DisclaimerUnofficial, and not affiliated with Anthropic. Nearly all of this is read straight out of what ships: npm bundles, captured prompts, published docs. Anthropic's own notes go in verbatim, marked as theirs. The rest is my reading, and every entry carries the strings behind it. If one looks wrong, vote it down and say why.
When a model session is set to xhigh (max) effort, a new advisory message is now shown in the model selector: "May use excessive tokens resulting in long response times or overthinking. Use sparingly for the hardest tasks."
Evidence
New constant (search for "May use excessive tokens resulting in long response times or overthinking")
Strings lifted out of the shipped bundle, so the claim above can be checked against them.