Getting it into your agent
One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.
npx skills add borhen68/SkillEngine --skill api-and-interface-designgit clone --depth 1 https://github.com/borhen68/SkillEngineWrote this? Show the measurements
A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.
[](https://agentmods.dev/skills/borhen68/skillengine/api-and-interface-design)<a href="https://agentmods.dev/skills/borhen68/skillengine/api-and-interface-design"><img src="https://agentmods.dev/badge/skills/borhen68/skillengine/api-and-interface-design/github.svg" alt="Measured on agentmods" height="20"></a>Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.
<a href="https://agentmods.dev/skills/borhen68/skillengine/api-and-interface-design"><img src="https://agentmods.dev/badge/skills/borhen68/skillengine/api-and-interface-design.svg" alt="Reviewed on agentmods" width="80" height="20"></a>What it costs to keep this loaded
Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.
| Model | Per session | Once invoked |
|---|---|---|
| Fable 5.1 | $0.00049 | $0.02550 |
| Opus 5.5 | $0.00020 | $0.01020 |
| Sonnet 5.5 | $0.00010 | $0.00510 |
| Haiku 4.5 | $0.00005 | $0.00255 |
Grade A, and why
api-and-interface-design scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 1mo ago.
A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.
Nothing flagged
None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.
This is a copy
83% identical to api-and-interface-design — 110 lines differ, which has more behind it and is treated as the original. This page carries a canonical link to it rather than competing with it.
How it starts
The opening of the file, as written. The whole thing — 322 lines — stays where its author put it; the contents beside it link to each section on GitHub.
API and Interface Design
Overview
Every API is a promise. Once published, it becomes a contract that other code depends on — and breaking that contract creates cascading failure across every consumer. The cost of a poorly designed API isn't just the initial implementation; it's the years of backwards-compatible maintenance, workarounds, and migration efforts that follow.
The interface contract: A good interface makes the right thing easy and the wrong thing impossible. It encodes assumptions explicitly, fails predictably, and evolves gracefully. This applies to REST APIs, GraphQL schemas, module boundaries, component props, function signatures, and any surface where one piece of code talks to another.
Real-world impact: Hyrum's Law states that with enough consumers, every observable behavior becomes a de facto contract. Design interfaces defensively from day one — it's infinitely cheaper than retrofitting constraints later.
When to Use
- Designing new API endpoints
- Defining module boundaries or contracts between teams
- Creating component prop interfaces
- Establishing database schema that informs API shape
- Changing existing public interfaces
Core Principles
Hyrum's Law
With a sufficient number of users of an API, all observable behaviors of your system will be depended on by somebody, regardless of what you promise in the contract.
This means: every public behavior — including undocumented quirks, error message text, timing, and ordering — becomes a de facto contract once users depend on it. Design implications:
- Be intentional about what you expose. Every observable behavior is a potential commitment.
- Don't leak implementation details. If users can observe it, they will depend on it.
- Plan for deprecation at design time. See
deprecation-and-migrationfor how to safely remove things users depend on. - Tests are not enough. Even with perfect contract tests, Hyrum's Law means "safe" changes can break real users who depend on undocumented behavior.
What this file has done since we first saw it
Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.
- 1mo ago First seen · 322 lines · 49 tokens per session scan A 9ba8c8b8122f
api-and-interface-design is a skill published in the GitHub repository borhen68/SkillEngine (17 stars, last pushed 4mo ago), licensed MIT. It adds 49 tokens to every session and 2,550 once invoked, about $0.0002 per session on Opus 5.5. A static security scan graded it A with 0 findings. It is 83% identical to api-and-interface-design, differing in 110 lines, and is treated as a copy.
Other skills, from other repositories
api-and-interface-design
Guidance for designing stable, clearly documented interfaces between software parts, such as web APIs, GraphQL schemas, modules, and component properties. It covers defining the contract first, limiting accidental dependencies, and planning safe changes.
deprecation-and-migration
A guide for retiring old code, systems, or interfaces and moving users to replacements. Deprecation means marking something as no longer preferred; migration means switching its users safely.
api-and-interface-design
Guides stable API and interface design. Use when designing APIs, module boundaries, or any public interface. Use when creating REST or GraphQL endpoints, defining type contracts between modules, or establishing boundaries between frontend and backend.
api-design
Design stable, versioned, self-documenting APIs. Easy to use correctly, hard to use incorrectly. Apply Hyrum's Law from day one.
code-review-and-quality
A code-review guide for checking changes across correctness, readability, architecture, security, and performance before they are merged.
constraint-driven-development
A development method for writing a project's quality standards as a lasting, checkable contract. It can cover areas such as test coverage, security, performance, accessibility, and architecture.