EXHIBIT 10.1

December 2, 2025

M3-Brigade Acquisition VI Corp.
1700 Broadway, 19th Floor
New York, NY 10019

Re: Appointment of Director

Ladies and Gentlemen:

This letter (this “Letter Agreement”) is being delivered to you in connection with undersigned’s appointment as a director of M3-Brigade Acquisition VI Corp., a Cayman Islands exempted company (the “Company”). Reference is made to the Underwriting Agreement (the “Underwriting Agreement”) entered into by and between the Company and Cantor Fitzgerald & Co., as representative (the “Representative”) of the several underwriters (each, an “Underwriter” and collectively, the “Underwriters”), relating to an underwritten initial public offering (the “Public Offering”), of 34,500,000 of the Company’s units (including up to 4,500,000 units that were purchased to cover over-allotments) (the “Units”), each comprised of one of the Company’s Class A ordinary shares, par value $0.0001 per share (the “Class A Ordinary Shares”), and one-third of one redeemable warrant. Each whole warrant (each, a “Warrant”) entitles the holder thereof to purchase one Class A Ordinary Share at a price of $11.50 per share, subject to adjustment as described in the Prospectus (as defined below). The Units were sold in the Public Offering pursuant to a registration statement on Form S-1 and prospectus (the “Prospectus”) filed by the Company with the U.S. Securities and Exchange Commission (the “Commission”) and the Units are listed on The Nasdaq Global Market. Certain capitalized terms used herein are defined in paragraph 11 hereof.

For good and valuable consideration, the receipt and sufficiency of which are hereby acknowledged, the undersigned (the “Insider”) hereby agrees with the Company as follows:

1. The Insider agrees that if the Company seeks shareholder approval of a proposed Business Combination, then in connection with such proposed Business Combination, he shall (i) vote any Ordinary Shares (as defined below) owned by him in favor of any proposed Business Combination and (ii) not redeem any Ordinary Shares owned by it, him or her in connection with such shareholder approval. If the Company seeks to consummate a proposed Business Combination by engaging in a tender offer, the Insider agrees that he will not sell or tender to the Company any Ordinary Shares owned by him in connection therewith.

2. The Insider hereby agrees that in the event that the Company fails to consummate a Business Combination within 24 months from the closing of the Public Offering, or such later date approved by the Company’s shareholders in accordance with the Company’s amended and restated memorandum and articles of association (as it may be amended from time to time, the “Charter”), the Insider shall take all reasonable steps to cause the Company to, as promptly as reasonably possible but not more than ten (10) business days thereafter, redeem 100% of the Class A Ordinary Shares sold as part of the Units in the Public Offering (the “Offering Shares”), at a per-share price, payable in cash, equal to the aggregate amount then on deposit in the Trust Account (as defined below), including interest earned on the funds held in the Trust Account

(less income taxes payable) and up to $100,000 of interest to pay dissolution expenses), divided by the number of then outstanding Offering Shares, which redemption will constitute full and complete payment for the Offering Shares and completely extinguish all Public Shareholders’ (as defined below) rights as shareholders (including the right to receive further liquidating or other distributions, if any), subject to the Company’s obligations under Cayman Islands law to provide for claims of creditors and in all cases subject to the other requirements of applicable law. The Insider agrees to not propose any amendment to the Charter (A) to modify the substance or timing of the Company’s obligation to allow redemption in connection with a Business Combination or to redeem 100% of the Offering Shares if the Company does not complete a Business Combination within the required time period set forth in the Charter or (B) with respect to any other material provisions relating to shareholders’ rights or pre-initial Business Combination activity, unless the Company provides its Public Shareholders with the opportunity to redeem their Offering Shares upon approval of any such amendment at a per-share price, payable in cash, equal to the aggregate amount then on deposit in the Trust Account, including interest earned on the funds held in the Trust Account and not previously released to the Company to pay its taxes, divided by the number of then outstanding Offering Shares.

