An uncharacteristically restrained Senate yesterday agreed to lay aside pet projects and joined the House in passing a bare-bones stopgap spending bill to keep the federal government operating after current funding authority runs out at midnight.

"There's something new under the sun after all," Sen. John C. Stennis (D-Miss.) said after the Senate, which has been known to add scores of amendments to such measures, passed the bill by voice vote in 17 minutes without a single add-on.

The bill, which would fund most of the government at levels slightly above current spending through mid-November, will go to a House-Senate conference this morning.

Barring unforeseen hitches, congressional leaders expect the so-called "continuing resolution" to be sent to President Reagan for signature in time to avoid the governmental disruptions that stopgap funding disputes have caused repeatedly in recent years.

Administration officials pointedly have declined to say whether the bill will be signed, capitalizing on any uncertainty in order to get what they want for defense while squeezing domestic spending. Congressional leaders have indicated they do not expect a veto, however.

But there may be a price to pay later. In order to keep members from decorating the continuing resolution with their favorite spending proposals, Senate leaders had to offer them other opportunities, starting with a supplemental appropriations bill already in the pipeline.

"Oh, perish the day of accountability," said Senate Appropriations Committee Chairman Mark O. Hatfield (R-Ore.), who, with Stennis, the committee's ranking Democrat, steered the stripped-down measure through a reluctantly compliant Senate. A Republican leadership aide suggested that the supplemental bill probably will wind up "so weighted down you'll need a truck to get it out of the chamber."

But, unlike the continuing resolution, the supplemental does not face the kind of precise deadline that puts pressure on the White House to accept spending it doesn't want.

The stopgap bill is needed because only four of 13 spending bills for the 1984 fiscal year starting Saturday have been signed into law. Two others, covering military construction and the District of Columbia, have been passed and sent to Reagan. Without interim spending authority, unfunded departments and agencies would face at least technical shutdowns Monday.

Hatfield said yesterday he expects the rest of the spending bills, except for foreign aid, to be passed before the continuing resolution expires at a mid-November date to be set by the conference today. He said that even the agriculture bill, which caused the most trouble between Congress and the White House, appeared close to being resolved.

In other spending action yesterday, the House voted 327 to 92 to extend through mid-November a recession-spawned program that provides extra weeks of jobless benefits to the longtime unemployed who have exhausted regular benefits.

A bill to extend the program for 18 months, but with less generous benefits, was being considered in the Senate. Existing authority for the program expires today.

The unemployment benefits extension approved by the House would last until Nov. 15, when another bill to extend the popular program for those who have exhausted their regular benefits could be used by the Democrats as a vehicle for a tax increase.

It would keep benefits flowing to 643,000 unemployed workers now on the supplemental benefit rolls and increase the number of eligible persons to more than 1 million.