Plan a non-trivial code change rigorously — understand the problem, form a hypothesis, validate assumptions through real experiments and doc/code reads, and cross-check the proposed solution against product specs, architecture, and the existing codebase before writing any plan. Use when the user says "/blueprint", "blueprint this", "plan this thoroughly", "deep plan", "I want to be sure before we build this", or for any change where a wrong direction would burn meaningful time, tokens, or compute. Heavier and more deliberate than `/plan`; the goal is the global optimum, not a local one.