The Insider acknowledges that he has no right, title, interest or claim of any kind in or to any monies held in the Trust Account with respect to the Founder Shares held by him. The Insider hereby further waives, with respect to any Ordinary Shares held by him, if any, any redemption rights he may have in connection with (a) the consummation of a Business Combination, including, without limitation, any such rights available in the context of a shareholder vote to approve such Business Combination, or (b) a shareholder vote to approve an amendment to the Charter (A) to modify the substance or timing of the Company’s obligation to allow redemption in connection with a Business Combination or to redeem 100% of the Offering Shares if the Company has not consummated a Business Combination within the time period set forth in the Charter or (B) with respect to any other material provisions relating to shareholders’ rights or pre-initial Business Combination activity or in the context of a tender offer made by the Company to purchase Offering Shares (although the Insider and his respective affiliates shall be entitled to redemption and liquidation rights with respect to any Offering Shares it or they hold if the Company fails to consummate a Business Combination within the time period set forth in the Charter).

3. During the period commencing on the effective date of the Underwriting Agreement and ending 180 days after such date, the Insider shall not, without the prior written consent of the Representative, (i) sell, offer to sell, contract or agree to sell, hypothecate, pledge, grant any option to purchase or otherwise dispose of or agree to dispose of, directly or indirectly, or establish or increase a put equivalent position or liquidate or decrease a call equivalent position within the meaning of Section 16 of the Securities Exchange Act of 1934, as amended (the “Exchange Act”), and the rules and regulations of the Commission promulgated thereunder, with respect to, any Units, Ordinary Shares (including, but not limited to, Founder Shares), Warrants or any securities convertible into, or exercisable, or exchangeable for, Ordinary Shares owned by him, (ii) enter into any swap or other arrangement that transfers to another, in whole or in part, any of the economic consequences of ownership of any Units, Ordinary Shares (including, but not limited to, Founder Shares), Warrants or any securities convertible into, or exercisable, or exchangeable for, Ordinary Shares owned by it, him or her, whether any such transaction is to be settled by delivery of such securities, in cash or otherwise, or (iii) publicly

announce any intention to effect any transaction specified in clause (i) or (ii). The provisions of this paragraph will not apply to any transfer permitted under paragraph 7(c) hereof or if the release or waiver is effected solely to permit a transfer not for consideration and the transferee has agreed in writing to be bound by the same terms described in this Letter Agreement to the extent and for the duration that such terms remain in effect at the time of the transfer.

4. In the event of the liquidation of the Trust Account upon the failure of the Company to consummate its initial Business Combination within the time period set forth in the Charter, M3-Brigade Sponsor VI, LLC (the “Sponsor”) (which for purposes of clarification shall not extend to any other shareholders, members or managing managers of the Sponsor) (the Sponsor in such capacity, the “Indemnitor”) agrees to indemnify and hold harmless the Company against any and all loss, liability, claim, damage and expense whatsoever (including, but not limited to, any and all legal or other expenses reasonably incurred in investigating, preparing or defending against any litigation, whether pending or threatened) to which the Company may become subject as a result of any claim by (i) any third party for services rendered or products sold to the Company (except for the Company’s independent auditors) or (ii) any prospective target business with which the Company has entered into a written letter of intent, confidentiality or other similar agreement or Business Combination agreement (a “Target”); provided, however, that such indemnification of the Company by the Indemnitor (x) shall apply only to the extent necessary to ensure that such claims by a third party or a Target do not reduce the amount of funds in the Trust Account to below the lesser of (i) $10.00 per Offering Share and (ii) the actual amount per Offering Share held in the Trust Account as of the date of the liquidation of the Trust Account, if less than $10.00 per Offering Share is then held in the Trust Account due to reductions in the value of the trust assets, less taxes payable, (y) shall not apply to any claims by a third party or a Target which executed a waiver of any and all rights to the monies held in the Trust Account (whether or not such waiver is enforceable) and (z) shall not apply to any claims under the Company’s indemnity of the Underwriters against certain liabilities, including liabilities under the Securities Act of 1933, as amended. In the event that any such executed waiver by a third party or a Target is deemed to be unenforceable against such third party or Target, the Sponsor shall not be responsible to the extent of any liability for such third-party claims. The Indemnitor shall have the right to defend against any such claim with counsel of its choice reasonably satisfactory to the Company if, within 15 days following written receipt of notice of the claim to the Indemnitor, the Indemnitor notifies the Company in writing that it shall undertake such defense. For the avoidance of doubt, none of the Company’s officers or directors will indemnify the Company for claims by third parties, including, without limitation, claims by vendors and prospective target businesses.

