How to know if two ETFs are 'substantially identical' according to wash sale rules?
Nobody knows for sure what "substantially identical" means because the IRS hasn't officially defined it. Until they do so, it would come down to the decision of an auditor or a tax court. The rule of thumb that I have always heard is if the funds track the same index, they are probably substantially identical. I think most people wouldn't consider any pair of AGG, CMF, and NYF to be substantially identical, so you should be safe with your tax-loss harvesting strategy.