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/lexler/skill-factory/nullablesnpx skills add lexler/skill-factory --skill nullablesgit clone --depth 1 https://github.com/lexler/skill-factoryWhat 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 | $0.00060 | $0.01935 |
| Opus 5 | $0.00030 | $0.00967 |
| Sonnet 5 | $0.00012 | $0.00387 |
| Haiku 4.5 | $0.00006 | $0.00194 |
Grade A, and why
nullables 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 — 93 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Nullables: Testing Without Mocks
STARTER_CHARACTER = ⭕️
Nullables are production code with an off switch: classes with external I/O anywhere in their dependencies offer create() (real) and createNull() (I/O disabled, everything else runs normally). Tests are narrow (focused on one class), sociable (dependencies run for real), and state-based (assert outputs and state, never method calls). Don't use mocking libraries or DI frameworks — Nullables make them unnecessary.
The cut
Stub at the lowest point — the third-party edge — never your own code:
OrderService → PaymentClient → HttpClient → third-party lib
app code high-level low-level ✂ stubbed when nulled
wrapper wrapper
PaymentClientis a high-level wrapper: it abstracts one service and speaks domain language.HttpClientis a low-level wrapper: it abstracts one technology and is generic and highly reusable.- The low-level wrapper holds the fork:
create()wires the real library (node http, RestTemplate);createNull()wires an embedded stub — your code, returning canned data, doing no I/O. - Everything left of the cut is your code and runs for real in tests.
With mocks, you only mock code you own; with Nullables, you only stub code you don't own. Only the bottom layer has a stub — one per technology. Everything above runs real in tests, so a bug anywhere in your code turns tests red. Mocking your own classes breaks that chain: mocked code never runs, and its bugs hide behind green tests.
An invented internal seam is the same break in disguise: cutting at rows() → List<Row> instead of the driver puts your mapping loop below the seam, where nulled tests never run it. The test: any parsing, mapping, or normalization you wrote must sit above the cut. When the third-party API is a chain of objects, mirror it — one stub class can play the whole chain (see the low-level wrapper files).
Two channels, plus events
Every class that talks to infrastructure anywhere in its dependencies offers the same two factory methods:
What ships with it
8 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.
- credits.md 604 B
- references/architecture.md 2.9 KB
- references/building-high-level-wrappers.md 8.2 KB
- references/building-low-level-wrappers-dynamic.md 11 KB
- references/building-low-level-wrappers-static.md 14 KB
- references/consuming-nullables.md 8.1 KB
- references/migration.md 5.2 KB
- references/utilities.md 5.2 KB
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 · 93 lines · 60 tokens per session scan A 0ead4895567d
nullables is a skill published in the GitHub repository lexler/skill-factory (231 stars, last pushed 7d ago), licensed Apache-2.0. It adds 60 tokens to every session and 1,935 once invoked, about $0.0003 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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