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 PeterHdd/agent-skills --skill engineering-system-designergit clone --depth 1 https://github.com/PeterHdd/agent-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/peterhdd/agent-skills/engineering-system-designer)<a href="https://agentmods.dev/skills/peterhdd/agent-skills/engineering-system-designer"><img src="https://agentmods.dev/badge/skills/peterhdd/agent-skills/engineering-system-designer.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 | $0.00072 | $0.02773 |
| Opus 5 | $0.00036 | $0.01386 |
| Sonnet 5 | $0.00014 | $0.00555 |
| Haiku 4.5 | $0.00007 | $0.00277 |
Grade A, and why
engineering-system-designer 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 — 135 lines — stays where its author put it; the contents beside it link to each section on GitHub.
System Design Guide
Overview
This guide covers the process of turning product requirements into deployable, observable, and resilient distributed system architectures. Use it for greenfield architecture, scaling existing systems, design reviews, architecture decision records, or monolith-to-services migrations.
Design Process
1. Requirements
Clarify functional needs, non-functional targets (latency, throughput, durability), read/write ratio, peak traffic patterns, and geographic distribution. If the stakeholder cannot provide traffic numbers, estimate from user count: assume 10% DAU/MAU ratio, 5 requests per session, 80% of traffic in 8 hours (peak = 3x average).
2. Capacity estimation
Calculate QPS, storage growth, and bandwidth. Project at 1x, 5x, and 10x load. Identify the bottleneck resource. Use scripts/capacity_calculator.py for calculations. Always show your math — never state capacity without derivation.
3. High-level architecture
Map components, data stores, queues, caches, and external dependencies. Define sync vs async boundaries. Start with the fewest components possible — if 3 boxes solve it, do not draw 7.
4. Component deep-dive
Specify technology choices with justification. Define partitioning, replication, consistency model, and cache invalidation per store. Every technology choice must answer: "Why this over the simpler alternative?"
5. Data model and API design
Design schemas for primary access patterns. Define API contracts with error codes and rate limits. Plan migration strategy. Every table must have its top 3 query patterns listed with expected latency.
6. Failure modes
List every component failure and its blast radius. Define circuit breakers, retries, timeouts, and fallbacks. For each failure mode, state: what breaks, who is affected, how it is detected, and what the automatic recovery is.
7. Monitoring
Specify metrics, alerts, and dashboards required before launch. Every SLO must have a corresponding alert. Every alert must have a runbook link.
What ships with it
5 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.
- 6d ago First seen · 135 lines · 72 tokens per session scan A 1281b0850c1e
engineering-system-designer is a skill published in the GitHub repository PeterHdd/agent-skills (11 stars, last pushed 6mo ago), licensed MIT. It adds 72 tokens to every session and 2,773 once invoked, about $0.0004 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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