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 jamestorrevillas/dev-skills --skill system-designgit clone --depth 1 https://github.com/jamestorrevillas/dev-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/jamestorrevillas/dev-skills/system-design)<a href="https://agentmods.dev/skills/jamestorrevillas/dev-skills/system-design"><img src="https://agentmods.dev/badge/skills/jamestorrevillas/dev-skills/system-design.svg" alt="Measured on agentmods" 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 | $0.00070 | $0.01469 |
| Opus 5 | $0.00035 | $0.00734 |
| Sonnet 5 | $0.00014 | $0.00294 |
| Haiku 4.5 | $0.00007 | $0.00147 |
Grade A, and why
system-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 8d 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 — 181 lines — stays where its author put it; the contents beside it link to each section on GitHub.
System Design
Design Process (Always Follow This Order)
- Clarify requirements — functional AND non-functional
- Estimate scale — users, requests/sec, data volume, read/write ratio
- Define the API — what does the system expose?
- Design the data model — what data, how stored, how accessed?
- High-level architecture — major components and their relationships
- Deep dive — zoom into the hardest or most critical parts
- Identify bottlenecks — and how to address them
Requirements Clarification
Before designing anything, answer:
Functional: What does the system do? Scale: How many users? Requests/sec? Data volume? Growth rate? Availability: What's the acceptable downtime? (99.9% = 8.7h/yr, 99.99% = 52m/yr) Latency: What's the acceptable response time? (p99) Consistency: Strong vs eventual consistency? (CAP theorem) Read/Write ratio: Read-heavy? Write-heavy? Mixed? Geography: Single region? Global? Data sovereignty requirements?
Scalability Patterns
Horizontal vs Vertical Scaling
| Vertical | Horizontal | |
|---|---|---|
| Method | Bigger machine | More machines |
| Limit | Hardware ceiling | Theoretically unlimited |
| Cost | Expensive | More cost-efficient |
| Downtime | Required | Zero downtime possible |
| Complexity | Simple | Requires load balancing, distributed design |
Default: design for horizontal scaling from the start.
Load Balancing
- Round Robin — equal distribution, simple, doesn't account for load
- Least Connections — routes to server with fewest active connections
- Consistent Hashing — for distributed caches, minimizes redistribution on scaling
Caching Strategy
| Layer | Tool | What to Cache |
|---|---|---|
| CDN | Cloudflare, CloudFront | Static assets, public API responses |
| Application | Redis, Memcached | Session data, computed results, DB query results |
| Database | Query cache, materialized views | Expensive aggregations |
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.
- 8d ago First seen · 181 lines · 0 tokens per session scan A 92d00c3cb80c
system-design is a skill published in the GitHub repository jamestorrevillas/dev-skills (3 stars, last pushed 5mo ago), licensed MIT. It adds 70 tokens to every session and 1,469 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 2026-08-31.
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