The problem: Claude Code's /effort <level> changes both the current session's effort and the persisted default that every new session inherits. That coupling can't be disabled. So there's no built-in way to bump just the current session without also changing the default for all future sessions.
The fix: a small slash command, /default-effort [level], that patches only the persisted default. Running it does not affect your current session (the default is read at session start, not hot-reloaded — verified). With no argument it resets the default to medium.
/effort high # session + default both go high (built-in behavior)
/default-effort medium # default drops back to medium; current session stays high
/default-effort # same as: /default-effort medium