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 agentmods add skills/kid-sid/codex-spellbook/system-designnpx skills add kid-sid/codex-spellbook --skill system-designgit clone --depth 1 https://github.com/kid-sid/codex-spellbookWrote 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/kid-sid/codex-spellbook/system-design)<a href="https://agentmods.dev/skills/kid-sid/codex-spellbook/system-design"><img src="https://agentmods.dev/badge/skills/kid-sid/codex-spellbook/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 | $0.00039 | $0.05168 |
| Opus 5 | $0.00019 | $0.02584 |
| Sonnet 5 | $0.00008 | $0.01034 |
| Haiku 4.5 | $0.00004 | $0.00517 |
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 3d 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 — 589 lines — stays where its author put it; the contents beside it link to each section on GitHub.
System Design
A structured end-to-end guide for designing systems from scratch, making architecture decisions, estimating capacity, and building for scalability and reliability.
When to Activate
- Designing a new service, platform, or system from scratch
- Writing a design document, tech spec, or RFC
- Making a technology selection or architecture decision
- Writing an Architecture Decision Record (ADR)
- Reviewing a design for scalability, reliability, or security gaps
- Planning capacity for a feature expected to handle significant load
Design Process
Follow these six steps in order. Each step informs the next — skipping steps leads to rework.
- Clarify requirements — Gather functional requirements (what it does), non-functional requirements (scale, latency, availability, durability), and constraints (budget, existing stack, team size, timeline).
- Estimate scale — QPS, storage, bandwidth. See Capacity Estimation section.
- Define API contracts — What APIs does the system expose? What are the request/response shapes, authentication mechanisms, and versioning strategy? Reference the
api-designskill for REST/gRPC conventions. - Design the data model — Define entities, relationships, access patterns, and storage technology. Choose SQL vs NoSQL based on the Technology Selection Matrix.
- Design components — Produce a High-Level Design (HLD) diagram, assign service responsibilities, and define communication patterns (sync vs async).
- Identify bottlenecks — Find single points of failure, scaling limits, hot partitions, and cascading failure risks before the design is locked.
Quick Decision Framework
| Non-functional requirement | Design implication |
|---|---|
| High read throughput | Read replicas, caching layer |
| High write throughput | Sharding, async writes, CQRS |
| Low latency | CDN, in-process cache, co-location |
| High availability | Multi-AZ, load balancing, circuit breakers |
| Strong consistency | Single-leader DB, distributed transactions (use carefully) |
| Eventual consistency | Event-driven, CQRS + event sourcing acceptable |
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.
- 3d ago First seen · 589 lines · 39 tokens per session scan A a278fd53add4
system-design is a skill published in the GitHub repository kid-sid/codex-spellbook (21 stars, last pushed 3mo ago), licensed MIT. It adds 39 tokens to every session and 5,168 once invoked, about $0.0002 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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