How do founders run 'deep research' to get report-quality answers?

Deep research modes in ChatGPT, Gemini, Perplexity and Claude are agents: you give a detailed brief, approve the research plan, and 5 to 30 minutes later get a long cited report built from dozens or hundreds of sources. The quality depends almost entirely on your brief, so specify scope, geography, timeframe, output structure and what sources to prefer. Treat the report as a strong first draft from a junior analyst and spot-check the claims that matter.

A quick orientation. The real value is below: resources worth your time, from people who've actually done it.