Adicet Bio, Inc.

FOR THE special MEETING OF STOCKHOLDERS

general information

When are this Proxy Statement and the accompanying materials scheduled to be sent to stockholders?

We anticipate on or about November 17, 2025, we will begin mailing this Proxy Statement and the accompanying proxy card or, for shares held in street name (i.e., held for your account by a broker or other nominee), a voting instruction form will be mailed and made available to stockholders on the Internet on or about the same date.

Why did I receive a full set of the proxy materials, instead of a Notice of Internet Availability of Proxy Materials?

We are using the “Full Set Delivery” method of providing proxy materials to stockholders. Because we have elected to utilize the “Full Set Delivery” option, we are delivering to all stockholders of record paper copies of the Proxy Statement and form of proxy card, as well as providing access to those proxy materials on a publicly accessible website. The Proxy Statement, form of proxy card, and the other Special Meeting materials are available on the internet at https://materials.proxyvote.com. Please note that, while our proxy materials are available at https://materials.proxyvote.com and on our website, no other information contained on either website is incorporated by reference in or considered to be a part of this Proxy Statement.

How do I attend and participate in the Special Meeting?

To attend and participate in the Special Meeting, stockholders will need to access the live audio webcast of the meeting. To do so, stockholders of record as of November 14, 2025 will need to visitwww.virtualshareholdermeeting.com/ACET2025SMand use their control number found on the proxy card, and beneficial owners of shares held in street name will need to follow the same instructions.

You will need the 16-digit control number included on your proxy card. Instructions on how to connect to the Special Meeting and participate via the Internet, including how to demonstrate proof of stock ownership, are posted atwww.virtualshareholdermeeting.com/ACET2025SM. If you do not have your 16-digit control number, you will be able to access and listen to the Special Meeting but you will not be able to vote your shares or submit questions during the Special Meeting.

The live audio webcast of the Special Meeting will begin promptly at 8:00 a.m. Eastern Time. We encourage stockholders to login to this website and access the webcast before the Special Meeting’s start time. Online check-in will begin, and stockholders may begin submitting written questions, at 7:45 a.m. Eastern Time, and you should allow ample time for the check-in procedures.

How can I get help if I have trouble checking in or listening to the meeting online?

We will have technicians ready to assist you with any technical difficulties you may have accessing the virtual meeting or submitting questions. If you encounter any difficulties accessing the virtual meeting during the check-in or meeting time, please call the technical support number that will be posted on the Virtual Shareholder Meeting log in page.

Who is soliciting my vote?

Our board of directors is soliciting your vote for the Special Meeting.

When is the record date for the Special Meeting?

The record date for determination of stockholders entitled to vote at the Special Meeting is the close of business on November 14, 2025.

How many votes can be cast by all stockholders?

There were 153,255,581 shares of our common stock, par value $0.0001 per share, outstanding on November 14, 2025, all of which are entitled to vote with respect to all matters to be acted upon at the Special Meeting. Each stockholder of record is entitled to one vote for each share of our common stock held by such stockholder. There are no cumulative voting rights. None of our shares of undesignated preferred stock were outstanding as of November 14, 2025.

Who is entitled to vote?

Holders of record of our common stock at the close of business on November 14, 2025, the record date for the Special Meeting, are entitled to vote at the Special Meeting. If you are a holder of record of our common stock as of the record date, you may vote the shares that you held on the record date even if you sell such shares after the record date. Each outstanding share as of the record date entitles its holder to cast one vote for each matter to be voted upon and, with respect to the election of directors, one vote for each director to be elected.

Registered Stockholders. If shares of our common stock are registered directly in your name with our transfer agent, you are considered the stockholder of record with respect to those shares. As the stockholder of record, you have the right to grant your voting proxy directly to the individuals listed on the proxy card or vote on your own behalf at our virtual Special Meeting. Throughout this Proxy Statement, we refer to these registered stockholders as "stockholders of record."