5. [Reserved.]

6. The Insider hereby agrees and acknowledges that: (i) the Underwriters and the Company would be irreparably injured in the event of a breach by the Insider of his obligations under paragraphs 1, 2, 3, 4, 7(a), 7(b), and 9 as applicable, of this Letter Agreement, (ii) monetary damages may not be an adequate remedy for such breach and (iii) the non-breaching party shall be entitled to injunctive relief, in addition to any other remedy that such party may have in law or in equity, in the event of such breach.

(a) The Insider agrees that he shall not Transfer any Founder Shares (or any Class A Ordinary Shares issuable upon conversion thereof) until the earlier of (A) one year after the completion of the Company’s initial Business Combination and (B) subsequent to the Business Combination, (x) if the closing price of the Class A Ordinary Shares equals or exceeds $12.00 per share (as adjusted for share splits, share capitalizations, reorganizations, recapitalizations and the like) for any 20 trading days within any 30-trading day period commencing at least 150 days after the Company’s initial Business Combination or (y) the date on which the Company completes a liquidation, merger, amalgamation, capital stock exchange, reorganization or other similar transaction that results in all of the Company’s Public Shareholders having the right to exchange their Class A Ordinary Shares for cash, securities or other property (the “Founder Shares Lock-up Period”).

(b) The Insider agrees that he shall not Transfer any Private Placement Warrants (or any Class A Ordinary Shares underlying the Private Placement Warrants), until 30 days after the completion of a Business Combination (the “Private Placement Warrants Lock-up Period”, together with the Founder Shares Lock-up Period, the “Lock-up Periods”).

(c) Notwithstanding the provisions set forth in paragraphs 7(a) and 7(b), Transfers of the Founder Shares, Private Placement Warrants and the Class A Ordinary Shares underlying the Private Placement Warrants that are held by the Insider or any of his permitted transferees (that have complied with this paragraph 7(c)), are permitted:

(i) to (1) members of the Sponsor, (2) the directors or officers of the Company, the Sponsor, the members of the Sponsor or the Representative, (3) any affiliates or family members of the directors or officers of the Company, the Sponsor, the members of the Sponsor or the Representative, (4) any members or partners of the Sponsor, the members of the Sponsor, the Representative or their respective affiliates, or any affiliates of the Sponsor, the members of the Sponsor, the Representative, or any employees of such affiliates;

(ii) by gift to a member of such individual’s immediate family or to a trust, the beneficiary of which is a member of such individual’s immediate family, an affiliate of such individual or to a charitable organization;

(iii) by virtue of laws of descent and distribution upon death of such individual;

(iv) pursuant to a qualified domestic relations order;

(v) in the case of a trust by distribution to one or more permissible beneficiaries of such trust;

(vi) by private sales or transfers made in connection with any forward purchase agreement or any financing transaction or similar arrangement, in connection with an extension of the timeframe for the Company to consummate a Business Combination or in connection with the consummation of an initial Business Combination at prices no greater than the price at which the securities were originally purchased;

(vii) to the Company for no value for cancellation in connection with the consummation of its initial Business Combination;

(viii) in the event of the Company’s liquidation prior to the completion of an initial Business Combination;

(ix) by virtue of the laws of the Cayman Islands, the laws of the State of Delaware or the Sponsor’s limited liability company agreement upon dissolution of the Sponsor; or

(x) in the event of the Company’s liquidation, merger, capital stock exchange or other similar transaction which results in all of the Company’s shareholders having the right to exchange their Class A Ordinary Shares for cash, securities or other property subsequent to the Company’s completion of an initial Business Combination;

provided, however, that in the case of clauses (i) through (vii) or (ix), these permitted transferees must enter into a written agreement with the Company agreeing to be bound by the transfer restrictions herein and the other restrictions contained in this Agreement (including provisions relating to voting, the Trust Account and liquidating distributions).

