[PopAlongKid]

>The IRS has my 1098s. They know the mortgage interest, principal, and origination date.

But they don't know what you used the proceeds for. Not all mortgage interest is deductible. Nor do they know what use you make of the property that secures the mortgage (primary residence, rental, other). They also don't know about prior year points you paid to originate a mortgage.

[__derek__]

The IRS can reasonably assume my primary residence address from my prior year's tax return, then match that to the 1098 with the same address. The other, less common situations are covered by my comment on the home office and rental-property plumber: some things will require corrections with additional details to substantiate them.

[PopAlongKid]

>But they don't know what you used the proceeds for. Not all mortgage interest is deductible

You failed to address this important point. There is a difference between acquisition debt and equity debt that the IRS has no information about your situation. Also the "primary residence address" is not required to be provided on the tax return, so no they can't "reasonably assume".