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 joao-aires/agent-skills-public --skill system-designgit clone --depth 1 https://github.com/joao-aires/agent-skills-publicWrote 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/joao-aires/agent-skills-public/system-design)<a href="https://agentmods.dev/skills/joao-aires/agent-skills-public/system-design"><img src="https://agentmods.dev/badge/skills/joao-aires/agent-skills-public/system-design/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/joao-aires/agent-skills-public/system-design"><img src="https://agentmods.dev/badge/skills/joao-aires/agent-skills-public/system-design.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 | $0.00051 | $0.01260 |
| Opus 5.5 | $0.00020 | $0.00504 |
| Sonnet 5.5 | $0.00010 | $0.00252 |
| Haiku 4.5 | $0.00005 | $0.00126 |
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 yesterday.
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.
Design the system around the outcome
Use this phase after business/product refinement and before dependent implementation planning. Revisit it when a new feature changes boundaries, persistence, identity, integrations, deployment or important quality attributes. For a small change, a brief assessment of the affected contract can be sufficient. Preserve accepted architecture in existing projects.
Read the product context and repository evidence. Use system-mapping for the current system and seek available security/privacy expertise for authority and data flows. Distinguish the current architecture, proposed changes and unresolved assumptions. Ask about missing product/authority decisions; investigate technical facts and make reversible choices within scope.
Make decisions the user can assess
Establish constraints
Start with the journey the system must support and follow the information needed to complete it. Identify who supplies that information, which component owns the durable record, which consumers use it, and which external systems can affect completion. This reveals where trust, consistency and failure need explicit treatment.
Translate quality goals into conditions that influence the design. Identify what must remain available when a dependency fails, which data may be lost, how quickly the user needs a response, and whether work continues after the client disconnects. Include supported clients, offline behavior, synchronization, expected concurrency, budget, deployment environment and the team's operating capabilities where relevant. Distinguish accepted requirements from provisional estimates; unmeasured demand and unknown platform support remain open inputs.
Compare plausible options
Compare options that differ in a consequential responsibility, such as where state is owned, when work completes, or who operates a dependency. Explain how each option serves the journey under normal operation and under the failures that matter. Include the cost of deployment, diagnosis and recovery, not only implementation convenience.
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.
- yesterday Changed · +30 lines a2eb6c4faa60
- 2d ago Changed · +2 lines 4bf838419f80
- 3d ago Changed · -2 lines 43c6cfbc0937
- 5d ago First seen · 51 lines · 51 tokens per session scan A 289ab107aa13
system-design is a skill published in the GitHub repository joao-aires/agent-skills-public (2 stars, last pushed yesterday), licensed MIT. It adds 51 tokens to every session and 1,260 once invoked, about $0.0002 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 2026-10-17.
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