8. The Insider represents and warrants that he has never been suspended or expelled from membership in any securities or commodities exchange or association or had a securities or commodities license or registration denied, suspended or revoked. The Insider’s biographical information furnished to the Company is true and accurate in all respects and does not omit any material information with respect to the Insider’s background. The Insider’s questionnaire furnished to the Company is true and accurate in all respects. The Insider represents and warrants that: he is not subject to or a respondent in any legal action for, any injunction, cease-and-desist order or order or stipulation to desist or refrain from any act or practice relating to the offering of securities in any jurisdiction; he has never been convicted of, or pleaded guilty to, any crime (i) involving fraud, (ii) relating to any financial transaction or handling of funds of another person, or (iii) pertaining to any dealings in any securities and he is not currently a defendant in any such criminal proceeding.

9. Except as disclosed in the Prospectus, neither the Insider nor any affiliate of the Insider, shall receive from the Company any finder’s fee, reimbursement, consulting fee, non-cash payments, monies in respect of any repayment of a loan or other compensation prior to, or in connection with any services rendered in order to effectuate, the consummation of the Company’s initial Business Combination (regardless of the type of transaction that it is), other than the following, none of which will be made from the proceeds held in the Trust Account prior to the completion of the initial Business Combination: repayment of a loan and advances up to an aggregate of $300,000 made to the Company by the Sponsor to cover offering-related and organizational expenses; cash compensation to an affiliate of the Sponsor, for any financial advisory, placement agency or other similar investment banking services that such affiliate may provide to our company, in connection with our initial Business Combination upon the consummation of our initial business combination, and reimbursement to such affiliate for any out-of-pocket expenses incurred by it in connection with the performance of such services; reimbursement for any reasonable out-of-pocket expenses related to identifying, investigating,

negotiating and consummating an initial Business Combination, and repayment of non-interest bearing loans which may be made by the Sponsor or an affiliate of the Sponsor or certain of the Company’s officers and directors to finance transaction costs in connection with an intended initial Business Combination, provided, that, if the Company does not consummate an initial Business Combination, a portion of the working capital held outside the Trust Account may be used by the Company to repay such loaned amounts so long as no proceeds from the Trust Account are used for such repayment. Up to $1,500,000 of such loans may be convertible into warrants of the post-business combination entity at a price of $1.50 per warrant at the option of the lender. Such warrants would be identical to the Private Placement Warrants, including as to exercise price, exercisability and exercise period.

10. The Insider has full right and power, without violating any agreement to which he is bound (including, without limitation, any non-competition or non-solicitation agreement with any employer or former employer), to enter into this Letter Agreement and to serve as a director on the board of directors of the Company and hereby consents to being named in the public filings of the Company as a director of the Company.

11. As used herein, (i) “Business Combination” shall mean a merger, capital stock exchange, asset acquisition, stock purchase, reorganization or similar business combination, involving the Company and one or more businesses; (ii) “Ordinary Shares” shall mean the Class A Ordinary Shares and Class B ordinary shares, par value $0.0001 per share (the “Class B Ordinary Shares”); (iii) “Founder Shares” shall mean the 8,625,000 Class B Ordinary Shares issued and outstanding and any Class A Ordinary Shares issued upon conversion of such Class B Ordinary Shares; (iv) “Initial Shareholders” shall mean the Sponsor and any Insider that holds Founder Shares; (v) “Private Placement Warrants” shall mean the 5,333,333 warrants that the Representative and Sponsor purchased for an aggregate purchase price of $8,000,000, or $1.50 per warrant, in a private placement that occurred simultaneously with the consummation of the Public Offering; (vi) “Public Shareholders” shall mean the holders of Class A Ordinary Shares issued as part of the Units in the Public Offering; (vii) “Trust Account” shall mean the trust fund into which a portion of the net proceeds of the Public Offering and the sale of the Private Placement Warrants were deposited; and (viii) “Transfer” shall mean the (a) sale of, offer to sell, contract or agreement to sell, hypothecate, pledge, grant of any option to purchase or otherwise dispose of or agreement to dispose of, directly or indirectly, or establishment or increase of a put equivalent position or liquidation with respect to or decrease of a call equivalent position within the meaning of Section 16 of the Exchange Act, and the rules and regulations of the Commission promulgated thereunder with respect to, any security, (b) entry into any swap or other arrangement that transfers to another, in whole or in part, any of the economic consequences of ownership of any security, whether any such transaction is to be settled by delivery of such securities, in cash or otherwise, or (c) public announcement of any intention to effect any transaction specified in clause (a) or (b).

