I suppose it matters what type of activity you have. For most people doing equity-only trades in a single brokerage, the IRS can handle things themselves. Ther

I suppose it matters what type of activity you have. For most people doing equity-only trades in a single brokerage, the IRS can handle things themselves. There are ways to make things exceedingly complex: 1. Have activity cross-brokerage (e.g. cross-account wash sales) as no entity cleanly tracks this. 2. Have straddles (e.g. put options on equity) which are under complex rules and AFAIK brokerages don't handle correctly (often failing to even identify them). 3. Constructively sale stock using options/shorting as rules are ambiguous and brokers don't implement the entirety of it.