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 HoangNguyen0403/agent-skills-standard --skill system-design-methodologygit clone --depth 1 https://github.com/HoangNguyen0403/agent-skills-standardWrote 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/hoangnguyen0403/agent-skills-standard/system-design-methodology)<a href="https://agentmods.dev/skills/hoangnguyen0403/agent-skills-standard/system-design-methodology"><img src="https://agentmods.dev/badge/skills/hoangnguyen0403/agent-skills-standard/system-design-methodology/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/hoangnguyen0403/agent-skills-standard/system-design-methodology"><img src="https://agentmods.dev/badge/skills/hoangnguyen0403/agent-skills-standard/system-design-methodology.svg" alt="Reviewed on agentmods" width="80" height="20"></a>- NVIDIA SkillSpector pass
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 | $0.00062 | $0.00969 |
| Opus 5 | $0.00031 | $0.00485 |
| Sonnet 5 | $0.00012 | $0.00194 |
| Haiku 4.5 | $0.00006 | $0.00097 |
Grade A, and why
system-design-methodology 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 12d 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 — 81 lines — stays where its author put it; the contents beside it link to each section on GitHub.
System Design Methodology
Priority: P0 (CRITICAL)
Requirements before solutions. Never draw a full architecture before numbers justify it.
Phase 0 - Classify Depth (always first)
- Quick sketch: exploratory ask, no scale numbers available, answer needed now. Assume defaults, label each one
ASSUMED, skip gates. - Full session: real build, migration, or budget commitment. Run every phase gate.
- State depth and mode (new design | review existing | interview practice) in one line, then continue.
- Escalate quick to full when a hard constraint or irreversible choice appears.
Phase 1 - Intake (gate)
- Parse request: verbs to use cases, nouns to entities, adjectives to constraints.
- Ask max 3 blocking questions per turn, each with a recommended default. See intake checklist.
- Required before design: DAU/actors, top 3 use cases, read:write ratio, latency SLO, consistency need, retention, peak shape, budget, team size.
- Freeze scope: list what is explicitly out of scope.
Phase 2 - Estimation (gate)
- Compute QPS, storage, bandwidth, and working-set memory via
system-design-estimation. - Present the numbers, name the one quantity that shapes the design, confirm before drawing anything.
Phase 3 - High-Level Design (incremental)
- Price the null option first: do nothing, buy it, or let an existing service absorb it. Rejecting it needs a stated reason, not silence.
- Start with the smallest system satisfying functional requirements: client, API, service, store.
- Add one component at a time. For each, state
constraint -> component -> costin one line. No component without a named constraint. - Define API surface (one endpoint per functional requirement) and data ownership before optimizing.
- Diagram only after the component set is agreed, per
system-design-diagramming.
Brownfield Path (review-existing mode)
- Map current state before proposing anything: components, owners, traffic, incidents.
- Measure, do not assume: pull real QPS, data volume, and p99 from the running system.
- Find the binding constraint - the one that fails first at the next growth step.
- Design the smallest change that moves it, then re-measure. A rewrite needs a structural constraint the current shape cannot satisfy.
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.
- 12d ago First seen · 81 lines · 62 tokens per session scan A 96b741320f37
system-design-methodology is a skill published in the GitHub repository HoangNguyen0403/agent-skills-standard (565 stars, last pushed 3d ago), licensed MIT. It adds 62 tokens to every session and 969 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 2026-08-30.
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