The Day the Single Means Died:
Why § 112(f) Safe Harbor Has Sharp Edges

September 19, 2025

Patent law loves a safe harbor, but the one in § 112(f) has always come with rocks just beneath the surface. In Immervision v. Apple, the District of Delaware reminded us just how sharp those edges can be. The court invalidated a claim that relied entirely on an “optical means” limitation, holding that § 112(f) protects functional claiming only when the limitation is part of a true combination. A claim built on a single functional element, however artfully drafted, is not a combination at all. It’s a black box. And under Immervision, it’s dead on arrival.

From Halliburton to the 1952 Act: The First Death of Functional Claims

The skepticism toward functional claiming is nothing new. In 1946, the Supreme Court in Halliburton Oil Well Cementing Co. v. Walker struck down claims to a “tuning means,” explaining that defining an invention solely by what it does leaves competitors “at sea” as to its scope. Congress responded in 1952 by softening Halliburton’s absolutism, enacting what became § 112, ¶ 6. The compromise was precise: functional language was permitted, but only for “an element in a claim for a combination.” Congress never intended to legalize the stand-alone black box.

Williamson and the Fall of Magic Words

For decades, courts treated § 112(f) as optional unless the words “means for” appeared. That strong presumption fell in 2015 with Williamson v. Citrix Online. There, the Federal Circuit held that “distributed learning control module” was nothing more than a nonce word — a placeholder that conveyed no structure. The court’s en banc opinion rewrote the playbook: what matters is not the presence of magic words, but whether the term recites “sufficiently definite structure.” If it doesn’t, § 112(f) applies, no matter how cleverly phrased.

Disclosure or Bust: The Algorithm Cases

Once a claim term falls under § 112(f), the patentee is bound to whatever structure the specification discloses. And here, the Federal Circuit has shown little patience. In Biomedino, LLC v. Waters Technologies, a “control mechanism” without identified hardware was indefinite. In Aristocrat Technologies v. International Game Technology, software claims collapsed because the patentee offered only functional descriptions, not algorithms. And in WMS Gaming v. IGT, the court underscored the rule: when the function is performed by a computer, the disclosure must include an algorithm. Anything less is fatal.

Immervision: The Court That Called the Bluff

Against this backdrop, Immervision pushed the doctrine back to its statutory roots. The patent’s panoramic lens claim, defined by an “optical means,” was more than just overbroad. It was a single means claim. Section 112(f) requires an element “in a claim for a combination.” Without a combination, there is no safe harbor.

The court didn’t wait for expert reports or trial. It resolved the issue at the pleadings stage under Rule 12(c), holding that the question was statutory and legal, not factual. That meant no “clear and convincing” evidence was needed to strike down the claim. In one stroke, the court not only invalidated the claim but also handed defendants a new early-stage tool.

A PTO in Lockstep

The USPTO is echoing the same skepticism. In 2024, it reminded examiners that “unit,” “mechanism,” and “module” should presumptively be treated as functional terms, subject to § 112(f), unless the record shows concrete structure. The agency’s guidance aligns with the courts: functional shortcuts are on borrowed time.

What Practitioners Should Take Away

The message is clear. For prosecutors, drafting claims that rest on a single functional hook is asking for trouble. Claims must be true combinations, supported by structural disclosure — especially algorithms for software. For litigators, Immervision is an invitation to attack early. A well-timed Rule 12 motion can cut off a case before costs spiral. And for in-house counsel, portfolios heavy with functional recitations should be stress-tested now. What looks broad on paper may be brittle when challenged.

The Day the Single Means Died

Immervision is more than a quirky case about camera lenses. It is a reminder that the safe harbor of § 112(f) was never meant to cover single-element claims dressed up in functional language. From Halliburton’s prohibition, to Congress’s 1952 compromise, to Williamson’s suspicion of nonce words, the through-line has always bent toward structure. Immervision just made the point impossible to ignore: if your claim lives and dies by one functional limitation, it will die.

The day the single means died was not July 24, 2025 in Delaware. It was written into the statute in 1952 and foreshadowed in 1946. Immervision simply carved the headstone.