/api-design

name: api-design
description: Design stable, versioned, self-documenting APIs. Easy to use correctly, hard to use incorrectly. Apply Hyrum's Law from day one.
category: build
applies-to: [claude, gemini, cursor, copilot, any]
version: 1.0.0

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

1. Write the usage examples before writing the implementation. 2. Ask: Is this easy to use correctly? Is it hard to use incorrectly? 3. Apply the principle of least surprise — the API should do what it looks like it does. 4. 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

5. Every observable behavior of your API will be depended upon by someone. 6. Document what IS and IS NOT guaranteed:

• Stable: return type, error codes, semantic behavior
• Unstable: response time, field ordering, internal implementation

7. 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

8. Version from day one: `/api/v1/`, `Content-Type: application/vnd.myapi.v1+json` 9. Breaking changes require a new version. 10. Maintain old versions for at least 6 months with deprecation notices. 11. 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

12. Every endpoint: purpose, inputs, outputs, error codes — documented. 13. Error messages tell the caller what went wrong AND how to fix it. 14. Schema validation on all inputs with meaningful error messages. 15. 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](../security-hardening/SKILL.md)
• Hyrum's Law: https://www.hyrumslaw.com/