Anthropic made a big mistake - The Software Archæologist

When Claude Code launched for real in June 2025, usage of the Anthropic models was included in the Pro and Max plans, for a flat monthly or annual subscription. These plans quickly became very popular when users realised that the effective cost per token was much lower compared to Anthropic's API pricing. ... In contrast, other coding agents such as Amp only provided the ability to connect to Claude models via the much more expensive pay-per-token API. It turns out that logging into third-party coding agents with an Anthropic OAuth token was a bit of a loophole. This was evident from the fact that it would only work if the client-supplied system prompt contained a specific phrase identifying itself as Claude Code. Nevertheless, many (presumably) unsuspecting Anthropic customers used OpenCode in this way; from their perspective, they were simply using the same service that they were already paying for, just in the comfort of their preferred coding harness. However, Anthropic clearly didn't see it this way. On 9 January 2026, Anthropic unceremoniously closed the loophole, changing their API to detect and reject requests from third-party clients. The renowned vibe-coder Peter Steinberger soon posted about it on the website formerly known as Twitter, and disgruntled Anthropic customers expressed their discontent in a GitHub issue, requesting the decision to be reversed, many threatening to cancel their Claude subscription otherwise. It's notable that Anthropic has not formally announced this change in ToS enforcement, neither ahead of time nor after the fact. The only quasi-announcement of this change was this thread, posted by an Anthropic employee on their personal account the day after the changes took effect, presumably in response to customer complaints. The stated motivation for the change was the allegation that "third-party harnesses using Claude subscriptions create problems for users and generate unusual traffic patterns [...] making it really hard for us to help debug when they have questions about rate limit usage or account bans and they don’t have any other avenue for this support." … Which brings us to the final point: without anticipating it, Anthropic just found itself in a classic prisoner's dilemma with OpenAI -- and OpenAI just defected. Not only are they officially supporting OpenCode users to use their Codex subscriptions and usage limits in OpenCode, they are extending the same support to other open-source coding harnesses such as OpenHands, RooCode, and Pi. And it's not just a theoretical announcement either: support for connecting ChatGPT Pro/Plus subscriptions with OpenCode has already shipped.