As a defense to accusations of patent infringement, accused infringers sometimes file inter partes reviews (IPRs) in the US Patent Office. IPRs allow one to challenge the validity of a patent as anticipated or obvious in view of the prior art. The risk, however, is that if the challenger loses, it will be estopped (i.e., barred) from raising in federal court litigation invalidity arguments that it raised or could have raised in the IPR. In a recent case, the Court of Appeals for the Federal Circuit (the appeals court that hears appeals of all patent cases) raised the bar for a patent owner to show that an accused infringer is estopped from raising certain invalidity arguments.
Ironburg Inventions sued Valve Corporation in federal court for patent infringement. Valve responded by filing an IPR in the Patent Office. In its IPR petition, Valve relied on certain prior art references that it argued should have rendered Ironburg’s patent claims invalid. Valve succeeded in invalidating only some claims of the Ironburg patent, but many claims survived.
Following the IPR decision, the case returned to the federal court, and Valve introduced four newly discovered prior art references as new grounds for invalidating the remaining claims of Ironburg’s patent. In response, Ironburg invoked IPR estoppel, a legal doctrine that, as explained above, blocks IPR petitioners from raising new invalidity grounds in federal court that had been “raised or reasonably could have been raised” during the IPR. Valve then challenged the scope of estoppel, leading to two precedential decisions from the Federal Circuit that redefined how the “reasonably could have been raised”standard should be applied.
The first decision, issued in 2023, clarified that the burden of proving what “reasonably could have been raised” in a past IPR is the responsibility of the patent owner pursuing the IPR estoppel rather than the patent challenger who filed the IPR petition. The Federal Circuit reasoned that the inquiry into what “reasonably could have been raised” is an objective one, focused on demonstrating what prior art an “ordinarily skilled searcher” could have found rather than what the petitioner actually did (or did not) find in its own subjective search.
In June 2026, the Federal Circuit further raised the bar by holding that it is not enough for the patent owner to show that the newly asserted prior art references were “merely accessible” to the petitioner. Instead, patent owners must also demonstrate that these references were practically discoverablethrough a reasonably diligent search performed at the time of the IPR petition. In this case, Ironburg showed that the newly asserted references appeared among the results of a broad classification-based search similar to that employed by Valve. Valve countered that such a search strategy yielded over 26,000 results, which would have been impractical to manually review. Ironburg then showed that targeted keyword searches narrowed the results to reveal the specific references ultimately asserted by Valve, but Valve successfully contended that Ironburg’s keyword selection was tainted by hindsight bias and thus not truly reflective of a pre-petition, blinded search. The Federal Circuit sided with Valve, concluding that “for purposes of the skilled searcher test, the patentee’s burden to show discoverability of invalidity grounds is not automatically satisfied by proving the findability of the prior art reference(s) on which the invalidity ground is based.”
Taken together, these two appellate decisions have rewritten the playbook on IPR estoppel. Patent owners must now do more than show mere availability of prior art by proving that the prior art could have been feasibly discovered without the use of “scorched-earth tactics” or the tempting bias of hindsight. At the same time, these rulings provide greater flexibility for patent challengers, who now retain a more viable path for asserting a critical, needle-in-a-haystack piece of prior art that may have been overlooked earlier in the legal dispute.
One wonders, however, whether, with the advent of AI-assisted searching, it could be easier for a patent owner to argue that it would have been practical—without a scorched-earth search and without the benefit of hindsight—to find the new prior art at the start. That argument will likely be made in future cases. The case covered is Ironburg Inventions Ltd. v. Valve Corporation, decided June 18, 2026. For more information, please contact the authors or your attorneys at McCarter & English.