api-design— agent skill

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

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

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

1. 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

1. Version from day one: /api/v1/, Content-Type: application/vnd.myapi.v1+json
2. Breaking changes require a new version.
3. Maintain old versions for at least 6 months with deprecation notices.
4. 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

1. Every endpoint: purpose, inputs, outputs, error codes — documented.
2. Error messages tell the caller what went wrong AND how to fix it.
3. Schema validation on all inputs with meaningful error messages.
4. 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)

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/