Valid question. It's because they have a separate product intended for use with general tools: Their API.

Their subscription plans aren't actually "Claude Code Plans". They're subscription plans for their tool suite, which includes claude code. It's offered at a discount because they know the usage of this customer base.

OpenCode used a private API to imitate Claude Code and connect as if it was an Anthropic product, bypassing the need to pay for the API that was for this purpose.

Anthropic has been consistent on this from the start. The subscription plans were never for general use with other tools. They looked the other way for a while but OpenCode was openly flaunting it, so they started doing detection and blocking.

OpenCode and maintainers have gone on the offense on Twitter with some rather juvenile behavior and now they're trying to cheekily allow a plugin system so they can claim they're not supporting it while very obviously putting work into supporting it.

Most of the anger in this thread comes from people who want their monthly subscription to be usable as a cheaper version of the public API, even though it was never sold as that.