An attended agent can ask you questions; a background agent inherits every ambiguity you left in the request. "Go clean up the auth module" comes back as a 40-file PR that renamed things you liked — and the postmortem is always the same: the brief was two sentences. Background Agent Task Brief Writer turns rough intent into a complete, dispatchable brief: an outcome-shaped goal, explicit scope and non-goals, hard constraints stated as prohibitions, acceptance criteria the agent can verify itself by command, a verification plan it runs before declaring done, stop-and-escalate rules for the situations where it should not proceed, and the handback format with required evidence. It resolves ambiguities or assigns them explicitly, right-sizes the agent's freedom to the risk, includes a diff size-guard that prevents runaway changes, and keeps the whole brief to one page. The download includes three reference files: the task-brief template, a guide to writing machine-checkable acceptance criteria, and a fully worked, annotated sample brief. It writes the brief; it does not dispatch or monitor the agent. Works with Claude Code, Cursor, Codex CLI, Gemini CLI, cloud background agents, and any SKILL.md agent.