(1) ATG may be deemed to be the beneficial owner of 3,471,778 shares of Primus common stock, which represents shares of Primus common stock subject to the Voting Agreements described in this Schedule 13D. ATG expressly disclaims beneficial ownership of any of the shares of Primus common stock covered by the Voting Agreements.

(2) Based upon 23,855,394 shares of Primus common stock outstanding as of August 9, 2004 (as represented by Primus in the Merger Agreement).

*SEE INSTRUCTIONS BEFORE FILLING OUT!

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Table of Contents

Item 1. Security and Issuer.

This statement relates to shares of common stock, par value $.025 per share (“Primus Common Stock”), of Primus Knowledge Solutions, Inc., a Washington corporation (“Primus”). The address of Primus’ principal executive offices is 1601 Fifth Avenue, Suite 1900, Seattle, WA 98101.

Item 2. Identity and Background.

This Schedule 13D is being filed by Art Technology Group, Inc. (“ATG”). ATG is a Delaware corporation, and the address of its principal business and principal executive office is 25 First Street, Cambridge, MA 02141. ATG’s common stock trades on the Nasdaq National Market under the symbol “ARTG.” ATG delivers software solutions to help consumer-facing organizations create an interactive experience for their customers and partners via the Internet and other channels.

For information with respect to the identity and background of each director and executive officer of ATG, see Schedule 1 attached hereto. To ATG’s knowledge, except as otherwise indicated on Schedule 1, each person named in Schedule 1 is a citizen of the United States.

During the last five years, none of ATG, or to ATG’s knowledge, any person named in Schedule 1, has been convicted in a criminal proceeding (excluding traffic violations or similar misdemeanors).

During the last five years, none of ATG, or to ATG’s knowledge, any person named in Schedule 1, was a party to a civil proceeding of a judicial or administrative body of competent jurisdiction and as a result of such proceeding was or is subject to a judgment, decree or final order enjoining future violations of, or prohibiting or mandating activities subject to, federal or state securities laws or finding any violation of such laws.

Item 3. Source and Amount of Funds or Other Consideration.

As an inducement to ATG’s entering into the Agreement and Plan of Merger by and among ATG, Autobahn Acquisition, Inc., a wholly owned subsidiary of ATG, and Primus, dated as of August 10, 2004 (the “Merger Agreement”), certain stockholders of Primus, including certain officers and directors (each a “Stockholder” and together the “Stockholders”), entered into individual Voting Agreements, each dated as of August 10, 2004 (each a “Voting Agreement” and together the “Voting Agreements”), with ATG covering a total of 3,471,778 shares of Primus common stock.

No separate consideration was paid by ATG in connection with the Voting Agreements. However, the shares of Primus common stock subject to the Voting Agreements are covered by the Merger Agreement. References to, and the

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descriptions of, the Merger Agreement and the Voting Agreements are qualified in their entirety by reference to the complete text of such agreements, which are filed as exhibits to this Schedule 13D and incorporated by reference herein.

Item 4. Purpose of Transaction.

ATG entered into the Merger Agreement and the related Voting Agreements with the intent of acquiring control of, and the entire common equity interest in, Primus.

Merger Agreement: ATG, Autobahn Acquisition, Inc., a wholly owned subsidiary of ATG, and Primus have entered into the Merger Agreement pursuant to which the parties have agreed that Autobahn Acquisition Inc. will merge with and into Primus, with Primus being the surviving corporation in the merger. The Merger Agreement provides that each outstanding share of Primus common stock will be cancelled in the merger and converted into the right to receive between 1.2976 and 1.4169 shares of common stock of ATG. This exchange ratio is subject to adjustment within this range based upon Primus’ achievement of specified minimum working capital requirements at closing, but in no event will the exchange ratio be less than 1.2976 ATG shares for each diluted share of Primus. Upon the completion of the merger, ATG will own all of the outstanding shares of Primus common stock.

Upon completion of the merger, the Board of Directors of Primus will be the Board of Directors of Autobahn Acquisition Inc., and certain officers of Primus will be replaced with officers of Autobahn Acquisition, Inc. The Articles of Incorporation and Bylaws of Primus following the merger will be the Articles of Incorporation and Bylaws of Autobahn Acquisition, Inc. as in effect immediately before the completion of the merger.

As a result of the merger, Primus Common Stock will cease to be authorized for listing on the Nasdaq Small Cap Market, and will become eligible for termination of registration pursuant to Section 12(g)(4) of the Exchange Act.

Voting Agreements: In connection with the Merger Agreement and as an inducement to ATG’s willingness to enter into the Merger Agreement, ATG has entered into individual Voting Agreements with certain stockholders of Primus, including certain officers and directors. The aggregate number of shares of Primus Common Stock covered by the Voting Agreements is 3,471,778.

Under the terms of the Voting Agreements, each Stockholder agreed to vote (or cause to be voted), and granted an irrevocable proxy to ATG to vote, their shares of Primus common stock (i) in favor of the adoption and approval of the Merger Agreement and the merger, and (ii) against any other extraordinary transaction, such as another merger or business combination, liquidation, or other change of control of Primus or any subsidiary of Primus. The Voting Agreements generally provide that the Stockholder may not sell, transfer, assign, pledge or otherwise dispose of their shares of Primus common stock, and that the Stockholder will not take any action that has or could have the effect of preventing the Stockholder from performing

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under the Voting Agreement. The Voting Agreements terminate on the earlier to occur of the closing of the merger and the termination of the Merger Agreement.

References to, and the descriptions of, the Merger Agreement and the Voting Agreements are qualified in their entirety by reference to the complete text of such agreements, which are filed as exhibits to this Schedule 13D and incorporated by reference herein.

Item 5. Interest in Securities of the Issuer.

Item 6. Contracts, Arrangements, Understandings or Relationships With Respect To Securities of the Issuer.

The information set forth in Items 2, 4 and 5 is hereby incorporated by reference. Other than as disclosed in this Schedule 13D or as set forth in the Merger Agreement and Voting Agreements, to ATG’s knowledge, there are no contracts, arrangements, understandings or relationships among the persons named in Item 2 and between such persons and any person with respect to any securities of the issuer, including but not limited to transfer or voting of any of the securities, finder’s fees,

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joint ventures, loan or option arrangements, puts or calls, guarantees of profits, division of profits or loss, or the giving or withholding of proxies.

Item 7. Materials To Be Filed As Exhibits.

The following documents are filed as exhibits:

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SIGNATURE

     After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete, and correct.

Dated: August 19, 2004

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SCHEDULE 1

*All individuals listed in the above table are employed by Art Technology Group, Inc. (“ATG”). The address of ATG’s principal place of business is 25 First St., Cambridge, MA 02141.

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