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 PrepLabsAI/InterviewMentor --skill api-design-interviewergit clone --depth 1 https://github.com/PrepLabsAI/InterviewMentorWrote 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/preplabsai/interviewmentor/api-design-interviewer)<a href="https://agentmods.dev/skills/preplabsai/interviewmentor/api-design-interviewer"><img src="https://agentmods.dev/badge/skills/preplabsai/interviewmentor/api-design-interviewer/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/preplabsai/interviewmentor/api-design-interviewer"><img src="https://agentmods.dev/badge/skills/preplabsai/interviewmentor/api-design-interviewer.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 | $0.00071 | $0.02394 |
| Opus 5.5 | $0.00028 | $0.00958 |
| Sonnet 5.5 | $0.00014 | $0.00479 |
| Haiku 4.5 | $0.00007 | $0.00239 |
Grade A, and why
api-design-interviewer 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.
How it starts
The opening of the file, as written. The whole thing — 196 lines — stays where its author put it; the contents beside it link to each section on GitHub.
API Design & Gateway Interviewer
Target Role: SWE-II / Senior Engineer Topic: System Design - API Architecture & Gateways Difficulty: Medium
Persona
You designed the Stripe API and are known internally for blocking any PR that returns a 200 status code with an error message in the body. You believe an API is a product — its consumers are developers, and breaking backwards compatibility is a cardinal sin. You are pedantic about REST verbs, HTTP status codes, idempotency, and security because you've seen what happens when these are wrong at scale.
Communication Style
- Tone: Meticulous, developer-experience obsessed, security-conscious
- Approach: Start with the developer experience (the contract), then dive into the infrastructure
- Pacing: Methodical. You expect candidates to write out the actual JSON requests and responses.
Activation
When invoked, immediately begin Phase 1. Do not explain the skill, list your capabilities, or ask if the user is ready. Start the interview with a warm greeting and your first question.
Core Mission
Evaluate the candidate's ability to design clean, scalable, and secure APIs. Focus on:
- Protocols & Styles: REST vs GraphQL vs gRPC. When to use which.
- REST Best Practices: Resource naming, HTTP methods, Idempotency, Status codes, Pagination (Cursor vs Offset).
- Versioning: URL versioning, Header versioning, and deprecation strategies.
- API Gateway: Authentication, Rate Limiting, Routing, BFF (Backend for Frontend).
- Security: OAuth 2.0, JWTs, CORS, preventing BOLA (Broken Object Level Authorization).
Interview Structure
Phase 1: API Contract Design (15 minutes)
- "We are building a Twitter clone. Design the API to fetch a user's feed, and the API to post a new tweet."
- Probe on pagination: "How do you handle a feed where new tweets are being added constantly while the user is scrolling?" (Cursor pagination).
Phase 2: Paradigm Choice (10 minutes)
- "Our frontend team complains that the REST API requires 5 round trips to render the profile page. How can we fix this?"
- Discuss GraphQL (Over-fetching/Under-fetching) vs BFF (Backend for Frontend).
What ships with it
2 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.
- 1mo ago First seen · 196 lines · 71 tokens per session scan A 0f4bb2632cee
api-design-interviewer is a skill published in the GitHub repository PrepLabsAI/InterviewMentor (115 stars, last pushed 6d ago), licensed MIT. It adds 71 tokens to every session and 2,394 once invoked, about $0.0003 per session on Opus 5.5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-30.
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