Additional Proxy Soliciting Materials (Form DEFA14A)

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 14A

(Rule 14a-101)

INFORMATION REQUIRED IN PROXY STATEMENT

SCHEDULE 14A INFORMATION

Proxy Statement Pursuant to Section 14(a) of the

Securities Exchange Act of 1934

Filed by the Registrant ☒

Filed by a Party other than the Registrant ☐

Check the appropriate box:

Agenus Inc.

(Name of Registrant as Specified In Its Charter)

(Name of Person(s) Filing Proxy Statement, if other than the Registrant)

Payment of Filing Fee (Check the appropriate box):

These materials are important and require your immediate attention. They require the stockholders Agenus Inc. to make important decisions. If you are in doubt as to how to make such decisions, please contact your financial, legal or other professional advisor. If you have any questions, you may contact Investor Relations at Agenus Inc., 3 Forbes Road, Lexington, Massachusetts 02421, or telephone or e-mail Investor Relations at 781-674-4400 or [email protected].

SUPPLEMENT DATED MAY 13, 2026 TO

PROXY STATEMENT DATED APRIL 30, 2026

This document is a supplement dated May 13, 2026 (the "Supplement") to the proxy statement dated April 30, 2026 and first mailed to stockholders of Agenus Inc. (the "Company" or "Agenus") on or about April 30, 2026 (the "Proxy Statement") in connection with an annual and special meeting (as the same may be adjourned or postponed, the "Special Meeting") of common stockholders of the Company to be held on June 16, 2026.

INTRODUCTION AND EXPLANATORY NOTE

The sole purpose of this Supplement is to supplement the Proxy Statement with corrective information concerning Proposal 4 contained in the Proxy Statement and the number of options which are eligible to be exchanged as set forth therein. The Proxy Statement inadvertently indicates that there are 8,697,165 options outstanding which are eligible to be exchanged; however, there are in fact 2,103,891 options outstanding which are eligible to be exchanged. Accordingly, this Supplement (i) amends certain information on page 45 of the Proxy Statement under the heading "Dilution, Burn Rate, and Overhang" and (ii) amends and restates in its entirety the disclosure set forth beginning on page 50 of the Proxy Statement under the heading "PROPOSAL 4 - TO APPROVE THE ONE-TIME OPTION EXCHANGE (WITH NON-EMPLOYEE DIRECTOR AND EXECUTIVE PREMIUM STRIKE)," in each case as set forth herein.

Except as described in this Supplement, the information provided in the Proxy Statement relating to Proposal 4 and the other proposals set forth therein continues to apply, and the information provided in the Proxy Statement is not amended, supplemented, or otherwise modified. This Supplement and the documents referred to in this Supplement should be read in conjunction with the Proxy Statement, the annexes and exhibits to the Proxy Statement, and the documents referred to in the Proxy Statement, each of which should be read in its entirety. To the extent that information in this Supplement differs from, updates, or conflicts with information contained in the Proxy Statement, the information in this Supplement supersedes the information in the Proxy Statement.

Proxies already received will continue to be voted as instructed unless otherwise revoked or changed by a subsequent proxy in the manner described on page 6 of the Proxy Statement under the heading "VOTING PROCEDURES-How can I change my vote?" If you have already submitted a proxy to vote your shares and you do not wish to change your vote, you do not need to resubmit a new proxy.

Capitalized terms used herein, but not otherwise defined, shall have the meanings ascribed to such terms in the Proxy Statement.

CORRECTION TO THE PROXY STATEMENT

Dilution, Burn Rate, and Overhang

The disclosure on page 45 of the Proxy Statement under the heading "Dilution, Burn Rate, and Overhang" is amended as follows.

Each reference to the number "8,697,165" is hereby deleted and replaced with the number "2,103,891."

PROPOSAL 4 - TO APPROVE THE ONE-TIME OPTION EXCHANGE (WITH NON-EMPLOYEE DIRECTOR AND EXECUTIVE PREMIUM STRIKE)

The following disclosure amends and restates the disclosure set forth beginning on page 50 of the Proxy Statement under the heading "PROPOSAL 4 - TO APPROVE THE ONE-TIME OPTION EXCHANGE (WITH NON-EMPLOYEE DIRECTOR AND EXECUTIVE PREMIUM STRIKE)."

