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 CraftOS-dev/CraftBot --skill property-based-testinggit clone --depth 1 https://github.com/CraftOS-dev/CraftBotWrote 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/craftos-dev/craftbot/property-based-testing)<a href="https://agentmods.dev/skills/craftos-dev/craftbot/property-based-testing"><img src="https://agentmods.dev/badge/skills/craftos-dev/craftbot/property-based-testing/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/craftos-dev/craftbot/property-based-testing"><img src="https://agentmods.dev/badge/skills/craftos-dev/craftbot/property-based-testing.svg" alt="Reviewed on agentmods" width="80" height="20"></a>- NVIDIA SkillSpector pass
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 | $0.00049 | $0.01407 |
| Opus 5.5 | $0.00020 | $0.00563 |
| Sonnet 5.5 | $0.00010 | $0.00281 |
| Haiku 4.5 | $0.00005 | $0.00141 |
Grade A, and why
property-based-testing 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 — 124 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Property-Based Testing Guide
Use this skill proactively during development when you encounter patterns where PBT provides stronger coverage than example-based tests.
When to Invoke (Automatic Detection)
Invoke this skill when you detect:
- Serialization pairs:
encode/decode,serialize/deserialize,toJSON/fromJSON,pack/unpack - Parsers: URL parsing, config parsing, protocol parsing, string-to-structured-data
- Normalization:
normalize,sanitize,clean,canonicalize,format - Validators:
is_valid,validate,check_*(especially with normalizers) - Data structures: Custom collections with
add/remove/getoperations - Mathematical/algorithmic: Pure functions, sorting, ordering, comparators
- Smart contracts: Solidity/Vyper contracts, token operations, state invariants, access control
Priority by pattern:
| Pattern | Property | Priority |
|---|---|---|
| encode/decode pair | Roundtrip | HIGH |
| Pure function | Multiple | HIGH |
| Validator | Valid after normalize | MEDIUM |
| Sorting/ordering | Idempotence + ordering | MEDIUM |
| Normalization | Idempotence | MEDIUM |
| Builder/factory | Output invariants | LOW |
| Smart contract | State invariants | HIGH |
When NOT to Use
Do NOT use this skill for:
- Simple CRUD operations without transformation logic
- One-off scripts or throwaway code
- Code with side effects that cannot be isolated (network calls, database writes)
- Tests where specific example cases are sufficient and edge cases are well-understood
- Integration or end-to-end testing (PBT is best for unit/component testing)
Property Catalog (Quick Reference)
| Property | Formula | When to Use |
|---|---|---|
| Roundtrip | decode(encode(x)) == x |
Serialization, conversion pairs |
| Idempotence | f(f(x)) == f(x) |
Normalization, formatting, sorting |
| Invariant | Property holds before/after | Any transformation |
| Commutativity | f(a, b) == f(b, a) |
Binary/set operations |
| Associativity | f(f(a,b), c) == f(a, f(b,c)) |
Combining operations |
| Identity | f(x, identity) == x |
Operations with neutral element |
| Inverse | f(g(x)) == x |
encrypt/decrypt, compress/decompress |
| Oracle | new_impl(x) == reference(x) |
Optimization, refactoring |
| Easy to Verify | is_sorted(sort(x)) |
Complex algorithms |
| No Exception | No crash on valid input | Baseline property |
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.
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 · 124 lines · 49 tokens per session scan A f482f34ce65e
property-based-testing is a skill published in the GitHub repository CraftOS-dev/CraftBot (395 stars, last pushed today), licensed MIT. It adds 49 tokens to every session and 1,407 once invoked, about $0.0002 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 2026-09-03.
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