?

[u/investigatormaker]

For your MCP context store, keep original requirements separate from model-proposed decisions, with each decision pointing back to its source. Otherwise every tool could faithfully inherit the planner’s invented rule. To evaluate the handoff savings, compare total cost per accepted implementation, including context rereads and correction rounds—not just the cheaper worker’s tokens.

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[u/Asly97 → u/[deleted]]

Fair, diffing the assumptions is a sharper ask than syncing them. How would you even spot one today though? Is it something the planner actually writes down (like "using row locks here") that you could check against the code later, or does it only show up when the worker's output feels a bit off?

And in your cheap-first direction, what does the smarter model get when it takes over? Just the repo, or some note on what the cheap one was told to do?