Todo/task-tracking tools (TaskCreate/Get/Update/List, TodoWrite) are no longer available on Opus 4.8, Sonnet 5, Fable 5, Mythos 5, and newer models; set CLAUDE_CODE_ENABLE_TODO_TOOLS=1 to bring them back"

Anthropic appears to agree frontier models don't need in-session planning tools.

https://github.com/anthropics/claude-code/issues/80487