The long-running fight between Ironburg Inventions Ltd. and Valve Corporation over a back-button game controller has, for the second time, turned not on infringement but on a procedural bar. On June 18, 2026, a Federal Circuit panel reversed a district court order that had estopped Valve from raising two invalidity grounds at trial, finding the evidence underlying the estoppel either too thin or tainted by hindsight. The panel remanded for the district court to consider the invalidity of U.S. Patent No. 8,641,525 in light of the prior art Valve had been barred from asserting.

The procedural posture is the heart of the story. Under 35 U.S.C. § 315(e)(2), a party that pursues an inter partes review to a final written decision cannot later assert in court any invalidity ground it "raised or reasonably could have raised" during the IPR. Valve filed an IPR petition against the ’525 patent in 2016. After a third party, Collective Minds Gaming, later filed its own IPR raising two new obviousness grounds — one based on a published application referred to as Kotkin, the other on a combination labeled Willner-Koji-Raymond — Valve amended its district court invalidity contentions to add those same grounds. Ironburg moved to estop Valve from using them.

This was the case’s second trip to the Federal Circuit. In Ironburg I (2023), the court held that the burden to prove what a skilled searcher reasonably could have discovered rests with the party asserting estoppel — here, Ironburg — and that the district court had wrongly relied on the fact that Collective Minds found the references, because nothing in the record showed whether that third party’s search was reasonably diligent rather than "scorched earth." The court vacated and remanded for Ironburg to try to meet its burden.

The Kotkin ground: classification searches and an unreviewable haystack

On remand, the district court permitted limited additional discovery and again granted estoppel. For Kotkin and a Japanese reference, Koji, it relied on Valve’s own pre-petition searches by Landon IP, reasoning that the classification codes under which those references were indexed had been searched. The Federal Circuit held that reasoning insufficient. The panel noted that searching the relevant classifications in the Japanese-language database returned 26,333 references, and that the district court treated mere presence in a searched classification as the end of the inquiry — without any narrowing step such as keyword or citation searching to reduce the set to a manageable size.

We hold only that something more is required for estoppel when a classification search alone returns an unreviewable number of search results—a demand not met here.Ironburg Inventions Ltd. v. Valve Corp., No. 2024-2088 (Fed. Cir. June 18, 2026)

The panel was careful about the boundaries of its holding. It declined to adopt Valve’s broader view of the discoverability standard, and it did not hold that manual review of search results is always required to establish discovery. It also declined to resolve whether "discovery" requires that a searcher read and understand a reference, or whether the skilled-searcher inquiry is a question of law or fact — concluding that the district court’s error was apparent under either standard. Because the record on remand had not been developed further on the Kotkin point, the panel found that nothing warranted additional fact-finding and reversed the estoppel of that ground.

The Willner-Koji-Raymond ground: hindsight bias

The second ground turned on evidence Ironburg developed on remand from a search firm, Cardinal IP. The court held that this evidence failed to establish discoverability because it was infected by hindsight. The district court had struck one forward-and-backward citation search for pulling in references that postdated Valve’s 2016 petition, but the panel found that an earlier search in the same set used the same citation technique without date restrictions and shaped the searcher’s subsequent keywords and classification choices. A later declaration applying date filters to only the one struck search, submitted five months afterward, did not cure the problem.

The panel also faulted a supplemental search that located the Raymond reference, pointing to deposition testimony that it was undertaken "because there was another reference that had not yet been found" — which the court treated as evidence of hindsight. Because Section 315(e)(2) speaks in terms of invalidity "grounds," not individual references, the panel explained that a finding that one reference in the Willner-Koji-Raymond combination was not discoverable meant the entire ground could not have been reasonably raised, so estoppel did not attach to it.

A concurrence on what a searcher must show

Judge Stark, who joined the opinion in full, wrote separately to map the developing skilled-searcher framework. He distinguished two steps a patentee must prove to estop a challenger: first, that the prior art references were findable by a skilled searcher conducting a diligent search; and second, that the searcher reasonably would have been expected to discover the invalidity ground based on those references. He framed Kotkin as a reference that was merely "accessible" — reachable through a classification search — but not "findable," because the search returned tens of thousands of other references. He contrasted it with Willner, which appeared on the face of the challenged patent and was not disputed on appeal.

What the court decided

The panel — Judges Chen, Hughes, and Stark, with Judge Hughes writing — reversed the district court’s judgment and remanded. The estoppel of the Kotkin ground rested on what the court called "undeveloped, overgeneralized evidence," and the estoppel of the Willner-Koji-Raymond ground failed to adequately account for hindsight. On remand, the district court is to consider the invalidity of the ’525 patent in light of both grounds. The court assessed costs to Valve.

The decision leaves several questions in the IPR-estoppel area expressly open, including whether the skilled-searcher inquiry is reviewed de novo or for clear error and what "discovery" of a reference entails. What the panel held is narrower: where the only evidence of findability is a classification search returning an unreviewable number of results, that alone does not carry the estoppel burden, and search evidence shaped by post-petition information is not probative of what a diligent searcher would have found at the relevant time. The asserted patent, U.S. Patent No. 8,641,525, returns to the district court with its previously barred invalidity defenses back in play.