Simple rule: Plan Mode is for thinking, Build Mode is for doing. Plan Mode (1 credit/message, never touches code). Use it when: - You're planning a new feature or unsure of the approach - You want to investigate a bug safely without risking more broken code - A fix has failed 2-3 times and you're going in circles - You want to compare approaches before committing to one Build Mode (default, variable cost based on complexity). Use it when: - You know exactly what you want and are ready to implement - You want the AI to autonomously write code, fix bugs, or refactor across multiple files The cleanest workflow: use Plan Mode to figure out the approach, then click "Approve" when you're happy with the plan and it automatically switches to Build Mode to execute. Many users spend 60-70% of their time in Plan Mode before touching a single line of code.