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Migration #22 adds done_when and blocked_on to tasks, both NOT NULL DEFAULT ''. "He has not written one" and "there is nothing to write" are the same state here, so no caller has to tell NULL from empty. blocked_on is a canonical Nexus entity id, never a name. It names a person and identity lives in Nexus, so free text here would be a second answer to a question Nexus already owns. The caller resolves before it writes. Both columns round-trip through ipc.TaskAPI: on ipc.Task, settable at intake through CaptureTaskReq, and writable afterwards through the new SetTaskFields, which is deliberately not one-way — he may sharpen a criterion, and a blocker clears when the person answers. SetTaskStatus now refuses candidate → open when done_when is empty (ErrTaskNoDoneWhen, mapped across the wire), the same refusal ParseTaskCapture makes for a capture marker with nothing after it: confirming work whose finish line nobody wrote is how a board fills with rows that can never leave it. Dropping such a candidate stays legal, and the /tasks confirm button now says what is missing instead of surfacing a not-found. One caller skips the gate. CaptureTask promoting a candidate he stated out loud would otherwise be denied intake rather than asked for a criterion, and a direct open capture never carried one either. The gate belongs to the deliberate promotion on /tasks, where V-511 puts a form.