Hey, reporters: don’t believe Paul Ryan’s budget numbers until they’re fully confirmed: by details, and where necessary by Congressional Budget Office and Joint Tax Committee projections.

This is, to be sure, advice reporters should use for all such proposals. But it’s particularly apt in the case of the House Republicans, who have been caught again and again using phony numbers, most recently in the debate over whether to repeal health reform.

Indeed, looking through Paul Ryan’s budget document (which has a lot more rhetoric than it has hard numbers), I see at least two obvious causes for skepticism. The first is that Ryan is counting on considerable budget savings — $1.4 trillion over the decade — by repealing the Affordable Care Act. Of course, the CBO has said that ACA will be a net budget winner over the next decade, and a major winner after that. Is Ryan using honest CBO numbers, but just juggling the presentation? Or is he simply ignoring neutral evaluations of the effects of health care reform? Ryan couldn’t explain the numbers during his press conference today.

The second clue is that Ryan refers (on page 59 of the proposal) to “dynamic” scoring of taxes. This, of course, is the same supply-side nonsense that Republicans have been peddling since 1981. It’s easy to show major deficit reductions if you slash taxes but pretend that it will yield revenue increases, but in reality, of course, it increases the deficit.

Tim Fernholz, meanwhile, points out another flaw: It’s one thing to set up budget targets or “caps” for the future, but quite another to actually hit them. It’s fair to credit Ryan and the Republicans with supporting real cuts even if they are talking about specific measures that have no chance of winning the support of the Democratic Senate and President Obama. But it’s a lot harder to know what to make of budget caps unaccompanied by specific program cuts.

As all budget wonks know, government spending tends to be unpopular in the abstract but popular when broken down to specific programs. That’s why promising “across-the-board” sequesters (pages 36-37) is a great budget gimmick for Congresses now, but never seems to work down the road. Note too that the Congressional Budget Office might accept a cap number for discretionary spending because there’s nothing to project about that number — but unless Republicans supply details, that projection is only as meaningful as the caps are foolproof. Which is to say, not very.

Bottom line: Paul Ryan’s budget claims to produce increasingly lower deficits than the president’s budget every year, getting to $773B by 2021. Reporters should treat that number as only that — a claim — until Ryan can show that the numbers are real.