Street Name Stockholders. If shares of our common stock are held on your behalf in a brokerage account or by a bank or other nominee, you are considered to be the beneficial owner of shares that are held in "street name," and the proxy materials were forwarded to you by your broker or nominee, who is considered the stockholder of record with respect to those shares. As the beneficial owner, you have the right to direct your broker, bank or other nominee as to how to vote your shares. Beneficial owners are also invited to attend our virtual Special Meeting. However, since a beneficial owner is not the stockholder of record, you may not vote your shares of our common stock on your own behalf at the Special Meeting unless you follow your broker's procedures for obtaining a legal proxy. Note you should also be receiving a voting instruction form for you to use from your broker. Throughout this Proxy Statement, we refer to stockholders who hold their shares through a broker, bank or other nominee as "street name stockholders."

How do I vote?

If you are a stockholder of record, there are several ways for you to vote your shares.

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By Internet. You may vote atwww.proxyvote.com, 24 hours a day, seven days a week, by following the instructions at that site for submitting your proxy electronically or scanning the QR code with your smartphone. You will be required to enter the 16-digit control number provided on your proxy card or voting instruction form. Votes submitted through the Internet prior to the commencement of the Special Meeting.

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By Telephone. You may vote using a touch-tone telephone by calling 1-800-690-6903, 24 hours a day, seven days a week. You will be required to enter the 16-digit control number provided on your proxy card or voting instruction form. Votes submitted by telephone prior to the commencement of the Special Meeting.

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By Mail. You may vote by mail by completing, signing, and dating the enclosed proxy card and returning it in the enclosed prepaid envelope. Votes submitted through the mail must be received prior to December 18, 2025.

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During the Special Meeting. If you attend the Special Meeting online, you may vote your shares online while virtually attending the Special Meeting by visitingwww.virtualshareholdermeeting.com/ACET2025SM. You will need your control number provided on your proxy card in order to be able to vote during the Special Meeting.

If you do not wish to vote in person or will not be attending the Special Meeting, you may vote by proxy. You can vote by proxy over the Internet or by mailing your proxy as described in the proxy materials. In order to be counted, proxies submitted by Internet must be received by the cutoff time of 11:59 p.m. Eastern Time on December 18, 2025. Proxies submitted by mail must be received before the start of the Special Meeting. If you complete and submit your proxy before the Special Meeting, the persons named as proxies will vote the shares represented by your proxy in accordance with your instructions. If you submit a proxy without giving voting instructions, your shares will be voted in the manner recommended by our board of directors on all matters presented in this Proxy Statement, and as the persons named as proxies may determine in their discretion with respect to any other matters properly presented at the Special Meeting. You may also authorize another person or persons to act for you as proxy in a writing, signed by you or your authorized representative, specifying the details of those proxies’ authority. The original writing must be given to each of the named proxies, although it may be sent to them by electronic transmission if, from that transmission, it can be determined that the transmission was authorized by you.

Even if you plan to participate in our virtual Special Meeting, we recommend that you also vote by proxy so that your vote will be counted if you later decide not to participate in the Special Meeting. If you submit a proxy via the Internet, by telephone, or by mail, your voting instructions authorize the proxy holders in the same manner as if you signed, dated, and returned your proxy card. If you submit a proxy via the Internet, by telephone, or by mail, you do not need to return your proxy card.

If any other matters are properly presented for consideration at the Special Meeting, including, among other things, consideration of a motion to adjourn the Special Meeting to another time or place (including, without limitation, for the purpose of soliciting additional proxies), the persons named in your proxy and acting thereunder will have discretion to vote on those matters in accordance with their best judgment. We do not currently anticipate that any other matters will be raised at the Special Meeting.

How do I revoke my proxy?

You may revoke your proxy by (1) following the instructions on the proxy materials and entering a new vote by mail that we receive before the start of the Special Meeting or over the Internet by the cutoff time of 11:59 p.m. Eastern Time on December 18, 2025, (2) attending and voting at the Special Meeting (although attendance at the Special Meeting will not in and of itself revoke a proxy), or (3) by filing an instrument in writing revoking the proxy or another duly executed proxy bearing a later date with our Corporate Secretary. Any written notice of revocation or subsequent proxy card must be received by our Corporate Secretary prior to the taking of the vote at the Special Meeting. Such written notice of revocation or subsequent proxy card should be hand delivered to our Corporate Secretary or sent to our principal executive offices at Adicet Bio, Inc., 131 Dartmouth Street, 3 rd Floor, Boston, Massachusetts 02116, Attention: Corporate Secretary.

