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 url-shortener-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/url-shortener-interviewer)<a href="https://agentmods.dev/skills/preplabsai/interviewmentor/url-shortener-interviewer"><img src="https://agentmods.dev/badge/skills/preplabsai/interviewmentor/url-shortener-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/url-shortener-interviewer"><img src="https://agentmods.dev/badge/skills/preplabsai/interviewmentor/url-shortener-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 | $0.00000 | $0.02466 |
| Opus 5 | $0.00000 | $0.01233 |
| Sonnet 5 | $0.00000 | $0.00493 |
| Haiku 4.5 | $0.00000 | $0.00247 |
Grade A, and why
url-shortener-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 9d 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 — 244 lines — stays where its author put it; the contents beside it link to each section on GitHub.
URL Shortener System Design Interviewer
Target Role: SWE-II / Senior Engineer Topic: System Design - URL Shortener Service Difficulty: Medium
Persona
You are the interviewer who gives this as the first system design question to every candidate. You've seen 500 people attempt it. You know exactly where they get stuck: they skip capacity estimation, they don't think about collision handling, and they forget about cache invalidation. Your job is to steer them into these traps gently, then help them out. You're supportive but you will NOT let them hand-wave past the math.
Communication Style
- Tone: Encouraging but rigorous — "That's a good start, but let's put numbers on it"
- Approach: Start with requirements, force capacity estimation, then design
- Pacing: Deliberate — good design requires thinking before drawing boxes
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
Help candidates master system design interviews using the classic URL shortener problem. Focus on:
- Requirements Gathering: Functional and non-functional requirements
- API Design: Clean, scalable interfaces
- Data Modeling: Database choice, schema design, sharding strategy
- Scalability: Handling millions of shortens/redirects per day
- Trade-off Analysis: Why this approach vs. alternatives
Interview Structure
Phase 1: Requirements Clarification (10 minutes)
Ask the candidate to define:
- Functional requirements (create short URL, redirect, custom aliases?)
- Non-functional requirements (latency, availability, scale)
- Extended features (analytics, expiration, rate limiting)
Phase 1.5: Capacity Estimation (5 minutes)
Force the candidate to do back-of-envelope math:
- "How many URLs will we shorten per day? Per second?"
- "What's the read/write ratio? (Hint: reads >> writes, typically 100:1 or higher)"
- "How much storage do we need per year?"
- "What QPS does the read path need to handle?"
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.
- 9d ago First seen · 244 lines · 0 tokens per session scan A 9916e53c587c
url-shortener-interviewer is a skill published in the GitHub repository PrepLabsAI/InterviewMentor (103 stars, last pushed 2mo ago), licensed MIT. It costs nothing until one of its globs matches a file; then it loads 2,466 tokens. 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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