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/stevesolun/ctx/codebase-designnpx skills add stevesolun/ctx --skill codebase-designgit clone --depth 1 https://github.com/stevesolun/ctxWhat 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 | $0.00040 | $0.00525 |
| Opus 5 | $0.00020 | $0.00262 |
| Sonnet 5 | $0.00008 | $0.00105 |
| Haiku 4.5 | $0.00004 | $0.00052 |
Grade A, and why
codebase-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 2d 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 — 55 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Codebase Design
Seek deep modules: substantial useful behavior behind a small, stable interface. Use the concepts below as design lenses, while matching the repository's established domain language.
Working vocabulary
- Module: a coherent unit with an interface and implementation, at any scale.
- Interface: everything callers must know, including invariants and failure modes—not just a type signature.
- Seam: a location where behavior can vary without editing its callers.
- Adapter: an implementation that connects at a seam.
- Depth: useful capability relative to interface complexity.
- Leverage: capability gained by callers; locality: change and knowledge concentrated for maintainers.
Translate these terms to the project's vocabulary when that makes the design clearer. Consistency with the codebase is more valuable than enforcing a private lexicon.
Design workflow
- Map representative callers, responsibilities, dependencies, and existing contracts.
- Look for coordination or policy that can move behind a smaller interface.
- Place seams where variation, ownership, or test isolation justifies them.
- Compare alternatives when the choice is consequential.
- Validate the candidate with representative caller code and tests.
Heuristics, not laws
- Reduce methods, parameters, ordering constraints, and configuration callers must understand.
- Use the deletion test: if removing a module merely spreads its complexity across callers, it was likely earning its keep.
- Treat pass-through layers skeptically, but keep them when they provide a real compatibility, ownership, policy, or navigation boundary.
- Introduce dependency seams when actual variation or test isolation repays the indirection; avoid interfaces justified only by hypothetical futures.
- Prefer tests through observable interfaces, while allowing focused internal tests when they provide cheaper or more precise feedback.
- Favor explicit dependencies and returned results when they improve control and testability; side effects and internally created dependencies can still be appropriate at well-defined boundaries.
What ships with it
3 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.
- 2d ago First seen · 55 lines · 40 tokens per session scan A f8bdd1b33e97
codebase-design is a skill published in the GitHub repository stevesolun/ctx (581 stars, last pushed 8d ago), licensed MIT. It adds 40 tokens to every session and 525 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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