The belief is that the subscriptions are subsidized by them (or just heavily cut into profit margins) so for whatever reason they're trying to maintain control over the harness - maybe to gather more usage analytics and gain an edge over competitors and improve their models better to work with it, or perhaps to route certain requests to Haiku or Sonnet instead of using Opus for everything, to cut down on the compute.
Given the ample usage limits, I personally just use Claude Code now with their 100 USD per month subscription because it gives me the best value - kind of sucks that they won't support other harnesses though (especially custom GUIs for managing parallel tasks/projects). OpenCode never worked well for me on Windows though, also used Codex and Gemini CLI.
You can point Claude Code at a local inference server (e.g. llama.cpp, vLLM) and see which model names it sends each request to. It's not hard to do a MITM against it either. Claude Code does send some requests to Haiku, but not the ones you're making with whatever model you have it set to - these are tool result processing requests, conversation summary / title generation requests, etc - low complexity background stuff.
Now, Anthropic could simply take requests to their Opus model and internally route them to Sonnet on the server side, but then it wouldn't really matter which harness was used or what the client requests anyway, as this would be happening server-side.
Actually curious to hear what others think about why Anthropic is so set on disallowing 3rd party tools on subscriptions.
So they have to move up the stack to higher margin business solutions. Which is why they offer subsidized subscription plans in the first place. It’s a marketing cost. But they want those marketing dollars to drive up the stack not commodity inference use cases.
When setting your token limits, their economics calculations likely assume that those optimizations are going to work. If you're using a different agent, you're basically underpaying for your tokens.
Build the single pane of glass everyone uses. Offer it under cost. Salt the earth and kill everything else that moves.
Nobody can afford to run alternative interfaces, so they die. This game is as old as time. Remember Reddit apps? Alternative Twitter clients?
In a few years, CC will be the only survivor and viable option.
It also kneecaps attempts to distill Opus.
It’s not like moving from android to iOS.
API = way more expensive, allowed to use on your terms without anthropic hindering you.
One-price-per-month subscriptions (Claude Code Pro/MAX @ $20/$100/$200 a month) use a different authentication mechanism, OAUTH. The useful difference is you get a lot more inference than you can for the same cost using the API but they require you to use Claude Code as a client.
Some clients have made it simple to use your subscription key with them and they are getting cease and desist letters.