Understanding Hyrum’s Law: When Observable Behavior Becomes Unbreakable Contract

In the complex world of software development, APIs and libraries act as contracts between different pieces of software. We expect them to behave as documented. However, a principle known as Hyrum’s Law highlights a common and often challenging reality that goes beyond the written specifications. This concept is crucial for anyone involved in building, maintaining, or consuming software interfaces.

What is Hyrum’s Law?

Hyrum’s Law, often attributed to Hyrum Wright from Google, states:

“With a sufficient number of users of an API, it does not matter what you promise in the contract: all observable behaviors of your system will be depended on by somebody.”

In simpler terms, once you release software (like an API or a library) that others use, they won’t just rely on the features you officially documented and promised. They will inevitably start depending on any behavior they can observe, whether it’s an intended feature, a side effect, an implementation detail, or even a bug.

Consider a library function designed to calculate a square root. Its contract promises to return the correct mathematical result. But if, perhaps for debugging purposes, the function also consistently prints a specific message to the console, some users might build tools or processes that expect that message. If the developers later remove that “unintended” console output, those user systems will break, even though the function still correctly calculates the square root as promised by the official contract. The observable behavior became part of an implicit contract.

The Challenge of Unintended Dependencies

This phenomenon creates significant challenges. A developer might make a seemingly harmless internal change – optimize an algorithm, refactor code for clarity, fix a minor bug, or even alter the format of log messages never meant for external consumption. If users have unknowingly relied on the previous observable behavior (e.g., the specific timing of the old algorithm, the structure of the buggy output, or the exact format of those logs), these changes can lead to unexpected failures in their applications.

What was intended as an improvement or a benign fix from the developer’s perspective becomes a breaking change for the user who depended on the undocumented aspect.

Real-World Consequences: Examples from Software History

History is filled with examples illustrating Hyrum’s Law:

Microsoft Excel’s Calculation Quirks: Older versions of Excel had minor floating-point calculation bugs. Some users built complex spreadsheets that, intentionally or not, relied on these slightly incorrect results. When Microsoft attempted to fix these bugs, it risked breaking these existing spreadsheets, demonstrating how even errors can become depended-upon features.
Java’s HashMap Ordering: Before Java 8, the order of elements within a HashMap wasn’t guaranteed by the official contract, but in common implementations, it was often deterministic based on insertion order and hash codes. Some developers wrote code that implicitly relied on this predictable order. When Java 8 introduced changes to improve HashMap performance and security (which altered the internal ordering), this previously working code broke because it depended on an observable, yet undocumented, behavior.
Undocumented API Endpoints: Developers sometimes discover and use undocumented features or endpoints of popular APIs (like early versions of mapping services). When the API provider later changes, removes, or restricts access to these undocumented aspects as part of an update or policy enforcement, applications relying on them inevitably fail.

These examples show that any observable aspect, regardless of documentation or intention, can become a dependency for someone, somewhere.

Key Implications for Software Teams

Understanding Hyrum’s Law has several important implications for development practices:

Backward Compatibility Burden: Any change to an observable behavior, even if not part of the official API, carries the risk of breaking users. This makes evolving software much harder, as maintainers might feel obligated to preserve unintended behaviors.
Refactoring Risks: Improving internal code structure (refactoring) becomes riskier. A change intended only to clean up code might alter subtle observable behaviors (like performance characteristics or error message details) that users depend on.
API Design Discipline: It emphasizes the need for careful API design. Developers must consciously decide what behaviors are exposed and supported, minimizing the “surface area” of observable implementation details. Interfaces should be as minimal as possible.
Expanded Testing Scope: Testing shouldn’t just cover the documented contract. Ideally, systems should be designed to minimize reliance on implementation details, but where unavoidable, critical observable behaviors might need testing to prevent unintentional breaking changes.
Accumulation of Technical Debt: The need to maintain backward compatibility with unintended behaviors can lead to keeping old, inefficient, or buggy code around longer than desired, increasing technical debt.

Strategies for Managing Hyrum’s Law

While Hyrum’s Law describes an inherent challenge, teams can adopt strategies to mitigate its negative impacts:

Mindful API Design: Design APIs with a clear distinction between the public, supported contract and internal implementation details. Use language features (like access modifiers) and design patterns (like interfaces) to hide internals where possible.
Robust Testing: Implement comprehensive test suites that verify the intended contract thoroughly. While testing every observable behavior is impossible, focus on stability for the documented public interface.
Explicit Documentation: Clearly document what is part of the guaranteed contract and, if necessary, explicitly state what is not supported or guaranteed (e.g., “The order of elements returned is not guaranteed and may change in future versions”).
Controlled Evolution: Use semantic versioning to signal breaking changes. Employ deprecation policies, giving users advance warning before removing or significantly altering any observable behavior, whether it was initially intended or not.
Clear Communication: Foster open communication channels with users to understand how they are using the software and to inform them about upcoming changes.

Balancing Stability and Progress

Hyrum’s Law highlights the tension between stability and evolution. Strictly adhering to preserving all observable behavior would make improving or fixing software nearly impossible. The key is not to prevent change but to manage it thoughtfully. Awareness of Hyrum’s Law encourages developers to be more deliberate about API design, more cautious about changes, and more communicative with their users.

Conclusion

Hyrum’s Law serves as a critical reminder for software engineers: the true contract of your software is defined not just by your documentation, but by everything your users can observe and potentially rely upon. Recognizing this helps teams build more robust, maintainable systems by encouraging careful design, thorough testing, clear communication, and a measured approach to software evolution.

At Innovative Software Technology, we understand the intricate challenges posed by principles like Hyrum’s Law in modern software development. Our expert teams leverage this understanding to deliver high-quality software solutions focused on long-term maintainability and reliability. We specialize in designing robust APIs with well-defined contracts, minimizing unintended dependencies that can lead to future breakage. Our comprehensive testing strategies go beyond basic functionality to ensure stability, while our experience in managing legacy systems helps navigate the complexities of evolving software without disrupting users who rely on established behaviors. By prioritizing clear documentation, controlled evolution, and proactive technical debt management, Innovative Software Technology empowers businesses to build and maintain reliable software systems that adapt gracefully to change, ensuring both stability for today and flexibility for tomorrow. Partner with us to build software resilient to the unspoken contracts highlighted by Hyrum’s Law.