Summary

We seek stockholder approval for a one-time exchange of Eligible Options (as defined below) held by employees, consultants, and non-employee directors, under our 2019 EIP, our Amended and Restated 2009 Equity Incentive Plan, and our 2015 Inducement Equity Plan, for new stock options covering the same number of shares - with replacement options held by our non-employee directors and executive officers priced at a 50% premium to fair market value on the Exchange Date. The Board recommends that stockholders approve Proposal 4.

Principal Considerations

How the Exchange Will Work

Eligible Holders

All employees (including executive officers), consultants, and non-employee directors of Agenus who hold Eligible Options on the Exchange Date will automatically participate in the Option Exchange, with no action required on their part. A participant must remain in continuous service with us through the Exchange Date to receive replacement options. Any individual whose service terminates before the Exchange Date - whether by voluntary resignation, retirement, involuntary termination, layoff, death, or disability - retains their Eligible Options under their existing terms and will not be eligible to receive replacement options.

Eligible Options

An "Eligible Option" is any outstanding stock option that (i) was granted on or before September 30, 2024; (ii) was granted under our 2019 EIP, our 2009 Plan, or the 2015 Inducement Equity Plan; (iii) has an exercise price of $2.50 or greater and greater than the closing price of our common stock on the Exchange Date; and (iv) vests based on continued service. As of April 27, 2026, Eligible Holders held outstanding options to purchase 2,103,891 shares of common stock, with a weighted-average exercise price of $46.27, of which 100.0% had exercise prices exceeding the closing price of our common stock on that date. The final composition of Eligible Options will be determined as of the Exchange Date. The weighted-average remaining life of the Eligible Options is 5.75 years.

One-for-One Share Exchange

The Option Exchange is a one-for-one exchange on a share-by-share basis. Each Eligible Option will be canceled and replaced with a new option covering the same number of shares, with a new exercise price determined under the tiered pricing structure described below and a new ten-year term. The aggregate number of outstanding options will not increase as a result of the Option Exchange.

Tiered Pricing Structure - the Non-employee Director and Executive Premium

The exercise price of each replacement option will be determined by the tier to which the holder belongs, as follows:

Vesting and Term

Replacement options granted in exchange for currently vested and exercisable options will be vested and exercisable on the grant date. Replacement options granted in exchange for unvested time-based options will vest on the same remaining schedule as the Eligible Options they replace, subject to continued service. All replacement options - Tier 1 and Tier 2 - will have a ten-year term from the Exchange Date.

Exchange Ratio; Aggregate Share Count Unchanged

The Option Exchange is a one-for-one exchange on a share-by-share basis. The aggregate number of shares subject to outstanding options will not increase. The Option Exchange does not increase the aggregate number of shares that may be granted under the 2019 EIP, the 2009 Plan, or the 2015 Inducement Equity Plan. The share pool increase in Proposal 3, if approved, is entirely separate from and independent of the Option Exchange.

Process

We anticipate that the Option Exchange will occur on the date this Proposal 4 is approved by stockholders (the "Exchange Date"). Subject to applicable requirements, we intend to provide Eligible Holders with a communication explaining the structure of the Exchange and their rights and obligations. Because the Exchange is a one-for-one, automatic exchange and not a voluntary tender offer, individual elections are not required.

Why the Compensation Committee Selected This Structure

The Compensation Committee evaluated several alternatives for responding to the current underwater option position, including: (i) issuing new equity awards in addition to the existing underwater options (increasing dilution); (ii) exchanging underwater options for full-value awards such as restricted stock units (changing the nature of the incentive and typically requiring a significant discount exchange ratio); and (iii) increasing cash compensation. The Committee concluded that a one-for-one option exchange, with a 50% strike premium for non-employee directors and executive officers, was the structure most consistent with the interests of stockholders because it requires no incremental cash outlay, restores retention and motivation value across the broader team, and - through the 50% non-employee director and executive premium - requires approximately 50% stock price appreciation from the Exchange Date before non-employee directors and executives realize economic value from their replacement options.

