1. The necessity—and limits—of the doctrine of equivalents

Under the established framework for patent infringement, an accused technical solution falls within the scope of a patent if it contains features identical or equivalent to every technical feature recited in the asserted claim. Conversely, it falls outside the scope if it omits one or more claimed features, or if one or more corresponding features are neither identical nor equivalent.[1]

The doctrine of equivalents operates within this framework. Even where an accused solution does not fall within the literal wording of a claim, a corresponding feature may still be treated as equivalent if it uses substantially the same means, performs substantially the same function, achieves substantially the same result, and could have been conceived by a person skilled in the art without inventive effort.[2]

The doctrine is essential to effective patent protection. Without it, an implementer could potentially avoid the literal scope of a claim through an insubstantial modification, thereby depriving the patent of meaningful protection.

The same flexibility, however, creates a risk of overreach. A patentee may emphasise the significance of a technical distinction during prosecution to secure grant, only to minimise that distinction in subsequent infringement proceedings and seek to recapture the excluded subject matter through equivalence. Patent law therefore requires both a doctrine that prevents protection from becoming unduly narrow and a countervailing rule that prevents its unwarranted expansion.

2. What prosecution history estoppel restricts

Article 6 of the Supreme People’s Court Interpretation on Several Issues Concerning the Application of Law in the Trial of Patent Infringement Disputes provides that, where a patent applicant or patentee has surrendered a technical solution in patent grant or invalidation proceedings through an amendment to the claims or specification, or through observations, a court will not support an attempt to bring that solution back within the scope of the patent in subsequent infringement proceedings.[3]

Viewed within the infringement framework, the paradigm case for prosecution history estoppel is not one in which the accused solution already falls within the literal claim language. It is one in which the patentee seeks to use the doctrine of equivalents to cover subject matter that the claim does not expressly encompass. In that situation, the court must consider not only whether the corresponding feature meets the ordinary technical test for equivalence, but also whether the relevant subject matter was clearly surrendered during grant or validity proceedings.

Estoppel does not redefine the literal wording of a claim, nor does it determine whether the ordinary technical criteria for equivalence are satisfied. Instead, it asks whether, even if those criteria could otherwise be met, the patentee may recapture as an equivalent a technical solution previously surrendered during prosecution or validity proceedings.

The two doctrines therefore perform complementary functions. The doctrine of equivalents prevents patent protection from becoming unduly narrow; prosecution history estoppel prevents the patentee from extending equivalent protection to surrendered subject matter. Together, they balance effective protection against certainty of patent boundaries.

3. The analytical structure of the “Dog Shoe” decision

The patent in (2025) Zui Gao Fa Zhi Min Zhong No. 805 concerned a fastening structure for a dog shoe. The dispute focused principally on the “strap”, the position of that strap, and the extension line of the strap’s tensile force as recited in claim 1. The accused product used an integrated fastening device consisting of a rotary dial, a disc and crossed filaments. The patentees argued that, despite differences in form, the mechanism continued to function as a strap and exerted the relevant tensile force on both sides of the shoe. On that basis, they contended that the corresponding features were identical or equivalent.

The Supreme People’s Court did not apply prosecution history estoppel in isolation from the feature-by-feature comparison. Instead, it integrated the doctrine into its analysis of the disputed features.

3.1 The “strap” feature: estoppel within the equivalence inquiry

The Court first construed the “strap” feature by reference to the claim and specification. The specification contemplated multiple forms, including laces and hook-and-loop fasteners. The feature was therefore not confined to a single physical form, but the strap had to be capable of being placed under tension in a fastened state. By contrast, the accused product used an integrated dial, disc and filament mechanism that compressed and covered a relatively large area of the dog’s foot when tightened.

When considering whether the accused fastening device was equivalent to the claimed “strap”, the Court examined the prosecution history. In responding to the first office action, the applicant had distinguished the prior art on the basis that it “fasten[ed] the ankle and broadly wrap[ped] the top of the foot”, whereas the claimed invention narrowed the area covered by the strap.

The Court held that, through those observations, the applicant had expressly surrendered technical solutions involving broad coverage and restraint of the top of the foot, and had obtained grant on that basis. Prosecution history estoppel therefore prevented the patentees from bringing such a solution back within the patent’s scope as an equivalent.

Estoppel was thus not an additional step detached from the technical comparison. It formed part of the Court’s determination of whether the accused fastening device could legally be treated as equivalent to the claimed strap. The issue was not merely whether the two structures were technically similar, but whether equivalent protection remained available for subject matter expressly surrendered during prosecution.

3.2 Strap position and direction of force: separate technical findings

The Court addressed the strap position and the extension line of the tensile force primarily through a separate technical comparison. It found that the accused fastening structure covered the entire top of the foot and extended above the dewclaw, rather than being positioned between the dewclaw and the toe joint as required by the claim. The combined action of the dial, eyelets and crossed filaments also produced a force pattern materially different from the extension line specified in the patent.

The Court therefore held that these corresponding features were neither identical nor equivalent. Taken together, the accused solution contained multiple features that were neither identical nor equivalent to the claimed features and consequently fell outside claim 1 and dependent claim 3.

