api-and-interface-design
Contract-first design, Hyrum's Law, One-Version Rule, error semantics, boundary validation. Use when designing APIs, module boundaries, or public interfaces.
What this skill does
# API and Interface Design ## Overview Design APIs and module boundaries with contract-first thinking. Apply Hyrum's Law (all observable behaviors will be depended on), the One-Version Rule (prefer one way to do something), and clear error semantics. Public interfaces deserve more care than internal ones. ## When to Use - Designing REST or GraphQL API endpoints - Creating module boundaries within an application - Building SDKs, libraries, or shared packages - Defining function signatures that cross module boundaries ## Core Principles ### Contract-First Design Define the contract before implementation: - Input shapes with validation rules - Output shapes with all possible variants - Error responses with clear semantics - Versioning strategy ### Hyrum's Law Every observable behavior of your API will eventually be depended on by someone. Design accordingly: - Be explicit about what is guaranteed vs what is implementation detail - Document only what you commit to maintaining - Hide internal details behind stable interfaces ### One-Version Rule Prefer one way to accomplish a given task: - Avoid multiple endpoints that do the same thing with different syntax - Avoid multiple function signatures for the same operation - If you must provide convenience methods, make one canonical path clear ### Error Semantics Errors should be: - Specific enough to be actionable - Consistent across the API surface - Accompanied by enough context to debug - Categorized: client errors (4xx) vs server errors (5xx) ### Boundary Validation Validate all input at system boundaries: - External API endpoints - Module boundaries - Database queries - File parsing ## Process ### Step 1: Define the Contract Write the API contract before any implementation: - Endpoint paths and HTTP methods - Request and response schemas - Error response formats - Pagination patterns if applicable ### Step 2: Validate the Contract Review for: - Consistency with existing API patterns - Naming clarity and predictability - Completeness of error cases - Backward compatibility implications ### Step 3: Implement Against the Contract - Implementation should match the contract exactly - If the contract needs to change, update the contract first - Add contract tests that verify implementation matches ### Step 4: Document the Public Surface - Document only guaranteed behavior, not implementation details - Include examples for common use cases - Document error cases and recovery strategies ## Common Rationalizations | Rationalization | Reality | |---|---| | "I can refine the API after it works" | Breaking changes after adoption are expensive and disruptive. | | "This is an internal API, it does not matter" | Internal APIs become external APIs the moment another team depends on them. | | "Adding more parameters is backward compatible" | Adding parameters increases the surface area that consumers may depend on. | ## Verification - [ ] Contract is defined before implementation - [ ] All error cases have defined responses - [ ] Boundary validation covers all external inputs - [ ] Public surface is documented with examples - [ ] Contract tests verify implementation matches specification ## Anti-Rationalization Table | Excuse | Counter | |--------|---------| | "I'll define the contract as I implement" | Without a contract, implementation drifts. Define it first to catch design flaws early. | | "This is an internal API, Hyrum's Law doesn't apply" | Internal APIs become external the moment another team depends on them. Design accordingly. | | "Adding one more endpoint is quick" | Every new endpoint increases the surface area consumers depend on. One-version rule prevents confusion. | | "Error handling can be added later" | Inconsistent error semantics are hard to retrofit. Define error responses upfront. | | "The contract tests are too much overhead" | Contract tests catch drift between spec and implementation. They prevent silent breaking changes. |
Related in Design
contribute
IncludedLocal-only OSS contribution command center. Auto-refreshes the user's in-flight PR and issue state on invoke so conversations start with full context — no need to brief Claude on what's in flight. Helps the user find issues to contribute to on GitHub, builds per-repo dossiers of what each upstream expects (CLA, DCO, branch convention, AI policy, draft-first, review bots, issue templates), runs deterministic gates before any external action so AI-assisted contributions don't reach maintainers as slop. State is markdown-only: candidate files at ~/.contribute-system/candidates/, repo dossiers at ~/.contribute-system/research/, append-only event log at ~/.contribute-system/log.jsonl. No database, no cloud calls. Use when the user asks about their PRs / issues / contributions, wants to find new work to take on, claim an issue, build/refresh a repo's dossier, or draft a Design Issue or PR. Trigger with "/contribute", "what's my PR status", "find a contribution", "claim issue X", "draft a Design Issue for Y", "refresh dossier for Z".
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mcp
IncludedModel Context Protocol (MCP) server development and tool management. Languages: Python, TypeScript. Capabilities: build MCP servers, integrate external APIs, discover/execute MCP tools, manage multi-server configs, design agent-centric tools. Actions: create, build, integrate, discover, execute, configure MCP servers/tools. Keywords: MCP, Model Context Protocol, MCP server, MCP tool, stdio transport, SSE transport, tool discovery, resource provider, prompt template, external API integration, Gemini CLI MCP, Claude MCP, agent tools, tool execution, server config. Use when: building MCP servers, integrating external APIs as MCP tools, discovering available MCP tools, executing MCP capabilities, configuring multi-server setups, designing tools for AI agents.
react-native-skia
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plaid
IncludedProduct Led AI Development — guides founders from idea to launched product. Six capabilities: Idea (discover a product idea), Validate (pressure-test the idea against fatal flaws, problem reality, competition, and 2-week MVP feasibility), Plan (vision intake + document generation), Design (translate image references into a design.md spec), Launch (go-to-market strategy), and Build (roadmap execution). Use when someone says "PLAID", "plaid idea", "help me find an idea", "product idea", "idea from my business", "idea from my expertise", "plaid validate", "validate my idea", "pressure-test", "is this idea good", "find fatal flaws", "validate the problem", "plan a product", "define my vision", "generate a PRD", "product strategy", "plaid design", "design from image", "translate image to design", "create design.md", "extract design tokens", "plaid launch", "go-to-market", "launch plan", "GTM strategy", "launch playbook", "plaid build", "build the app", "start building", or "execute the roadmap".
nextjs-framer-motion-animations
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