After I get assigned on options, TWS displays the cost basis (avg price column) as just the strike price, but it should be the strike price plus the cost basis of the options.
If you sell a put and later get exercised on the option, you have 2 transactions. Your closed option trade will show a P/L based upon entry & exit. Your open stock position will show a P/L based upon entry. Your stock purchase was at the strike price so that's your "average price". You need to look back at the option trade to see what your P/L was on THAT trade.
This would be contrary to the IRS rules, where the assignment is not a taxable event, but the premium received/paid on the options is rolled into the cost basis of the stock.
No, as I said. The option trade was a SEPARATE trade so will have a separate P/L. You received cash in your account for selling the put so that's your profit on the option trade. Keeping it simple, at what price did you buy the stock? That will determine P/L on stock transaction.
Since I got assigned instead of actually closing the option position, the P&L on the option itself should be undefined and the P&L on the stock should use a cost basis of the strike price +/- the option premium.
You'll see a loss for your option position and an equal gain on your stock position (assuming that the stock price is not moving and not taken into account the time decay of your option). Your total P&L for these 2 positions will be netted as zero.
You're just refusing to look at them as 2 transactions. Option trade closed when exercised by broker. P/L = premium received, not "undefined". You have no options left so why "undefined"? Trade over. Stock trade opened when purchased at strike price. P/L should be based upon purchase price, not +/- option result. Simple