You'll notice
The 1M-context restriction that blocks certain Sonnet model switches now also blocks opusplan[1m]
What
Claude Code has a check that prevents switching to a 1-million-token context window for certain models, previously applied to sonnet[1m], sonnet-4-6[1m], and sonnet-5[1m]. That same restriction now also applies to the model string opusplan[1m].
Why
This closes a gap where opusplan[1m] could be selected without the same 1M-context restriction that already applied to the equivalent Sonnet variants.