One provision of the tax package imposes a new tax on all earned income above $51,300, up to $125,000.
It attracted little attention because it is not part of the income-tax code. Instead, the provision increases the coverage of the Medicare tax, which now costs individuals and businesses 1.45 percent of their payrolls.
Like its twin, the Social Security retirement tax of 6.2 percent, the Medicare tax now ends when an individual's earnings reach $51,300. (That ceiling is for this year. Next year, it will rise with inflation to $54,300.)
Under the new provision, the retirement tax will stop at the normal threshold, while the Medicare tax will continue to be applied until an individual earns $125,000. Someone earning $70,000, for instance, would pay an additional $226.65 in payroll taxes -- 1.45 percent times $15,700, the amount exceeding $54,300.
The budget agreement also includes a provision sought by business lobbyists since 1986: It restores a lower tax rate for capital gains, the profits on the sale of such assets as stocks, bonds, real estate and art.
Special low rates for capital gains were wiped out by the Tax Reform Act of 1986, in part on the grounds that all income should be taxed the same regardless of its source. The current proposal sets the top rate for gains at 28 percent, while salary income can be taxed as high as 31 percent.
The differential is not large, but tax experts say it is great enough to change investor behavior.
"Many of my clients are on the phone as we speak, planning what to do," said Victoria Alther, a senior manager at the accounting firm Ernst & Young. "I don't think the differential will make or break a deal, but it will affect the timing."
Although some earlier versions of capital-gains cuts had limited the benefits to financial assets, such as stocks, this one appeared also to embrace real estate, timber, antiques and other assets that were covered by the pre-1986 tax code.