The Inventorship Squeeze: Invalidity, Unenforceability, and Why Prosecution Just Got More Expensive

In the space of a few months, the U.S. Court of Appeals for the Federal Circuit and the U.S. District Court for the District of Massachusetts delivered two decisions that, read together, change how patent applicants and their counsel should approach the deceptively simple question of who invented what. Fortress Iron, LP v. Digger Specialties, Inc., No. 2024-2313 (Fed. Cir. Apr. 2, 2026), holds that if an inventorship error cannot be corrected under 35 U.S.C. § 256, the patent is invalid — full stop, no intent required. Inline Plastics Corp. v. Lacerta Group, Inc., No. 1:18-cv-11631 (D. Mass. Nov. 13, 2025), holds that if an inventorship omission was intentional, the entire patent family is unenforceable for inequitable conduct — the Therasense penalty at full weight.

Neither result is entirely new. What is new is the two-track vulnerability the cases now define. Because Fortress Iron’s invalidity holding operates independently of intent while Inline Plastics’s unenforceability holding operates on intent, they cover different fact patterns without overlapping — and applicants must now defend against both. This article explains the two paths, the practical differences that matter, and what practitioners might do differently.

The Invalidity Path: Fortress Iron

Fortress Iron involved railing patents and the mid-litigation discovery that two Chinese-vendor employees, Lin and Huang, were true coinventors. Lin was added under Section 256(a); Huang could not be located, so the district court denied Section 256(b) correction and the Federal Circuit affirmed. The court read Section 256(b)’s savings clause by necessary implication: if inventorship can be corrected, the patent is not invalidated; if it cannot, it is invalid. Section 100(f) defines “inventor” as the individuals collectively — all true inventors, not most — and, extending Chou v. University of Chicago, an omitted coinventor is a “party concerned” entitled to notice and hearing before correction. As Judge Lourie framed it, inventors “are where it all begins, even if they eventually assign their interests to others, such as employers.” Inventor identification is not paperwork; it is constitutional.

The takeaway: intent is irrelevant on this path. A patent can be invalidated by nothing more than the procedural failure to give an omitted inventor notice and hearing under Section 256(b). Good faith at prosecution is no defense; if a true inventor exists whom the applicant cannot later locate, the Fortress Iron backstop is triggered.

The Unenforceability Path: Inline Plastics

Inline Plastics sued Lacerta Group in 2018 over five patents on tamper-resistant thermoformed food containers. After a Federal Circuit remand, a November 2025 bench trial found by clear and convincing evidence that Inline had intentionally omitted two 4Sight consultants — Stuart Leslie and Richard Curtiss — as joint inventors. Inventorship is a “critical requirement for obtaining a patent” and therefore material (PerSeptive Biosystems, Inc. v. Pharmacia & Upjohn, Inc.), and intent to deceive can be inferred from circumstantial evidence when it is the “single most reasonable inference” under Therasense. The consequence is unenforceability of the entire family, not just the asserted patents.

The finding rested on inconsistent, uncorroborated testimony from the named inventors set against contemporaneous October 2002 4Sight drawings depicting the tear-strip feature. Under the rule of reason, those third-party documents outweighed the inventors’ post-hoc conception claims (Blue Gentian, LLC v. Tristar Products, Inc.; Plastipak Packaging, Inc. v. Premium Waters, Inc.), and the court concluded that deceptive intent was the single most reasonable inference. As Stark v. Advanced Magnetics, Inc. confirms, even where Section 256 would permit correction, a patent remains vulnerable to unenforceability when named inventors intentionally omit a co-inventor. Intent is everything on this path — but the threshold is reachable by circumstantial pattern.

Why the Two Paths Together Matter

The classical response to inventorship anxiety has been the Section 256 safety net: get it wrong, fix it later. Fortress Iron narrows that net dramatically — correction requires notice and hearing to all parties concerned, and if a party concerned cannot be found, correction fails and the patent is invalid, no bad faith needed. The classical response to inequitable-conduct anxiety has been Therasense’s high bar, and Inline Plastics does not lower it; it shows the bar is reachable on circumstantial evidence when known third-party contributions are knowingly excluded. Applicants must now be right on inventorship for two independent reasons: wrong innocently, Fortress Iron invalidates; wrong intentionally, Inline Plastics makes the family unenforceable.

The paths lock together. Under pre-AIA Section 256, correction was available only where the error arose “without any deceptive intention,” so intent foreclosed correction outright. The America Invents Act struck that clause (Pub. L. 112-29, § 20), and current Section 256 imposes no deceptive-intent bar — but that liberalization does not rescue an applicant who misrepresented inventorship on purpose. Correction cures the incorrect-inventorship defect that drives the Fortress Iron path; it does nothing to cure inequitable conduct. Fixing the inventor block removes the Fortress Iron vulnerability while leaving the Inline Plastics one fully intact.

