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 aj-geddes/useful-ai-prompts --skill property-based-testinggit clone --depth 1 https://github.com/aj-geddes/useful-ai-promptsWrote 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/aj-geddes/useful-ai-prompts/property-based-testing)<a href="https://agentmods.dev/skills/aj-geddes/useful-ai-prompts/property-based-testing"><img src="https://agentmods.dev/badge/skills/aj-geddes/useful-ai-prompts/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/aj-geddes/useful-ai-prompts/property-based-testing"><img src="https://agentmods.dev/badge/skills/aj-geddes/useful-ai-prompts/property-based-testing.svg" alt="Reviewed on agentmods" width="80" height="20"></a>- Socket pass
- Snyk 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 | $0.00041 | $0.00670 |
| Opus 5 | $0.00020 | $0.00335 |
| Sonnet 5 | $0.00008 | $0.00134 |
| Haiku 4.5 | $0.00004 | $0.00067 |
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 9d 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 — 97 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Property-Based Testing
Table of Contents
Overview
Property-based testing verifies that code satisfies general properties or invariants for a wide range of automatically generated inputs, rather than testing specific examples. This approach finds edge cases and bugs that example-based tests often miss.
When to Use
- Testing algorithms with mathematical properties
- Verifying invariants that should always hold
- Finding edge cases automatically
- Testing parsers and serializers (round-trip properties)
- Validating data transformations
- Testing sorting, searching, and data structure operations
- Discovering unexpected input combinations
Quick Start
Minimal working example:
# test_string_operations.py
import pytest
from hypothesis import given, strategies as st, assume, example
def reverse_string(s: str) -> str:
"""Reverse a string."""
return s[::-1]
class TestStringOperations:
@given(st.text())
def test_reverse_twice_returns_original(self, s):
"""Property: Reversing twice returns the original string."""
assert reverse_string(reverse_string(s)) == s
@given(st.text())
def test_reverse_length_unchanged(self, s):
"""Property: Reverse doesn't change length."""
assert len(reverse_string(s)) == len(s)
@given(st.text(min_size=1))
def test_reverse_first_becomes_last(self, s):
"""Property: First char becomes last after reverse."""
reversed_s = reverse_string(s)
assert s[0] == reversed_s[-1]
assert s[-1] == reversed_s[0]
// ... (see reference guides for full implementation)
Reference Guides
Detailed implementations in the references/ directory:
| Guide | Contents |
|---|---|
| Hypothesis for Python | Hypothesis for Python |
| fast-check for JavaScript/TypeScript | fast-check for JavaScript/TypeScript |
| junit-quickcheck for Java | junit-quickcheck for Java |
What ships with it
5 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.
- 9d ago First seen · 97 lines · 41 tokens per session scan A f0e2aa98f376
property-based-testing is a skill published in the GitHub repository aj-geddes/useful-ai-prompts (338 stars, last pushed 6mo ago), licensed MIT. It adds 41 tokens to every session and 670 once invoked, about $0.0002 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 2026-09-03.
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