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 frontal-labs/mcp-server --skill api-design-principlesgit clone --depth 1 https://github.com/frontal-labs/mcp-serverWrote 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/frontal-labs/mcp-server/api-design-principles)<a href="https://agentmods.dev/skills/frontal-labs/mcp-server/api-design-principles"><img src="https://agentmods.dev/badge/skills/frontal-labs/mcp-server/api-design-principles/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/frontal-labs/mcp-server/api-design-principles"><img src="https://agentmods.dev/badge/skills/frontal-labs/mcp-server/api-design-principles.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 | $0.00044 | $0.00813 |
| Opus 5.5 | $0.00018 | $0.00325 |
| Sonnet 5.5 | $0.00009 | $0.00163 |
| Haiku 4.5 | $0.00004 | $0.00081 |
Grade A, and why
api-design-principles 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 yesterday.
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
100% identical to api-design-principles — 0 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 — 111 lines — stays where its author put it; the contents beside it link to each section on GitHub.
API Design Principles
Master REST and GraphQL API design principles to build intuitive, scalable, and maintainable APIs that delight developers and stand the test of time.
When to Use This Skill
- Designing new REST or GraphQL APIs
- Refactoring existing APIs for better usability
- Establishing API design standards for your team
- Reviewing API specifications before implementation
- Migrating between API paradigms (REST to GraphQL, etc.)
- Creating developer-friendly API documentation
- Optimizing APIs for specific use cases (mobile, third-party integrations)
Core Concepts
1. RESTful Design Principles
Resource-Oriented Architecture
- Resources are nouns (users, orders, products), not verbs
- Use HTTP methods for actions (GET, POST, PUT, PATCH, DELETE)
- URLs represent resource hierarchies
- Consistent naming conventions
HTTP Methods Semantics:
GET: Retrieve resources (idempotent, safe)POST: Create new resourcesPUT: Replace entire resource (idempotent)PATCH: Partial resource updatesDELETE: Remove resources (idempotent)
2. GraphQL Design Principles
Schema-First Development
- Types define your domain model
- Queries for reading data
- Mutations for modifying data
- Subscriptions for real-time updates
Query Structure:
- Clients request exactly what they need
- Single endpoint, multiple operations
- Strongly typed schema
- Introspection built-in
3. API Versioning Strategies
URL Versioning:
/api/v1/users
/api/v2/users
Header Versioning:
Accept: application/vnd.api+json; version=1
Query Parameter Versioning:
/api/users?version=1
Detailed patterns and worked examples
Detailed pattern documentation lives in references/details.md. Read that file when the navigation tier above is insufficient.
Best Practices
REST APIs
- Consistent Naming: Use plural nouns for collections (
/users, not/user) - Stateless: Each request contains all necessary information
- Use HTTP Status Codes Correctly: 2xx success, 4xx client errors, 5xx server errors
- Version Your API: Plan for breaking changes from day one
- Pagination: Always paginate large collections
- Rate Limiting: Protect your API with rate limits
- Documentation: Use OpenAPI/Swagger for interactive docs
What ships with it
5 files beside SKILL.md in the same directory: the scripts, references and assets a skill reads on demand. Not counted in the per-session cost; read them before you install if any of them is executable.
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.
- yesterday First seen · 111 lines · 44 tokens per session scan A 1f6166c23c0a
api-design-principles is a skill published in the GitHub repository frontal-labs/mcp-server (1 stars, last pushed yesterday), licensed Apache-2.0. It adds 44 tokens to every session and 813 once invoked, about $0.0002 per session on Opus 5.5. A static security scan graded it A with 0 findings. It is 100% identical to api-design-principles, differing in 0 lines, and is treated as a copy.
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