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if the amendment's purpose were unrelated to patentability, the court might consider whether it was the kind of reason that nonetheless might require estoppel. Id., at 40-41. Simply because estoppel has been discussed most often in the context of amendments made to avoid the prior art, see, e. g., id., at 30, it does not follow that amendments made for other purposes will not give rise to estoppel. Section 112 requires that the application describe, enable, and set forth the best mode of carrying out the invention. The patent should not issue if these requirements are not satisfied, and an applicant's failure to meet them could lead to the issued patent being held invalid in later litigation. Festo's argument that amendments made to comply with § 112 concern the application's form and not the invention's subject matter conflates the patent-ee's reason for making the amendment with the impact the amendment has on the subject matter. Estoppel arises when an amendment is made to secure the patent and the amendment narrows the patent's scope. If a § 112 amendment is truly cosmetic, it would not narrow the patent's scope or raise an estoppel. But if a § 112 amendment is necessary and narrows the patent's scope—even if only for better description—estoppel may apply. Pp. 735-737.
(d) Prosecution history estoppel does not bar the inventor from asserting infringement against every equivalent to the narrowed element. Though estoppel can bar challenges to a wide range of equivalents, its reach requires an examination of the subject matter surrendered by the narrowing amendment. The Federal Circuit's complete bar rule is inconsistent with the purpose of applying the estoppel in the first place—to hold the inventor to the representations made during the application process and the inferences that may be reasonably drawn from the amendment. By amending the application, the inventor is deemed to concede that the patent does not extend as far as the original claim, not that the amended claim is so perfect in its description that no one could devise an equivalent. The Court's view is consistent with precedent and PTO practice. The Court has consistently applied the doctrine in a flexible way, considering what equivalents were surrendered during a patent's prosecution, rather than imposing a complete bar that resorts to the very literalism the equivalents rule is designed to overcome. E. g., Goodyear Dental, supra, at 230. The Federal Circuit ignored Warner-Jenkinson's instruction that courts must be cautious before adopting changes that disrupt the settled expectations of the inventing community. See 520 U. S., at 28. Inventors who amended their claims under the previous case law had no reason to believe they were conceding all equivalents. Had they known, they might have appealed the rejection instead. Warner-Jenkinson struck the appropriate balance by placing the burden on the patentee to prove that an
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