DevelopersGlobal Overview
APIs are contracts. Once published, every behavior — documented or not — becomes something users depend on (Hyrum's Law). This skill enforces the discipline of designing APIs that are stable, self-documenting, and difficult to misuse.
When to Use
- Creating any public API endpoint
- Designing function/library APIs
- Extending or versioning existing APIs
Process
Step 1: Design the Interface First
- Write the usage examples before writing the implementation.
- Ask: Is this easy to use correctly? Is it hard to use incorrectly?
- Apply the principle of least surprise — the API should do what it looks like it does.
- Design for the caller, not the implementer.
Verify: You can write 3 example usages without looking at the implementation.
Step 2: Apply Hyrum's Law
- Every observable behavior of your API will be depended upon by someone.
- Document what IS and IS NOT guaranteed:
- Stable: return type, error codes, semantic behavior
- Unstable: response time, field ordering, internal implementation
- Be conservative in what you expose — you can always add, never remove.
Verify: Every public field and behavior is either documented as stable or marked as internal.
Step 3: Versioning Strategy
- Version from day one:
/api/v1/,Content-Type: application/vnd.myapi.v1+json - Breaking changes require a new version.
- Maintain old versions for at least 6 months with deprecation notices.
- Additive changes (new optional fields) are non-breaking.
Verify: API version is in the URL or headers. Deprecation policy is documented.
Step 4: Self-Documentation
- Every endpoint: purpose, inputs, outputs, error codes — documented.
- Error messages tell the caller what went wrong AND how to fix it.
- Schema validation on all inputs with meaningful error messages.
- OpenAPI/Swagger spec generated (not hand-written).
Verify: A new developer can use the API from documentation alone, without reading source code.
Common Rationalizations (and Rebuttals)
| Excuse | Rebuttal |
|---|---|
| "We'll document it later" | Undocumented APIs become black boxes. Document as you build. |
| "We can break it, it's internal" | Internal APIs become external. Design them well from the start. |
| "Versioning is premature" | Retrofitting versioning into an unversioned API is painful. Start versioned. |
Verification
- Interface designed before implementation
- Stable vs. unstable behaviors documented
- Versioning strategy in place
- All endpoints documented (OpenAPI/Swagger)
- Error messages actionable
- Breaking vs. non-breaking changes policy defined
References
- security-hardening skill
- Hyrum's Law: https://www.hyrumslaw.com/