Reasoning effort "xhigh" silently downgraded to "medium" (not "max") on models without xhigh (e.g. claude-opus-4.6, claude-sonnet-4.6)

Reasoning effort "xhigh" silently downgraded to "medium" (not "max") on models without xhigh (e.g. claude-opus-4.6, claude-sonnet-4.6) #5999