On December 11, 2025, the United States Court of Appeals for the Federal Circuit upheld the Patent Trial and Appeal Board’s (PTAB) decision to invalidate two computer font patents challenged by Duolingo Inc. and CoStar Realty Information Inc. This ruling effectively ends the legal dispute over these patents.
The patents in question were held by Modern Font Applications LLC, which had previously initiated litigation against Duolingo and CoStar, alleging infringement. In response, both companies filed petitions for inter partes review with the PTAB, seeking to invalidate the patents on grounds of obviousness and lack of novelty. The PTAB agreed with the petitioners, declaring the patents invalid.
Modern Font Applications appealed the PTAB’s decision to the Federal Circuit, arguing that the board had erred in its findings. However, the Federal Circuit panel found no reversible error in the PTAB’s analysis and conclusions, thereby affirming the invalidation of the patents.
This outcome underscores the challenges patent holders may face when defending the validity of their patents, particularly in the rapidly evolving field of software and digital applications. For companies like Duolingo and CoStar, the decision removes potential legal obstacles, allowing them to continue utilizing font technologies without the looming threat of infringement litigation.
The case highlights the importance of thorough patent examination and the role of inter partes review as a mechanism for challenging questionable patents. It also serves as a reminder for companies to diligently assess the validity of patents they may be accused of infringing, as successful challenges can lead to favorable outcomes in litigation.