<PAGE>

                                                                   Exhibit 10.15


               Cambridge Technology Partners (Massachusetts), Inc.
                             1991 Stock Option Plan
                    Executive Non-Qualified Option Agreement

Cambridge Technology Partners (Massachusetts), Inc. (the "Company"), a Delaware
corporation, hereby grants to the person named below an option to purchase all
or any part of the number of shares of Common Stock, $0.01 par value (the
"Common Stock"), of the Company (the "Option") under and subject to the
Company's 1991 Stock Option Plan (the "Plan"), exercisable on the following
terms and conditions and those set forth on the reverse side of this
certificate. This Option is not intended to qualify and shall not be treated as
an "incentive stock option" under Section 422(b) of the Internal Revenue Code of
1986, as amended from time to time (the "Code").

<TABLE>
<CAPTION>

<S>                                                             <C>
Option to Purchase: 572,296 SHARES (THE "OPTION SHARES")        Date of Grant: August 27, 1999

Granted to (the "Optionee"): JACK L. MESSMAN                    Option Price: $14.438

Social Security Number: ###-##-####                             Expiration Date:  AS SET FORTH IN SECTION 5

Vesting Start Date(the "Vesting Start Date"): August 27, 2000

Vesting Schedule: To vest:
</TABLE>


(a)  143,074 shares on August 27, 2000 and

(b)  an additional 12,500 shares each month thereafter (commencing September 27,
     2000) up to and including December 31, 2000, and

(c)  an additional 5,574 shares on January 1, 2001, and

(d)  an additional 12,500 shares each month thereafter (commencing February 27,
     2001) up to and including December 31, 2001, and

(e)  an additional 5,574 shares on January 1, 2002, and

(f)  an additional 12,500 shares each month thereafter (commencing February 27,
     2002) up to and including December 31, 2002, and

(g)  an additional 5,574 shares on January 1, 2003, and

(h)  an additional 12,500 shares each month thereafter (commencing February 27,
     2003) up to and including August 31, 2003.

(i)  In addition, (A) if the closing price of the Company's common stock on the
     Nasdaq Stock Market equals or exceeds $28.876, then 143,074 unvested shares
     (or such lesser amount to the extent that fewer than 143,074 shares remain
     unvested on such date) shall vest immediately and automatically, and (B) if
     the closing price of the Company's common stock on the Nasdaq Stock Market
     equals or exceeds $36.095, then 143,074 unvested shares (or such lesser
     amount to the extent that fewer than 143,074 shares remain unvested on such
     date) shall vest immediately and automatically.

     By acceptance of this Option, the Optionee agrees to the terms and
conditions on the reverse side of this certificate and in the Plan.

                                                James P. O'Hare
                                                Secretary
<PAGE>

               Cambridge Technology Partners (Massachusetts), Inc.

                              Terms and Conditions

1.   Plan Incorporated by Reference. This Option is issued pursuant to the terms
     of the Cambridge Technology Partners (Massachusetts), Inc. 1991 Stock
     Option Plan, as amended (the "Plan"). Capitalized terms used and not
     otherwise defined in this certificate have the meanings given to them in
     the Plan. This certificate does not set forth all of the terms and
     conditions of the Plan, which are incorporated herein by reference. The
     Management Resource Committee of the Board of Directors ("MRC") administers
     the Plan and its determinations regarding the operation of the Plan are
     final and binding. Copies of the Plan may be obtained upon written request
     without charge from AST StockPlan, Inc. 250 Broadway, 14th Floor, New York,
     NY, 10007, or on the Knowledge repository found on the internal Company web
     site address: http://w3.ctp.com/.

2.   Option Price. The price to be paid for each share of Common Stock issued
     upon exercise of the whole or any part of this Option is the Option Price
     set forth on the face of this certificate.

3.   Vesting Schedule. No portion of this Option may be exercised until the date
     on which such portion shall have vested. Except as set forth herein, and
     subject to the determination of the Company in its sole discretion to
     accelerate the vesting schedule hereunder due to other circumstances and
     subject to a reduction in the percentage of Option Shares vesting each
     month in the event that the Optionee becomes employed on less than a
     full-time basis (such new percentage shall be determined by the Company at
     the time the Optionee becomes employed on less than a full time basis and
     shall be set forth in a replacement option agreement to be executed at that
     time), this Option shall be vested and exercisable with respect to the
     percentage of the total number of Option Shares as listed on the vesting
     and exercise schedule attached to this certificate.

