swift-library-design
by JoannisThe swift-library-design skill helps you design Swift libraries and frameworks with stronger public APIs, compile-time safety, and performance-aware defaults. Use it for protocol-oriented architecture, associated types, result builders, DSLs, noncopyable types, and progressive disclosure when building reusable Swift packages or SDK-style modules.
This skill scores 84/100, which means it is a solid directory listing for users working on Swift library and framework design. The repository gives enough trigger guidance, concrete API-pattern examples, and performance-focused advice that an agent can apply it with less guesswork than a generic prompt, though it is not a fully packaged, install-command-driven skill.
- Clear triggerability: the frontmatter names specific use cases like public API design, protocol-oriented architecture, result builders, performance optimization, and noncopyable types.
- Good operational depth: the main skill body includes core principles plus multiple pattern sections, with code fences and repo-backed references for API and performance guidance.
- Useful agent leverage: progressive disclosure and performance concerns are explicitly covered, so an agent can choose patterns and constraints with concrete examples rather than inventing them.
- No install command and no supporting scripts, so adoption is manual and less packaged than directory users may expect.
- Some examples are truncated in the preview and the repo has no resource or rule files, so the workflow is helpful but not especially comprehensive.
Overview of swift-library-design skill
What swift-library-design is for
The swift-library-design skill helps you design Swift libraries and frameworks with stronger public APIs, better compile-time safety, and performance-aware defaults. It is most useful when you are deciding how types, protocols, generics, builders, and response patterns should fit together before you write or refactor the package.
Who should use it
Use the swift-library-design skill if you are building reusable Swift code for other teams, open-source consumers, or SDK-style integration. It is especially relevant for library authors working on protocol-oriented APIs, DSLs, @inlinable hot paths, or move-only / noncopyable resource handling.
What makes it different
This swift-library-design skill is not a generic Swift coding helper. It focuses on API shape, not just implementation, so it is better when the real problem is “what should the public surface look like?” rather than “write this function.” The strongest signals in the repo are protocol-oriented design, compile-time safety, performance by default, and progressive disclosure.
How to Use swift-library-design skill
Install and scope it correctly
Install with npx skills add Joannis/claude-skills --skill swift-library-design. Then use it for architecture decisions in a Swift package, module, or framework, not for unrelated app logic. The swift-library-design install step is worth it when your prompt needs reusable API design guidance rather than a one-off snippet.
Give it a design problem, not a vague task
The swift-library-design usage works best when you provide the library goal, the consumer, and the constraints. Good inputs include: what the package does, the primary caller, whether the API must be synchronous or async, performance limits, and whether the library must support test doubles or multiple backends.
Example prompt shape:
Design a Swift library API for streaming HTTP responses. I need a protocol-oriented surface, minimal allocations, and room for custom response generators. Prefer compile-time safety and a simple default path for first-time users.
Read the right files first
Start with SKILL.md, then inspect references/api-patterns.md and references/performance.md. Those files carry the most useful implementation guidance for this skill because they show the intended protocol patterns, return-type composition, and performance annotations. If you only skim one reference, read api-patterns.md first.
Turn rough ideas into better output
If your starting point is “design a Swift SDK,” add the missing decision points: public vs internal types, generic constraints, associated types, inlining tradeoffs, and the simplest ergonomic entry point. That lets the swift-library-design guide produce an API shape instead of broad advice, and it makes the swift-library-design for Frontend Development less likely to drift into irrelevant UI patterns.
swift-library-design skill FAQ
Is this only for server-side Swift?
No. The swift-library-design skill is for any reusable Swift library or framework, including client SDKs, command-line tools, and shared modules. It is most valuable when the code will be consumed across module boundaries.
How is this different from a normal prompt?
A normal prompt may give you code that compiles. The swift-library-design skill is aimed at better module boundaries: protocol design, constrained generics, response adaptation, and performance decisions like @inlinable and @usableFromInline. That usually produces APIs that are easier to evolve.
Should beginners use it?
Yes, if the goal is to learn how to structure a reusable API. It is less useful if you are still deciding basic Swift syntax or building a single-file app feature. Beginners get the most value by asking for a small surface area first, then expanding after the initial design is clear.
When should I not use it?
Skip it when you need view code, app architecture, or a quick implementation with no reusable surface. It is also a poor fit if you do not care about public API stability, extension points, or performance boundaries.
How to Improve swift-library-design skill
Provide the constraints the skill cannot guess
The best swift-library-design outputs come from specific inputs: target Swift version, whether the package is public, expected call volume, threading model, and whether ABI stability matters. If the library must be fast across module boundaries, say so; if source compatibility matters more than micro-optimization, say that too.
Ask for the API decision, not just the implementation
If you want stronger results, ask for the protocol hierarchy, the concrete type layout, and the ergonomics tradeoff. For example: “Should this use associated types or type erasure?” or “What should be public, @usableFromInline, or internal?” That framing forces the skill to address the decisions that matter most.
Watch for common failure modes
The main risk is over-generalization: a design that is elegant but too abstract, too many protocols, or performance annotations applied where they create maintenance cost. Another common miss is underspecifying the consumer story, which leads to an API that is technically solid but awkward to call.
Iterate with one concrete sample
After the first pass, give the skill a real usage example and ask it to redesign around that call site. If the initial output is a rough ResponseGenerator or builder pattern, refine it with one or two realistic inputs, expected outputs, and error cases. That is the fastest way to improve the swift-library-design guide output without bloating the public API.