Expected Participation

Eligible Options

As of April 27, 2026, Eligible Holders held outstanding options to purchase 2,103,891 shares of common stock with a weighted average exercise price of $46.27 per share, of which 100.0% were underwater, meaning the exercise price of the option exceeded $4.09 (the closing price of a share of our common stock on April 27, 2026). Of the stock options held by Eligible Holders, all of such outstanding options had an exercise price equal to or greater than $2.50 per share and otherwise met the criteria to be considered Eligible Options for purposes of the Option Exchange (assuming that the exercise prices are greater than the closing price of our common stock on the date of the Option Exchange). The weighted average remaining life of the Eligible Options is 5.75.

The following chart illustrates the breakdown of Eligible Options by their exercise price, as of April 27, 2026.

The following chart illustrates the breakdown of Eligible Options by the year in which they were granted, as of April 27, 2026.

The following table sets forth the number of Eligible Options expected to be held by our NEOs, other executive officers as a group, non-employee directors, employees, and consultants as of April 27, 2026. Exact numbers will be determined as of the Exchange Date. Dr. Armen and Dr. O'Day, as executive officers, will be subject to the Tier 1 premium strike.

No employee holds greater than 5% of the 2,103,891 total Eligible Options.

Accounting Impact

The incremental compensation cost associated with the Option Exchange will be measured as the excess, if any, of the fair value of each award of new stock option granted to participants in the Option Exchange, measured as of the date the new stock options are granted, over the fair value of the stock options surrendered in exchange for the new stock options, measured immediately prior to the cancellation. This incremental compensation cost will be recognized ratably over the vesting period of the new stock options, if any.

Material U.S. Federal Income Tax Consequences of the Option Exchange

The cancellation and grant of stock options pursuant to the Option Exchange should be treated as a non-taxable exchange because the new stock options will have an exercise price equal to or greater than the fair market value of our common stock on the grant date. Neither the Company, nor participants in the Option Exchange, should recognize any income for U.S. federal income tax purposes upon the grant of the new stock options. To the extent permissible and available for grant under our 2019 EIP, new stock options granted under the Option Exchange will be incentive stock options for U.S. federal income tax purposes to the maximum extent permitted under the Internal Revenue Code. Tax effects may vary in other countries; a more detailed summary of tax considerations will be provided to all participants in the Option Exchange documents.

Financial Statements

Our consolidated financial statements and other information required by Item 13(a) of Schedule 14A under the Exchange Act are incorporated by reference from our Annual Report on Form 10-K for the fiscal year ended December 31, 2025, filed with the SEC on March 16, 2026.

Interests of Certain Persons

Our executive officers and non-employee directors are eligible to participate in the Option Exchange to the extent they hold Eligible Options and continue in service through the Exchange Date, and therefore may be considered to have an interest in the approval of this Proposal 4. Notwithstanding this interest, the Board has concluded - and the Compensation Committee has recommended - that the Option Exchange is in the best interests of stockholders because: (i) the Tier 1 non-employee director and executive premium strike ensures that non-employee directors and executives benefit only after stockholders first benefit from at least approximately 50% appreciation; (ii) the exchange is one-for-one, preserving the aggregate option count; and (iii) the Option Exchange will restore retention and motivation value across the entire team that is now executing our core strategic plan.

Vote Required

To approve Proposal 4, a majority of the votes cast by stockholders present in person or by proxy and voting on the matter must vote FOR Proposal 4. If your shares are held by your broker, bank, or nominee in "street name," and you do not vote your shares, your broker, bank, or nominee has authority to vote your unvoted shares on Proposal 4. If the broker, bank, or nominee does not vote your unvoted shares, there will be no effect on the vote because these "broker non-votes" are not considered to be voting on the matter. Abstentions and "broker non-votes" will not be counted as votes cast or shares voting on Proposal 4, and will have no effect on the vote.

The Board of Directors recommends a vote FOR Proposal 4.