Okay, whichever one of you is a lawyer, please explain why this doesn’t qualify as acceptance by performance under U.S. contract law.
It is difficult to give a hard doctrinal answer to this question. As a practical matter, the question does not come up frequently, so is not covered by the standard treatises or leading cases. I'd be surprised if there is not some caselaw on the subject, but precisely because it is a rare question, the caselaw will be obscure. This is what I meant when I remarked in my prior post that it is "likely that the doctrinal basis is under-theorized as a matter of law."
However (as
@Steel_Wind would be sure to point out), no court is going to allow
anything to be acceptance by performance. For example, no court is going to enforce the agreement "You agree to give me $1 million. You accept this agreement by breathing" based on the claim that somebody accepted it by breathing. The fact that the doctrinal basis for this limitation is uncertain or under-specified does not mean that the limit does not exist. As
@Steel_Wind has noted, contract law does not attempt to be comprehensive; it attempts to settle only those claims that are actually brought before the court.
(If you doubt this, I would encourage you to post the following offer and agreement on your website: "WOTC, pursuant to this agreement, you agree to permanently transfer to me all intellectual property related to D&D for $1. WOTC may accept this offer by using the word 'fantasy' in any communication.")
Furthermore, as a practical matter, the question is even less relevant than the above implies. Offer and acceptance are questions of
fact that are analyzed by a court to determine if there was the mutual assent necessary to form an agreement. In court, vague and ambiguous facts are analyzed
afterwards to determine whether there was mutual assent by virtue of acceptance of an offer. However, on a prospective basis, it is relatively easy for a person to create facts showing that the person does not accept the offer, even if their activities would otherwise suggest they do accept, e.g. by putting the offeror on notice that the offer is not accepted.
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To repeat myself: There are exceptions to everything, including things I've said above. If you want to know all the complexity and nuance, don't look in a forum post. Moreover, even if the advice correctly applies to your situation, there may be consequences that apply to you that are not explored. The fact that I don't know what exceptions, complexity, nuance and consequences are applicable to your specific situation is one reason (among many) that this is not legal advice. So, I'll say what you hear so many lawyers say. This is not legal advice. I am not your lawyer. You can rely on my legal advice only when we have discussed your specific situation and you have entered into an engagement letter with me or my law firm, and have agreed to pay me or my law firm for the provision of legal advice.