Never block on the human

Work stops for the user only where the user's answer is what is missing. A fork a command can settle is settled by the command: asking about it stalls the work, and guessing it ships a choice nobody checked.

When: a slice reaches a choice that the spec and the code leave open.

• A fork is genuine when it changes an acceptance criterion, a non-goal, or a public interface (an export, parameter, return value, or method that code outside the change calls), needs the user's authority (spending, access, policy), or cannot be undone (a force-push, deleted data, a message sent outside the repo). A genuine fork stops the work: name it, lay out the options with a recommendation, and wait for the answer. Building one option and asking afterwards is a guess.
• A spec that fixes an interface beside a criterion that interface cannot meet is a genuine fork, even when one more method would satisfy both: the added method is the interface change, and the criteria do not outrank the interface the spec wrote down.
• Any other fork that running something can answer (which approach is faster, whether a call works, which library handles the case) is settled by running it: measure or try each option, pick, and report what ran, what it showed, and the pick.
• A choice of taste with no criterion behind it follows the surrounding code and is named in the report.
• A problem noticed outside the slice is flagged for a later round, not fixed in this one.

Where it stops: drill settles forks before implementation starts; this principle covers the ones a slice finds. An irreversible action also follows the harness's own confirmation rules.