Prosecution history estoppel prevents a patentee from using the doctrine of equivalents to recapture coverage it gave up during examination, whether by narrowing a claim through amendment to overcome a rejection or by making arguments to the examiner that clearly disclaim certain subject matter.

Under Festo v. Shoketsu Kinzoku, a narrowing amendment made for a reason related to patentability creates a presumption that estoppel applies to the surrendered territory. The patentee can rebut that presumption by showing the equivalent was unforeseeable at the time, the rationale underlying the amendment bore only a tangential relation to the equivalent, or some other reason shows the patentee could not reasonably have described the equivalent.

In a simulation, prosecution history estoppel operates as a constraint that tightens the doctrine-of-equivalents dial for particular claim limitations, based on the specific amendments and arguments made in the file wrapper for that patent.

Patent litigators review the full prosecution history early to map which claim limitations carry an estoppel risk, since it can eliminate an equivalents theory before the case even reaches a jury. Prosecuting attorneys, conversely, try to avoid unnecessary narrowing amendments and to preserve broader arguments where possible to limit future estoppel exposure.