3.3 Estoppel was not the sole basis for the non-infringement finding

Although prosecution history estoppel informed the Court’s treatment of the “strap” feature, it was not the sole basis for the finding of non-infringement. Even apart from that feature, the differences in strap position and direction of force independently prevented the accused solution from satisfying the all-elements rule.

The significance of the decision therefore lies not in a finding of non-infringement based exclusively on estoppel, but in its clear demonstration of how estoppel may operate within the equivalence analysis for a specific claim feature.

4. Why prosecution history may limit equivalent protection

The justification for prosecution history estoppel extends beyond preventing inconsistent litigation positions. More fundamentally, it preserves consistency between the grant process and subsequent infringement proceedings.

Patent rights emerge from an iterative process between the applicant and the patent office. To overcome objections relating to inventive step, sufficiency of disclosure or claim breadth, an applicant may need to explain or narrow the asserted technical solution. Where such a statement forms part of the basis for grant and results in the surrender of identifiable subject matter, it may subsequently constrain the scope of equivalent protection available to the patentee.

Allowing the patentee to disavow that limitation after grant and recover the same subject matter through the doctrine of equivalents would deprive the prosecution process of practical effect. It would also undermine the ability of market participants to assess patent risk and design around the patent by reference to the claims and publicly available prosecution record.

Not every statement made during prosecution gives rise to estoppel. Whether a technical solution has been surrendered requires a contextual analysis, including whether the amendment or statement was clearly limiting, whether it addressed a specific obstacle to grant, whether the surrendered subject matter can be identified, and whether there is a substantive connection between the limitation and the grant or maintenance of the patent.

Article 13 of the Supreme People’s Court Interpretation (II) on Several Issues Concerning the Application of Law in the Trial of Patent Infringement Disputes provides an important exception: where the patentee proves that a limiting amendment or statement made in grant or validity proceedings was expressly rejected, the amendment or statement will not be treated as having surrendered the relevant technical solution.[4]

An express rejection is not the same as silence or the absence of an express acceptance by the patent office. Even where the patent was ultimately granted for other reasons, the full prosecution record must be considered to determine whether the limiting amendment or statement was expressly rejected.

5. Practical implications for patent prosecution and portfolio management

For businesses and patent practitioners, the principal lesson of the “Dog Shoe” decision arises at the prosecution stage.

Office action responses are sometimes treated as purely instrumental documents directed at securing grant. Applicants may therefore accept broad distinctions or limitations so long as the patent is granted. In subsequent invalidation or infringement proceedings, however, those statements may be revisited and may directly affect the scope of equivalent protection that remains available.

When responding to an inventive-step objection, an applicant should consider not only whether a proposed statement will assist in securing grant, but also whether it may be understood as excluding an entire category of technical solutions and thereby reduce the ability of the patent to reach alternative implementations in future litigation.

At a minimum, the applicant and its patent counsel should consider:

1.whether it is genuinely necessary to negate an entire category of technical solutions in order to overcome the objection;

2.whether the response can be confined to the specific technical distinction at issue, without an unnecessarily broad disclaimer; and

3.whether the proposed statement may restrict future equivalence arguments concerning substitute structures or design variations.

For commercially significant patents, prosecution, validity review and infringement enforcement are not isolated stages. They form part of a single legal lifecycle. Clear and limiting technical distinctions made to secure grant may continue to shape the equivalent protection available in later litigation.

Conclusion

The significance of (2025) Zui Gao Fa Zhi Min Zhong No. 805 lies not in any expansion of prosecution history estoppel, but in the way the decision locates the doctrine within the analysis of individual claim features.

In assessing the “strap” feature, the Court relied on the prosecution history to conclude that the patentees had surrendered solutions involving broad coverage and restraint of the top of the foot and could not reclaim them through equivalence. The Court separately found that the accused product’s strap position and direction of force were neither identical nor equivalent to the corresponding claim features.

The decision confirms that prosecution history estoppel neither displaces the feature-by-feature comparison nor supplies the ordinary technical test for equivalence. Rather, it bars a patentee, within that analysis, from asserting equivalent protection over surrendered subject matter. In this way, the doctrine of equivalents and prosecution history estoppel operate together to balance effective patent protection against legal certainty.

Applicants should therefore treat office action responses as documents with consequences extending well beyond grant. An express limitation made to overcome an objection may later constrain the available scope of equivalent protection. For high-value patents, the balance between securing grant and preserving a commercially meaningful scope of protection should be considered from the outset.

References

[1] Article 7, Supreme People’s Court Interpretation on Several Issues Concerning the Application of Law in the Trial of Patent Infringement Disputes.

[2] Article 7(2), Supreme People’s Court Interpretation on Several Issues Concerning the Application of Law in the Trial of Patent Infringement Disputes.

[3] Article 6, Supreme People’s Court Interpretation on Several Issues Concerning the Application of Law in the Trial of Patent Infringement Disputes.

[4] Article 13, Supreme People’s Court Interpretation (II) on Several Issues Concerning the Application of Law in the Trial of Patent Infringement Disputes.