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1. The IRS isn't signing binding international agreements, they're agreeing to cooperate with foreign governments in the reporting of financial assets, which is well within their legal privilege.

2. The fourth amendment doesn't apply to data you're legally compelled to surrender to the government. By your logic, every form 1040 that's ever been filed is a fourth amendment violation.

3. The eighth amendment has almost never been used in a constitutional case involving excessive fines. When it has, the court has primarily used it to prevent pseudo-fines that aren't really called fines in order to skirt the eighth amendment.

Sure, FATCA might be annoying, but a legal challenge has almost nothing going for it. Don't get your hopes up.