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 cnfeat/top-pm-skills --skill system-design-interviewgit clone --depth 1 https://github.com/cnfeat/top-pm-skillsWrote 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/cnfeat/top-pm-skills/system-design-interview)<a href="https://agentmods.dev/skills/cnfeat/top-pm-skills/system-design-interview"><img src="https://agentmods.dev/badge/skills/cnfeat/top-pm-skills/system-design-interview/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/cnfeat/top-pm-skills/system-design-interview"><img src="https://agentmods.dev/badge/skills/cnfeat/top-pm-skills/system-design-interview.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 | $0.00069 | $0.01626 |
| Opus 5 | $0.00034 | $0.00813 |
| Sonnet 5 | $0.00014 | $0.00325 |
| Haiku 4.5 | $0.00007 | $0.00163 |
Grade A, and why
system-design-interview 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 6d 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 — 142 lines — stays where its author put it; the contents beside it link to each section on GitHub.
System Design Interview Skill
Structures a complete, interview-grade system design response — covering clarifying questions, requirements, capacity estimates, architecture, component design, and trade-offs. Works equally well for real architecture sessions.
Required Inputs
Ask for these if not provided:
- The system to design (e.g. "design a URL shortener", "design a notification service", "design Twitter's feed")
- Scope (interview prep / real architecture decision / practice run)
- Scale target (rough numbers: DAU, requests/sec, data volume — or "assume typical web scale")
- Constraints or priorities (e.g. prioritise availability over consistency, minimise cost, low-latency reads)
- Time available (interview context only: 30 / 45 / 60 minutes — skip for real architecture sessions)
- Emphasis (optional — any area to go deeper on, e.g. "focus on the DB design" or "spend more time on scaling")
Output Format
1. Clarifying Questions
Before designing, list 4–6 questions that would change the design. Examples:
- Read-heavy or write-heavy? (affects caching and DB choice)
- Global or single-region? (affects latency requirements)
- Strong or eventual consistency? (affects storage and replication)
- Acceptable latency targets? (p50 / p99)
- Any existing infrastructure constraints?
Then proceed with stated assumptions if answering an interview question.
2. Functional Requirements
Core features (must have):
- [Feature 1]
- [Feature 2]
- [Feature 3]
Out of scope (for this design):
- [What's deliberately excluded and why]
3. Non-Functional Requirements
| Requirement | Target |
|---|---|
| Availability | [e.g. 99.9% / 99.99%] |
| Latency | [e.g. p95 < 100ms for reads] |
| Throughput | [e.g. 10k writes/sec peak] |
| Consistency | [Strong / Eventual] |
| Durability | [e.g. 99.999% — no data loss] |
4. Capacity Estimation
Traffic:
- DAU: [X]
- Reads/sec: [X] (peak: [X])
- Writes/sec: [X] (peak: [X])
Storage:
- Per record size: [X bytes]
- Records per day: [X]
- 5-year storage: [X GB/TB]
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.
- 6d ago First seen · 142 lines · 69 tokens per session scan A 26d3170d3e74
system-design-interview is a skill published in the GitHub repository cnfeat/top-pm-skills (48 stars, last pushed 2mo ago), licensed MIT. It adds 69 tokens to every session and 1,626 once invoked, about $0.0003 per session on Opus 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 2026-09-03.
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