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 sjarmak/coding-agent-workflows --skill property-testinggit clone --depth 1 https://github.com/sjarmak/coding-agent-workflowsWrote 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/sjarmak/coding-agent-workflows/property-testing)<a href="https://agentmods.dev/skills/sjarmak/coding-agent-workflows/property-testing"><img src="https://agentmods.dev/badge/skills/sjarmak/coding-agent-workflows/property-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/sjarmak/coding-agent-workflows/property-testing"><img src="https://agentmods.dev/badge/skills/sjarmak/coding-agent-workflows/property-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 | $0.00112 | $0.02252 |
| Opus 5.5 | $0.00045 | $0.00901 |
| Sonnet 5.5 | $0.00022 | $0.00450 |
| Haiku 4.5 | $0.00011 | $0.00225 |
Grade A, and why
property-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 — 188 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Property-Based Testing
An example test pins the inputs the author thought of. A property test states what must hold for every input, lets the library generate inputs, and shrinks a failure to the smallest input that still breaks it. All Hegel libraries and Hypothesis share the same core model: internal shrinking on the choice sequence, health checks, and a database of failing examples that replays first on the next run.
When it is required
Pure logic gets a property test in the RED step, next to the example tests, not after. Missing one is a MEDIUM, non-blocking review finding. Shapes:
- parsers, encoders, decoders, serializers
- arithmetic, units, money, time, geometry
- ordering, dedup, merge, diff, set operations
- state machines, reducers, schedulers
- any function with an inverse or a reference implementation
I/O glue, controllers and wiring do not need one. A property test never
replaces the explicit boundary cases from common/testing.md; it adds the
inputs nobody listed.
Property catalog
| Property | Statement | Typical target |
|---|---|---|
| Round-trip | decode(encode(x)) == x |
codecs, serializers, parsers with printers |
| Idempotence | f(f(x)) == f(x) |
normalizers, formatters, dedup |
| Invariant | a fact about the output holds for every input | sort output is ordered and same length |
| Commutativity, associativity | argument order or grouping does not change the result | merge, union, money arithmetic |
| Monotonicity | larger input never gives smaller output | pricing, scoring, time arithmetic |
| Differential oracle | fast(x) == simple(x) |
optimized code against a naive version |
| Metamorphic | a known change to the input gives a known change to the output | search: adding a matching doc grows results |
| No crash | every generated input returns a value or a typed error | parsers on arbitrary bytes |
| State machine | every rule sequence keeps the model and the real system in agreement | caches, connection pools, editors |
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 · 188 lines · 112 tokens per session scan A ae4c0d77a68b
property-testing is a skill published in the GitHub repository sjarmak/coding-agent-workflows (7 stars, last pushed 10d ago), licensed MIT. It adds 112 tokens to every session and 2,252 once invoked, about $0.0004 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 2026-10-02.
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