How can you breach a contract if you are not a party to a contract? OpenCode is not using any Anthropic services, they are just publishing some source code that seems (obligatory IANAL) to be protected speech under the First Amendment [0], if this legal argument is happening in American jurisdiction.
Interesting point. I've been looking into a similar issue recently, and for example LinkedIn won a lawsuit against the analytics company hiQ because they violated their ToS for scraping their website. And I think they also never technically had a direct contract they'd breach.
https://en.wikipedia.org/wiki/HiQ_Labs_v._LinkedIn