In last-minute summit advice, Defense Secretary Caspar W. Weinberger has urged President Reagan to avoid a Geneva agreement pledging U.S. adherence to the unratified SALT II treaty because it "would limit severely your options for responding to Soviet treaty violations."

The defense secretary also opposed giving the Soviets any assurance "that you will continue to be bound" by current Strategic Defense Initiative (SDI) research limits because that "would diminish significantly the prospects that we will succeed in bringing our search for a strategic defense to fruition."

Weinberger's tough advice was carried in a three-page cover letter to the Pentagon's long-awaited study of Soviet violations that was delivered to the White House Wednesday. A copy of the letter and the study's executive summary were obtained by The Washington Post yesterday.

In that study, as previously reported, the Pentagon found that "Soviet violations are continuing and require an appropriate and proportionate response on our part."

The Joint Chiefs of Staff, Weinberger reported, "generally feel" that the appropriate response to the violations would be full funding of the administration's strategic modernization program and the improvements in military readiness already requested of Congress.

The defense secretary said he would present the president with his own "range of options for appropriate response" when Reagan returned from Geneva. They would include, he added, "responses we can take within various classified programs."

In making his presummit "points," Weinberger told Reagan "a pledge to continue to adhere to SALT II, even though the Soviets are violating it, could make it difficult, if not impossible, to do other things we should do to make up for their violations."

"We can," Weinberger went on, "of course continue to observe parts of SALT II, at your option."

The Soviet Union has proposed, according to U.S. and Soviet officials, that Reagan and Soviet leader Mikhail Gorbachev agree to a one-year extension of the treaty, which is to expire on Dec. 31.

Administration sources said yesterday that Reagan planned to raise the issue of Soviet violations and the need for Moscow to come into compliance with past treaties.

When the issue of continuing compliance came up earlier this year, Weinberger pressed for Reagan to deploy new missiles, thereby sending the United States over the treaty limits.

Weinberger, in his letter, said failure to respond to Soviet violations "can only encourage them to commit more."

He then praised Reagan, saying, "It is significant that you are the first president to report to Congress violations by the U.S.S.R. that have continued for 20 years."

Weinberger also told Reagan that he "will almost certainly come under great pressure" to limit SDI "research, development and testing to only that research allowed under the most restrictive interpretation" of the 1972 Antiballistic Missile (ABM) Treaty.

The Pentagon chief told Reagan that any such "agreement to limit the SDI program according to the narrow (and, I believe, wrong) interpretation of the ABM treaty" would harm the program.

Last month, Weinberger lost an interagency battle over this issue.

A Pentagon legal study of the ABM treaty and the negotiating record found that a country could carry out not only research but also testing and development of space-based missile defenses if they involved exotic weapons such as lasers and particle beams.

The president decided to continue his SDI program under the earlier, narrow interpretation of what the treaty allowed.