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/opagani/cassini-workshop/solid-principlesnpx skills add opagani/cassini-workshop --skill solid-principlesgit clone --depth 1 https://github.com/opagani/cassini-workshopWhat 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 | $0.00097 | $0.00991 |
| Opus 5 | $0.00048 | $0.00495 |
| Sonnet 5 | $0.00019 | $0.00198 |
| Haiku 4.5 | $0.00010 | $0.00099 |
Grade A, and why
solid-principles 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.
This is a copy
100% identical to solid-principles — 0 lines differ, which has more behind it and is treated as the original. This page carries a canonical link to it rather than competing with it.
How it starts
The opening of the file, as written. The whole thing — 83 lines — stays where its author put it; the contents beside it link to each section on GitHub.
SOLID Principles in TypeScript
Five principles of object-oriented design (Robert C. Martin) that keep code flexible, testable, and resistant to rot — adapted to idiomatic TypeScript, where interfaces, structural typing, union types, and plain functions often satisfy a principle without ceremony.
🎯 Why: Design for Change
The goal of writing software is to be able to change it safely. SOLID is the OO toolkit for that goal: SRP limits what changes when a reason to change appears, OCP lets you extend without editing tested code, LSP keeps substitutions safe, ISP keeps consumers from breaking on unrelated changes, DIP lets you swap implementations without rewriting policy. Every rule below earns its keep only if it shrinks the next diff.
How to use this skill
- Match the symptom in the decision guide below to a principle.
- Open
references/principles.mdfor the full before/after TypeScript example and the "when this is over-engineering" note for that principle. - Apply the smallest change that removes the actual pain. SOLID is a response to demonstrated change pressure, not a checklist to apply preemptively. A 30-line app does not need an interface per class.
Reference file
references/principles.md— all five principles (SRP, OCP, LSP, ISP, DIP) with motivation, a bad example, a refactor, TypeScript-specific notes, and when not to apply each.
The five principles
| Letter | Principle | One-line intent |
|---|---|---|
| S | Single Responsibility | A class/module should have one reason to change. |
| O | Open/Closed | Open for extension, closed for modification. |
| L | Liskov Substitution | Subtypes must be usable wherever their base type is expected. |
| I | Interface Segregation | Many small client-specific interfaces beat one fat one. |
| D | Dependency Inversion | Depend on abstractions, not concretions. |
Decision guide — symptom → principle
| You notice… | Apply | Letter |
|---|---|---|
| A class changes for unrelated reasons (DB + formatting + business rules) | Single Responsibility | S |
Adding a new "type" means editing a growing switch/if-else |
Open/Closed | O |
| A subclass throws on, no-ops, or weakens a method it inherited | Liskov Substitution | L |
| Implementers are forced to stub methods they don't use | Interface Segregation | I |
| Business logic imports a concrete DB/HTTP/SDK client directly | Dependency Inversion | D |
| Unit tests need a real database, network, or clock | Dependency Inversion | D |
| One change ripples across many unrelated files | Single Responsibility | S |
instanceof chains decide behavior |
Open/Closed (often Strategy/polymorphism) | O |
What ships with it
1 file 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.
- yesterday First seen · 83 lines · 97 tokens per session scan A 8d5f21c036da
solid-principles is a skill published in the GitHub repository opagani/cassini-workshop (0 stars, last pushed 2mo ago), licensed MIT. It adds 97 tokens to every session and 991 once invoked, about $0.0005 per session on Opus 5. A static security scan graded it A with 0 findings. It is 100% identical to solid-principles, differing in 0 lines, and is treated as a copy.
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