They recommend this in the announcement[1], but the way they suggest doing it is via a bogus /effort command that doesn't exist. See [2] for full details about thinking effort. It also recommends a bogus way to change effort by using the arrow keys when selecting a model, so don't use that either.
[1]: https://www.anthropic.com/news/claude-opus-4-6
[2]: https://code.claude.com/docs/en/model-config#adjust-effort-l...