4.   Exercise of Option.

     (a)  Optionee may exercise only vested portions of this Option and only in
          the following manner. From time to time prior to the earlier to occur
          of (i) the termination hereof in accordance with the provisions of
          this Option, or (ii) the Expiration Date (as set forth in Paragraph 5
          herein) with respect to a given portion of this Option, the Optionee
          may give written notice to the Company of his or her election to
          purchase some or all of the Option Shares for which this Option may be
          exercised at the time of such notice. Said notice shall specify the
          number of Option Shares to be purchased and shall be accompanied (i)
          by payment therefor in cash and (ii) by such agreement, statement or
          other evidence as the Company may require in order to satisfy itself
          that the issuance of the Option Shares being purchased pursuant to
          such exercise and any subsequent resale thereof will be in compliance
          with applicable laws and regulations, including without limitation all
          applicable federal and state securities laws and regulations. This
          Option shall not be exercisable for any fractional share.

     (b)  Certificates for the Option Shares so purchased will be issued to the
          Optionee upon compliance to the satisfaction of the Company with all
          requirements under applicable laws or regulations in connection with
          such issuance. Until the Optionee shall have complied with the
          requirements hereof and of the Plan, the Company shall be under no
          obligation to issue the Option Shares subject to this Option, and the
          determination of the MRC (as defined in the Plan) as to such
          compliance shall be final and binding on the Optionee. The Optionee
          shall not be deemed for any purpose to be the owner of any Option
          Shares subject to this Option until such Option Shares shall have been
          issued in accordance with the foregoing provisions.

     (c)  Notwithstanding any other provision hereof or of the Plan, no portion
          of this Option shall be exercisable (i) after its termination in
          accordance with the provisions hereof, (ii) after the Expiration Date
          applicable thereto (as set forth in Paragraph 5 herein), or (iii) at
          any time unless all necessary regulatory or other approvals have been
          received.

     (d)  To the extent that this Option is not exercised in full, it will be
          deemed to have been exercised first for any remaining Option Shares in
          the Installment (as defined in Paragraph 5 herein) which would
          otherwise expire on the next succeeding Expiration Date, then for any
          remaining Option Shares in the Installment which would otherwise
          expire on the second succeeding Expiration Date and so on, thereby
          reducing the number of Option Shares with respect to which this Option
          will expire on such Expiration Dates.

5.   Expiration Date of Option and Underlying Option Shares. This Option will
     expire and terminate in installments (each, an "Installment") on the
     following dates (each, an "Expiration Date" ): (a) August 27, 2005 with
     respect to the portion of this Option which vests during the period
     beginning on the Vesting Start Date and ending August 26, 2001; (b) August
     27, 2006 with respect to the portion of this Option which vests during the
     period beginning August 27, 2001 and ending August 26, 2002; (c) August 27,
     2007 with respect to the portion of this Option which vests during the
     period beginning August 27, 2002 and ending August 26, 2003; (d) August 27,
     2008 with respect to the portion of this Option which vests on or after
     August 27, 2003; provided, however, that if the vesting of any portion of
     this Option is accelerated because the closing price of the Company's
     common stock on the Nasdaq Stock Market equals or exceeds $28.876, then
     that portion of this Option shall not expire until August 27, 2007, and if
     the vesting of any portion of this Option is accelerated because the
     closing price of the Company's common stock on the Nasdaq Stock Market
     equals or exceeds $36.095, then that portion of this Option shall not
     expire until August 27, 2008.