If a broker, bank, or other nominee holds your shares, you must contact such broker, bank, or nominee in order to find out how to change your vote.

How is a quorum reached?

Our Amended and Restated Bylaws, or bylaws, provide that a majority of the shares entitled to vote, present in person or represented by proxy, will constitute a quorum for the transaction of business at the Special Meeting.

Under the General Corporation Law of the State of Delaware, shares that are voted “abstain” or “withheld” and broker “non-votes” are counted as present for purposes of determining whether a quorum is present at the Special Meeting. If a quorum is not present, the meeting may be adjourned until a quorum is obtained. There were 153,255,581 shares of common stock outstanding and entitled to vote on November 14, 2025, our record date. Therefore, a quorum will be present if 76,627,791 shares of our common stock are present in person or represented by executed proxies timely received by us at the Special Meeting. Shares present virtually during the Special Meeting will be considered shares of common stock represented in person at the meeting.

How is the vote counted?

Under our bylaws, any proposal other than an election of directors is decided by a majority of the votes properly cast for and against such proposal, except where a larger vote is required by law or by our Restated Certificate of Incorporation (Certificate of Incorporation) or Amended and Restated Bylaws. Abstentions and broker “non-votes” do not have an impact on such proposals. Proposal Nos. 1 and 2 require an affirmative vote of a majority of votes properly cast for and against such proposal, and abstention and broker “non-votes,” if any, will have no effect on these proposals.

If you authorize your proxy to vote your shares electronically via the Internet or by telephone, or, if you properly marked, signed, dated and returned the proxy card mailed to you, the shares that the proxy represents will be voted in the manner specified on the proxy. If you properly signed and returned a proxy card but no specification is made, your shares will be voted (i) for the proposal to approve of an amendment to our Certificate of Incorporation to combine outstanding shares of our Common Stock, into a lesser number of outstanding shares and effectuate the Reverse Stock Split and (ii) for the proposal to approve of an adjournment of the Special Meeting, if necessary.

If your shares are held in “street name” by a brokerage firm, your brokerage firm is required to vote your shares according to your instructions. Pursuant to the New York Stock Exchange (NYSE) rules, which are applicable to NYSE-member brokerage firms, if you do not give instructions to your brokerage firm, the brokerage firm will still be able to vote your shares with respect to “discretionary” items, but will not be allowed to vote your shares with respect to “non-discretionary” items. Proposal Nos. 1 and 2 are considered to be a discretionary item under the NYSE rules, and your broker or nominee will be able to vote on that item even if it does not receive instructions from you. A broker or nominee may not vote your shares with respect to these non-discretionary items if you have not provided instructions. This is called a broker “non-vote.” As there are no non-discretionary proposals for the Special Meeting, we do not anticipate receiving any broker “non-votes.” We strongly encourage you to submit your proxy with instructions and exercise your right to vote as a stockholder.

Who pays the cost for soliciting proxies?

We are making this solicitation and will pay the entire cost of preparing and distributing our proxy materials and soliciting votes. If you choose to access the proxy materials or vote over the Internet, you are responsible for any Internet access charges that you may incur. Our officers and employees may, without compensation other than their regular compensation, solicit proxies through further mailings, personal conversations, facsimile transmissions, e-mails, or otherwise. We have hired Broadridge Financial Solutions, Inc. to assist us in the distribution of proxy materials described above. Proxy solicitation expenses that we will pay include those for preparation, mailing, returning, and tabulating the proxies.

How can I know the voting results?

We will publish final results in a Current Report on Form 8-K to be filed with the SEC within four business days following the Special Meeting. If final voting results are not available to us in time to file a Form 8-K within four business days after the Special Meeting, we intend to file a Form 8-K to publish preliminary results and, within four business days after the final results are known to us, file an amended Form 8-K to publish the final results.