12. The Company will maintain an insurance policy or policies providing directors’ and officers’ liability insurance, and each Director shall be covered by such policy or policies, in accordance with its or their terms.

13. This Letter Agreement constitutes the entire agreement and understanding of the parties hereto in respect of the subject matter hereof and supersedes all prior understandings,

agreements, or representations by or among the parties hereto, written or oral, to the extent they relate in any way to the subject matter hereof or the transactions contemplated hereby. This Letter Agreement may not be changed, amended, modified or waived (other than to correct a typographical error) as to any particular provision, except by a written instrument executed by all parties hereto.

14. No party hereto may assign either this Letter Agreement or any of its rights, interests, or obligations hereunder without the prior written consent of the other parties. Any purported assignment in violation of this paragraph shall be void and ineffectual and shall not operate to transfer or assign any interest or title to the purported assignee. This Letter Agreement shall be binding on the Insider and his respective successors, heirs and assigns and permitted transferees.

15. Nothing in this Letter Agreement shall be construed to confer upon, or give to, any person or corporation other than the parties hereto any right, remedy or claim under or by reason of this Letter Agreement or of any covenant, condition, stipulation, promise or agreement hereof. All covenants, conditions, stipulations, promises and agreements contained in this Letter Agreement shall be for the sole and exclusive benefit of the parties hereto and their successors, heirs, personal representatives and assigns and permitted transferees.

16. This Letter Agreement may be executed in any number of original or facsimile counterparts and each of such counterparts shall for all purposes be deemed to be an original, and all such counterparts shall together constitute but one and the same instrument.

17. This Letter Agreement shall be deemed severable, and the invalidity or unenforceability of any term or provision hereof shall not affect the validity or enforceability of this Letter Agreement or of any other term or provision hereof. Furthermore, in lieu of any such invalid or unenforceable term or provision, the parties hereto intend that there shall be added as a part of this Letter Agreement a provision as similar in terms to such invalid or unenforceable provision as may be possible and be valid and enforceable.

18. This Letter Agreement shall be governed by and construed and enforced in accordance with the laws of the State of New York without giving effect to conflicts of law principles that would result in the application of the substantive laws of another jurisdiction. The parties hereto (i) all agree that any action, proceeding, claim or dispute arising out of, or relating in any way to, this Letter Agreement shall be brought and enforced in the courts of New York City, in the State of New York, and irrevocably submit to such jurisdiction and venue, which jurisdiction and venue shall be exclusive and (ii) waive any objection to such exclusive jurisdiction and venue or that such courts represent an inconvenient forum.

19. Any notice, consent or request to be given in connection with any of the terms or provisions of this Letter Agreement shall be in writing and shall be sent by express mail or similar private courier service, by certified mail (return receipt requested), by hand delivery or facsimile transmission.

20. This Letter Agreement shall terminate on the earlier of (i) the expiration of the Lock-up Periods or (ii) the liquidation of the Company; provided, however, that paragraph 4 of this Letter Agreement shall survive such liquidation.

[Signature Page Follows]

Sincerely,

By:	/s/ Frank M. Garrison, Jr.
	Name:	Frank M. Garrison, Jr.

Acknowledged and Agreed:

M3-BRIGADE ACQUISITION VI CORP.

By:	s/ Mohsin Y. Meghji
	Name:	Mohsin Y. Meghji
	Title:	Executive Chairman of the Board of Directors

[Signature Page to Letter Agreement]

M3-Brigade Acquisition VI Corp. 8-K