The default mode is not the asking one
Claude Code defaults to its automatic mode, which is reviewed by the engine rather than by you. It is doing what it says on the label: most tool calls run without a prompt, and only what it flags comes back to you.
No engine offers all five modes
The names in the picker are a shared vocabulary and each engine supports four of them. A mode an engine lacks is not offered rather than quietly translated into something near it, which is why the same menu is a different length per engine.
One engine enforces nothing on its own
Grok asks the client about every non-read tool call whatever mode it was told to use, so the client is the only thing deciding. Its read-only mode refuses outright rather than asking, and that refusal is the client speaking, not the engine.
A policy layer runs before the mode
Hard denies and pre-approved rules are evaluated ahead of any mode-driven prompt, for every engine. A call that never asked may have been settled before the mode was consulted at all.