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 joao-aires/agent-skills-public --skill property-based-testinggit clone --depth 1 https://github.com/joao-aires/agent-skills-publicWrote 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/joao-aires/agent-skills-public/property-based-testing)<a href="https://agentmods.dev/skills/joao-aires/agent-skills-public/property-based-testing"><img src="https://agentmods.dev/badge/skills/joao-aires/agent-skills-public/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/joao-aires/agent-skills-public/property-based-testing"><img src="https://agentmods.dev/badge/skills/joao-aires/agent-skills-public/property-based-testing.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 | $0.00183 | $0.00997 |
| Opus 5.5 | $0.00073 | $0.00399 |
| Sonnet 5.5 | $0.00037 | $0.00199 |
| Haiku 4.5 | $0.00018 | $0.00100 |
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 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.
Copies of this mod
1 near-identical copy found in the catalogue:
- property-based-testing — 100% identical, 0 lines differ
How it starts
The opening of the file, as written. The whole thing — 69 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Property-Based Testing
An example test asserts one point. A property asserts a rule over the whole input domain and lets the generator hunt for the counterexample. That trade is worth making when the code has an algebraic shape — an inverse, an invariant, an oracle — and not otherwise. Code with no such shape gets example tests; saying so is a valid outcome.
Check first whether the shape is missing or merely buried. A calculation wrapped in I/O, a string built by concatenation, an in-place mutation — each has a property and no seam to assert it through. See references/refactoring.md before concluding there is nothing to assert.
Property catalog
| Property | Formula | Where it applies |
|---|---|---|
| Roundtrip | decode(encode(x)) == x |
Serialization, conversion pairs |
| Inverse | f(g(x)) == x |
encrypt/decrypt, compress/decompress |
| Oracle | new(x) == reference(x) |
Optimization, refactoring, reimplementation |
| Idempotence | f(f(x)) == f(x) |
Normalization, formatting, sorting |
| Invariant | Holds before and after | Any transformation, contract state |
| Easy to verify | is_sorted(sort(x)) |
Complex algorithms with cheap checkers |
| Commutativity | f(a, b) == f(b, a) |
Binary and set operations |
| Associativity | f(f(a,b), c) == f(a, f(b,c)) |
Combining operations |
| Identity | f(x, e) == x |
Operations with a neutral element |
Strength ordering, weakest to strongest:
no crash → type preservation → invariant → idempotence → roundtrip / oracle.
Assert the strongest property the code supports. "No crash" alone rarely justifies the dependency — if that is all you can find, either a small rearrangement exposes something stronger, or the honest report is that this code is a poor PBT candidate. Rule out the first before settling for the second.
The two ways a property test asserts nothing
- Tautology.
assert add(a, b) == a + brestates the implementation; no bug they share can fail it. Pick a property that constrains the function without recomputing it. Note the exception:f(x) == f(x)is a genuine determinism property whenfis not obviously pure — serializers over dicts or sets, hashing, anything reading the clock. - Vacuity.
assume()that filters out nearly every input passes without exercising anything, and self-contradictoryassume()passes having run zero cases. Push constraints into the strategy so the generator produces valid inputs directly.
What ships with it
9 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.
- yesterday First seen · 69 lines · 183 tokens per session scan A 9448cacb65c6
property-based-testing is a skill published in the GitHub repository joao-aires/agent-skills-public (2 stars, last pushed yesterday), licensed MIT. It adds 183 tokens to every session and 997 once invoked, about $0.0007 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 2026-10-21.
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