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 kevmoo/dash_skills --skill dart-seal-type-hierarchiesgit clone --depth 1 https://github.com/kevmoo/dash_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/kevmoo/dash_skills/dart-seal-type-hierarchies)<a href="https://agentmods.dev/skills/kevmoo/dash_skills/dart-seal-type-hierarchies"><img src="https://agentmods.dev/badge/skills/kevmoo/dash_skills/dart-seal-type-hierarchies/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/kevmoo/dash_skills/dart-seal-type-hierarchies"><img src="https://agentmods.dev/badge/skills/kevmoo/dash_skills/dart-seal-type-hierarchies.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 | $0.00066 | $0.01305 |
| Opus 5.5 | $0.00026 | $0.00522 |
| Sonnet 5.5 | $0.00013 | $0.00261 |
| Haiku 4.5 | $0.00007 | $0.00130 |
Grade A, and why
dart-seal-type-hierarchies 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 1mo 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 — 158 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Seal Type Hierarchies
1. When to use this skill
Use this skill when:
- A package declares an abstract base type whose subtypes are all defined in the
same library, but the base type is not marked
sealed. - Code branches over the members of such a hierarchy with
ischecks or a non-exhaustiveswitch, so adding a subtype later fails silently at runtime instead of loudly at compile time. - Reviewing a new type hierarchy and deciding whether it should be open to external subtyping.
When NOT to use (Abstention Guardrails)
Do NOT seal a type when:
- Subtypes span multiple libraries:
sealedrequires every direct subtype to be declared in the same library as the base type. If subtypes live in other files (and are notpart ofthe same library), the code will not compile. Either move them or leave the hierarchy open. - The type is public API and external extension is intended: Sealing a type exported from a published package is a breaking change for any downstream package that extends or implements it. Plugin interfaces, visitor bases, and extension points are meant to be open. Do not seal them to win exhaustiveness.
- The base type is concrete and instantiated:
sealedimpliesabstract. If callers construct the base type directly, sealing it breaks them, and the fix is a larger refactor than this skill covers. - Fewer than two subtypes: A single subtype is specialization, not an algebraic hierarchy. Exhaustiveness checking buys nothing.
- SDK constraint below 3.0.0: Class modifiers do not exist before Dart 3.
Check
environment.sdkinpubspec.yamlfirst.
2. Why sealing is the correctness move
The value is not switch syntax. It is that the compiler starts rejecting incomplete branching:
seal the hierarchy -> exhaustiveness checking -> adding a subtype becomes
a compile error at every switch, instead of a silent fallthrough
An unsealed hierarchy with is cascades is not wrong today. It becomes wrong
the moment someone adds a subtype, and nothing tells them which call sites they
missed. Sealing is prophylactic: it converts a future runtime bug into a
present-day compile error.
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.
- 1mo ago First seen · 158 lines · 66 tokens per session scan A 608af5ced2b2
dart-seal-type-hierarchies is a skill published in the GitHub repository kevmoo/dash_skills (147 stars, last pushed yesterday), licensed Apache-2.0. It adds 66 tokens to every session and 1,305 once invoked, about $0.0003 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 2026-09-13.
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