Short answer
Reimplementing an API is generally lawful in the EU, and the Directive addresses interfaces directly:
"Protection in accordance with this Directive shall apply to the expression in any form of a computer program. Ideas and principles which underlie any element of a computer program, including those which underlie its interfaces, are not protected by copyright under this Directive." —
The phrase "including those which underlie its interfaces" is a deliberate legislative choice, and recital 11 confirms it was inserted "for the avoidance of doubt".
The EU question is therefore what did you copy, not whether copying was fair. Four different things get muddled together under the word "API":
- The interface specification — names, signatures, parameter orders, semantics. Largely ideas and principles.
- The implementing code behind it — protected expression. Copying it infringes.
- The documentation — a literary work in its own right, protected independently.
- Header files, stubs and declaration sets — the contested middle, where originality has to be assessed.
Importantly, the US Google v Oracle analysis does not transfer. That case turned on fair use, a doctrine EU copyright does not have.
What the Directive means by an interface
Recital 10 supplies the working definition, and it is more precise than everyday usage:
"The function of a computer program is to communicate and work together with other components of a computer system and with users and, for this purpose, a logical and, where appropriate, physical interconnection and interaction is required to permit all elements of software and hardware to work with other software and hardware and with users in all the ways in which they are intended to function. The parts of the program which provide for such interconnection and interaction between elements of software and hardware are generally known as 'interfaces'. This functional interconnection and interaction is generally known as 'interoperability'; such interoperability can be defined as the ability to exchange information and mutually to use the information which has been exchanged." —
Recital 11 then makes the protection position explicit, and extends the same reasoning to logic, algorithms and programming languages — while confirming that the expression of those ideas and principles is protected.
What SAS Institute settles
SAS Institute (C-406/10) is the closest EU authority, and its subject matter maps closely onto API reimplementation.
The national court asked whether reproducing in a second program the selection of statistical operations implemented in the first, the mathematical formulae used to describe them, the particular commands or combinations of commands by which they are invoked, the options provided, the keywords and syntax recognised, the defaults chosen, and the number of iterations performed, infringed copyright.
The Court held that the functionality of a computer program, the programming language, and the format of data files used to exploit certain of its functions do not constitute a form of expression of that program and are not protected by copyright in computer programs under the Directive.
Commands, keywords, syntax, options and defaults are, in substance, what an API surface consists of. That is why the case matters so much here.
Two qualifications the Court preserved. First, elements falling outside software-specific protection may still be assessed under the general InfoSoc framework where they constitute the author's own intellectual creation — the approach taken to graphical user interfaces in BSA (C-393/09). Second, the manual was analysed separately: reproducing keywords and syntax in documentation describing the second program raises a distinct question about copyright in the first program's manual.
Where reimplementation goes wrong
Copying implementation code. Article 4(1)(a) covers permanent or temporary reproduction in part or in whole; Article 4(1)(b) covers translation, adaptation, arrangement and any other alteration, and reproduction of the results. Taking the body of a method, not merely its signature, infringes.
Copying documentation. Writing your own reference documentation is essential. Lifting descriptions, examples and parameter explanations from the original is copying a literary work.
Copying header files or declaration sets wholesale. These sit closest to the line. A single declaration is unlikely to be an author's own intellectual creation; a large, elaborately structured set might raise the question the Court left open in SAS Institute. Writing declarations from a specification is safer than copying files.
Copying comments, error strings and test suites. Comments and messages are textual expression. Test suites are works in their own right.
Copying preparatory design material. Article 1(1) includes preparatory design work within "computer programs" where a program can result from it.
Decompilation: the interoperability route
Where the interface information is not available, Article 6 provides a narrow but real route:
"The authorisation of the rightholder shall not be required where reproduction of the code and translation of its form within the meaning of points (a) and (b) of Article 4(1) are indispensable to obtain the information necessary to achieve the interoperability of an independently created computer program with other programs, provided that the following conditions are met: (a) those acts are performed by the licensee or by another person having a right to use a copy of a program …; (b) the information necessary to achieve interoperability has not previously been readily available to the persons referred to in point (a); and (c) those acts are confined to the parts of the original program which are necessary in order to achieve interoperability." —
Article 6(2) restricts the use of information so obtained: not for goals other than interoperability of the independently created program, not to be given to others except as necessary for that interoperability, and not for developing, producing or marketing a program substantially similar in its expression or for any other infringing act. Article 6(3) adds the Berne-derived proviso against unreasonable prejudice to the rightholder's legitimate interests or conflict with normal exploitation.
Crucially, Article 8 makes contractual provisions contrary to Article 6 null and void. A licence clause banning reverse engineering cannot defeat the Article 6 right, though it may still restrict activity that Article 6 does not cover.
Before reaching for Article 6, note condition (b): if the interface is publicly documented, the information was readily available and decompilation is not indispensable. Most public APIs will not need this route at all.
Why the US case does not help
Google v Oracle held that Google's use of the Java SE API declaring code was fair use. The Court assumed copyrightability for the purposes of the analysis and decided the case on the fair-use factors.
EU copyright has no fair-use doctrine. It has a closed list of exceptions, and the software-specific regime in Articles 5 and 6, all subject to the three-step test. An EU court will not balance transformativeness against market harm; it will ask whether protected expression was reproduced, and if so whether an exception applies.
The practical effect is that EU analysis is narrower but more predictable. You do not argue that copying was justified; you establish that what you took was idea rather than expression.
Practical guidance
- Work from published specifications and documentation, not from source or header files.
- Write declarations from the specification rather than copying files.
- Write all documentation, comments, examples and error messages yourself.
- Keep the study team and the implementation team separate, and record the handover.
- Rely on Article 5(3) observation, study and testing — a right Article 8 protects from contractual exclusion.
- Use Article 6 decompilation only where the information is genuinely not readily available, confine it to necessary parts, and respect the Article 6(2) use restrictions.
- Check patents and trade marks separately; neither is affected by the copyright analysis.
- Check licence terms for restrictions Article 8 does not void, which may still sound in contract.
- Do not rely on US authorities.
Conclusion
Recreating an API in the EU is, as a matter of copyright, usually a question with a favourable answer: Article 1(2) says the ideas and principles underlying interfaces are not protected, and SAS Institute confirms that commands, keywords, syntax, options and defaults are not forms of expression of a program.
Infringement comes from what travels alongside the specification — implementation code, header files copied wholesale, documentation, comments and tests. Build from published specifications, write your own text, keep the record of how you did it, and the EU framework is considerably more predictable than the US fair-use debate suggests.
Sources
This article provides general legal information, not advice for a specific matter. Laws, facts and procedural deadlines may change the result. Follow the linked primary sources and obtain local advice before acting.