And two tracks understates it. An omitted inventor is not a passive problem to be fixed on the applicant’s timetable: she has standing to sue under Section 256 even without an ownership interest (Chou v. University of Chicago; reputational injury suffices under Shukh v. Seagate Technology, LLC), and if she has assigned to an adversary, proving her co-inventorship can hand that adversary a co-ownership defense (Smart Parts, Inc. v. WDP Ltd.).

A mismatch between the patent’s inventive entity and a related publication can also turn that publication into prior art against the patent itself. A co-authored reference is “by another” — and available as prior art — unless the relied-upon disclosure reflects the same inventive entity named on the patent, the inquiry turning on whether the non-overlapping author contributed significantly to the portions relied upon (Google LLC v. IPA Technologies Inc.; In re Land; Merck Serono S.A. v. Hopewell Pharma Ventures, Inc.). Inventorship errors can thus manufacture the very prior art that sinks the claims. Getting inventorship wrong is not a single correctable clerical risk but a multi-front exposure — and only some fronts can be cured by Section 256 at all.

Practical Recommendations

Treat inventorship documentation as a first-order deliverable. Run the inventor-identification exercise at every meaningful stage — provisional, non-provisional, continuation, amendment, issuance — and document it contemporaneously, mapping specific claim elements to specific contributions. The measure is the Pannu standard reaffirmed in HIP, Inc. v. Hormel Foods Corp.: a contribution must not be “insignificant in quality … measured against the dimension of the full invention,” and it is significant if it supplies the features that make the claims novel or non-obvious (Blue Gentian).

Squarely address third-party contributions. When consultants or vendors contribute to conception, do not assume the issue away — the 4Sight consultants were off-payroll and their drawings undisclosed, which is precisely what generated the specific-intent inference. Keep a running inventorship log for outside contributors.

Remember that inventorship governs prior-art qualification. To use a multi-authored reference as prior art, a challenger must show it is “by another” — that a non-overlapping author contributed enough to the relied-upon portions to be a joint inventor of them (Google LLC v. IPA Technologies Inc.). Evaluate the inventive contributions inside your prior-art references with the same rigor you apply to your own applications.

Resolve questions during prosecution rather than deferring to Section 256. Even a successful correction is not absolute: equitable doctrines like forfeiture can bar a patentee from relying on a corrected inventorship date to antedate prior art absent diligence (Implicit, LLC v. Sonos, Inc.). Implicit distinguished Egenera, Inc. v. Cisco Systems, Inc. (IPWatchdog coverage), where an intervening claim construction changed the inventorship analysis and justified revisiting it — inventorship can be re-engineered to move a prior-art date only when something other than an adverse ruling has genuinely shifted.

Separate ownership from inventorship. Under Stanford v. Roche, rights vest first in the inventor, and an assignment transfers only what the assignor actually holds. An assignment cannot cure a defective inventor determination — it presupposes the very title in question. Omit a true inventor and her undivided interest was never conveyed; name a non-inventor and he had nothing to assign (Ethicon, Inc. v. U.S. Surgical Corp.). And be wary of repackaging inventorship as a state-law conversion claim — such claims are frequently preempted when they seek “patent-like” damages (BearBox LLC v. Lancium LLC).

What to Watch

Whether the Federal Circuit hears an appeal of the Inline Plastics inequitable-conduct finding is worth tracking; the specific-intent inference is fact-intensive and will draw scrutiny. Whether Fortress Iron’s “party concerned” requirement extends to wrongly-named inventors (misjoinder) remains untested — Section 256(b) covers both, but no post-Fortress Iron decision has reached the point. And pleading a Section 256 claim can be decisive to forum: it is the “arising under” lever missing in my earlier analysis of Acorda Therapeutics v. Alkermes, and IQE, PLC v. Newport Fab, LLC shows it can pull accompanying state-law claims — and their anti-SLAPP exposure — into the Federal Circuit.

Inventorship Back in the Spotlight

Inventorship was long treated as a soft-edged, correctable, largely honorary designation — an error to clean up later rather than a validity condition to lock down at filing. Fortress Iron and Inline Plastics move it back to the center. Getting inventorship wrong now carries two independent enforcement penalties, one procedural and one intentional, and the assumption that either can be worked around at leisure is no longer safe. For practitioners, the diligence has to be front-loaded to prosecution; for applicants, the outside collaborators who contribute to conception cannot be quietly excluded and forgotten.