"Do the thing" fails opaquely. The agent hits something unexpected, quietly adapts, and reports success — and you find out three steps later that it adapted wrong.
A phased brief fixes this by splitting one instruction into three with a pause between each. Phase A: inspect and report, change nothing. Phase B: execute. Phase C: verify and report facts. The pause after Phase A is the whole trick — it turns an unexpected finding into a halt instead of an improvisation. Add a context block up front (IDs, paths, prior state), safety rules at the end, and a named report-back format, and you have a brief that fails loudly.
The strongest version makes your own assumptions falsifiable: state the premise and tell the agent to STOP and report if the probe contradicts it. An agent halting on a surprise is a successful run, not a failed one.
Run the seed: rewrite a vague instruction as a phased brief.