hegel

Hegel: property-based testing

Hegel generates random inputs for your code and shrinks failing cases to minimal counterexamples. Libraries exist for Rust (hegeltest), Go, C++, TypeScript, Java, and OCaml, all integrating with the standard test runner.

Learn the API from, in order: existing hegel tests in this repo, the library's documentation, and the hegel library source available in your environment (vendored or registry copy). Check the exact name and signature of everything you use. Do not guess syntax.

Ground every property in evidence — documented contracts, names and signatures, invariants the code itself asserts, existing tests. An undocumented but ordinary expectation (a round-trip, two equivalent spellings agreeing, a rescaled input giving the same answer) is evidence too: name the expectation, and when such a test fails, judge the failure by whether the behavior is defensible, not by whether anything promised it. Write one property per test, in the project's existing test files.

Techniques

Short, specific guides in this skill's techniques/ directory. Load one when you reach that stage, when you are stuck, or when a result surprises you:

• techniques/surfaces.md — choosing what to test: the full public API, sibling entry points, every instantiation
• techniques/directions.md — accept- and reject-direction properties; oracles that pin values, not just validity
• techniques/generators.md — generators: full domains, hostile inputs, targeted subsets
• techniques/scale.md — the scale probe for recursion and complexity bugs
• techniques/running.md — case counts and run configuration
• techniques/triage.md — investigating failures and reporting honestly
• techniques/adversarial-review.md — before finishing, a subagent reviews the code for missing properties you cannot see

Before you finish

Three steps, each required; skipping one leaves the job undone.

1. Surface coverage. Re-check your surface list (techniques/surfaces.md) against the library's public API index: every public module, and every sibling entry point and instantiation of a surface you tested, gets a test or a stated reason. Then check each tested surface against the technique menu above — anything unchecked gets a stated reason.

2. Adversarial review by a subagent — required, not optional. Launch a separate subagent (the Task tool, or your harness's equivalent) to find the properties you missed. Give it the implementation and the contract it must meet (doc comments, README, signatures) but not your tests — a reviewer holding your tests inherits your blind spots, which is the whole point of using a fresh agent. Tell it to assume the code is wrong until shown otherwise and to return the properties the contract implies (roundtrips, invariants, agreement between equivalent paths, the documented meaning of named values) and the input classes most likely to break them (empty, degenerate, boundary, maximal, malformed). Write what it returns as hegel tests, run them, and fix what they catch. You are not done until this review has run and you have acted on it. techniques/adversarial-review.md has the full brief.

3. Checklist. Review what you wrote against the hegel-review checklist.