6.   Termination of Employment. Except as set forth in Section 8(b), below, this
     Option, as to any unexercised portion hereof, shall terminate on the date
     three (3) months after the date on which the Optionee is no longer employed
     by the Company or a subsidiary as defined in the Code (and, except as set
     forth in clauses 6(a) and 6(b) and section 8 below, this Option shall not
     vest with respect to any additional Option Shares following the date on
     which the Optionee is no longer employed by the Company or a subsidiary as
     defined in the Code); provided, however, that (a) if such termination of
     employment results from the Optionee's permanent and total disability as
     defined in Section 22(e)(3) of the Code, this Option may be exercised,
     whether or not exercisable at the time of such termination, until the date
     twelve (12) months after such termination, or until the applicable
     Expiration Date with respect to any particular portion of this Option (as
     set forth in Paragraph 5 herein), whichever first occurs, and (b) if such
     termination of employment results from the Optionee's death, this Option
     may be exercised, whether or not exercisable at the time of such
     termination, by the Optionee's executors or administrators within
     twenty-four (24) months thereafter, or until the applicable Expiration Date
     with respect to any particular portion of this Option (as set forth in
     Paragraph 5 herein), whichever first occurs. No Option will confer upon the
     Optionee any right to continued employment by the Company or any subsidiary
     of the Company, nor will it interfere in any way with the Optionee's right
     or the Company's or any such subsidiary's right to terminate, or otherwise
     modify the terms of, the Optionee's employment at any time.

7.   Transferability. Except as otherwise permitted by the Plan, each of this
     certificate and this Option is personal to the Optionee, is non-assignable
     and is not transferable in any manner, by operation of law or otherwise,
     other than by will or by the laws of descent and distribution, and is
     exercisable, during the Optionee's lifetime, only by the Optionee.

8.   Effect of Certain Transactions.

     (a)  If (i) the Company is to be merged into another entity, or if one or
          more entities is to be merged into the Company or if there is to be a
          consolidation of the Company and one or more entities and, in any such
          case, the shares of Common Stock are to be converted into cash,
          securities or other property other than shares of Common Stock (an
          "Acquisition"), or (ii) if the Company is to be liquidated, or is to
          sell or otherwise dispose of substantially all of its assets to
          another entity (a "Sale"), or (iii) a person, entity or group acquires
          beneficial ownership of 50% or more of either the then outstanding
          shares of the Company's common stock or the combined voting power of
          the Company's then outstanding voting securities entitled to vote
          generally in the election of directors, or (iv) 50% or more of the
          directors of the Company are persons who are not Continuing Directors
          (defined below), or (v) the Company materially changes Optionee's
          authority or the conditions of Optionee's employment (including the
          election of
<PAGE>

          another President or Chief Executive Officer), then: (x) the time for
          exercise of any unexercised and unexpired portion of this Option,
          including the then unvested portion of this Option, shall be
          accelerated to immediately prior to the consummation of such event,
          and (y) solely in the event of an Acquisition or Sale, this Option
          shall terminate within 90 days after the effective date of such
          Acquisition or Sale; provided, however, that the foregoing clauses (x)
          and (y) shall not apply to any transaction in which the former
          stockholders of the Company immediately after such transaction hold or
          receive by reason of their prior ownership of shares of capital stock
          of the Company shares of capital stock of the resulting or surviving
          corporation constituting a majority of the voting power of all
          outstanding stock of such resulting or successor corporation. For
          purposes of this Section, a "Continuing Director" shall mean (i) any
          member of the Board of Directors of the Company as of the date of this
          Agreement or (ii) any person who subsequently becomes a member of the
          Board, if such person's nomination or election to the Board is
          recommended or approved by a majority of the Continuing Directors.

     (b)  If Optionee voluntarily retires from his employment with the Company
          or the Company terminates Optionee's employment without "cause" (as
          defined in Section 6 of the Employment Agreement between Optionee and
          the Company), then any unvested portion of this Option shall
          immediately become vested and fully exercisable on the effective date
          of the retirement, resignation or termination, and any unexpired
          portion of this Option shall expire on the fifth anniversary of such
          accelerated vesting date.

9.   Tax Withholding. The Optionee shall, not later than the date as of which
     the exercise of this Option or disposition of Option Shares becomes a
     taxable event for Federal income tax purposes, pay to the Company or make
     arrangements satisfactory to the MRC for payment of any Federal, state, and
     local taxes required by law to be withheld.

10.  Representations. By acceptance of this Option, the Optionee agrees,
     acknowledges and understands that a purchase of shares under this Option
     will not be made with a view to their distribution, as that term is used in
     the Securities Act of 1933 (the "Act") unless such distribution is in
     compliance with or exempt from the registration and prospectus requirements
     of the Act, and the Optionee agrees to sign a certificate to such effect at
     the time of exercising this option and agrees that the certificate for the
     shares so purchased may be inscribed with a legend to ensure compliance
     with the Act.