Stateful Invariant Testing
aviggiano/security
Build metric-driven Chimera/create-chimera-app stateful invariant testing campaigns for Solidity projects.
Writes, reviews, and debugs property-based tests — Hypothesis, fast-check, proptest, jqwik, rapid, and Echidna or Medusa for Solidity invariants.
Install Property Based Testing
$ npx skills add trailofbits/skills --skill property-based-testing -a claude-codeProject install by default; add -g for ~/.claude/skills/.
$ gh skill install trailofbits/skills property-based-testing --agent claude-codeProject scope by default; add --scope user for a personal install. Needs GitHub CLI 2.90.0 or later (public preview).
$ git clone --depth 1 https://github.com/trailofbits/skills.git skills-src && mkdir -p .claude/skills && cp -r skills-src/plugins/property-based-testing/skills/property-based-testing .claude/skills/property-based-testing && rm -rf skills-srcUse ~/.claude/skills/ instead of .claude/skills for a personal install. The folder must contain SKILL.md.
Claude Code skills documentation · loads skills from .claude/skills/
Install the "property-based-testing" agent skill from https://github.com/trailofbits/skills/tree/main/plugins/property-based-testing/skills/property-based-testing into .claude/skills/property-based-testing/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "property-based-testing", then confirm the skill loads.Claude Code copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.
$skill-installer install https://github.com/trailofbits/skills/tree/main/plugins/property-based-testing/skills/property-based-testingType this inside Codex. $skill-installer <name> installs a curated skill from openai/skills. The installer writes to $CODEX_HOME/skills (default ~/.codex/skills). Restart Codex if the skill does not show up.
$ npx skills add trailofbits/skills --skill property-based-testing -a codexProject install goes to .agents/skills/; add -g for ~/.codex/skills/.
$ gh skill install trailofbits/skills property-based-testing --agent codexProject scope by default (.agents/skills/); add --scope user for a personal install.
$ git clone --depth 1 https://github.com/trailofbits/skills.git skills-src && mkdir -p .agents/skills && cp -r skills-src/plugins/property-based-testing/skills/property-based-testing .agents/skills/property-based-testing && rm -rf skills-srcUse ~/.agents/skills/ instead of .agents/skills for a personal install.
Codex skills documentation · loads skills from .agents/skills/
Install the "property-based-testing" agent skill from https://github.com/trailofbits/skills/tree/main/plugins/property-based-testing/skills/property-based-testing into .agents/skills/property-based-testing/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "property-based-testing", then confirm the skill loads.Codex copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.
$ npx skills add trailofbits/skills --skill property-based-testing -a cursorProject install goes to .agents/skills/; add -g for ~/.cursor/skills/.
$ gh skill install trailofbits/skills property-based-testing --agent cursorProject scope by default (.agents/skills/); add --scope user for a personal install.
$ git clone --depth 1 https://github.com/trailofbits/skills.git skills-src && mkdir -p .cursor/skills && cp -r skills-src/plugins/property-based-testing/skills/property-based-testing .cursor/skills/property-based-testing && rm -rf skills-srcUse ~/.cursor/skills/ instead of .cursor/skills for a personal install.
Cursor skills documentation · loads skills from .cursor/skills/, .agents/skills/, .claude/skills/, .codex/skills/
Install the "property-based-testing" agent skill from https://github.com/trailofbits/skills/tree/main/plugins/property-based-testing/skills/property-based-testing into .cursor/skills/property-based-testing/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "property-based-testing", then confirm the skill loads.Cursor copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.
$ gemini skills install https://github.com/trailofbits/skills.git --path plugins/property-based-testing/skills/property-based-testing--scope user (default) or --scope workspace; --path is the subfolder of the repo that holds the skill; --consent skips the security confirmation prompt.
$ npx skills add trailofbits/skills --skill property-based-testing -a gemini-cliProject install goes to .agents/skills/; add -g for ~/.gemini/skills/.
$ gh skill install trailofbits/skills property-based-testing --agent gemini-cliProject scope by default (.agents/skills/); add --scope user for a personal install.
$ git clone --depth 1 https://github.com/trailofbits/skills.git skills-src && mkdir -p .gemini/skills && cp -r skills-src/plugins/property-based-testing/skills/property-based-testing .gemini/skills/property-based-testing && rm -rf skills-srcUse ~/.gemini/skills/ instead of .gemini/skills for a personal install, then run /skills reload.
Gemini CLI skills documentation · loads skills from .gemini/skills/, .agents/skills/
Install the "property-based-testing" agent skill from https://github.com/trailofbits/skills/tree/main/plugins/property-based-testing/skills/property-based-testing into .gemini/skills/property-based-testing/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "property-based-testing", then confirm the skill loads.Gemini CLI copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.
$ gh skill install trailofbits/skills property-based-testingInstalls for Copilot at project scope by default; add --scope user for a personal install. Preview a skill first with gh skill preview. Needs GitHub CLI 2.90.0 or later (public preview).
$ npx skills add trailofbits/skills --skill property-based-testing -a github-copilotProject install goes to .agents/skills/; add -g for ~/.copilot/skills/.
$ git clone --depth 1 https://github.com/trailofbits/skills.git skills-src && mkdir -p .github/skills && cp -r skills-src/plugins/property-based-testing/skills/property-based-testing .github/skills/property-based-testing && rm -rf skills-srcUse ~/.copilot/skills/ instead of .github/skills for a personal install. Commit .github/skills so cloud agent and code review can use it.
GitHub Copilot skills documentation · loads skills from .github/skills/, .claude/skills/, .agents/skills/
Install the "property-based-testing" agent skill from https://github.com/trailofbits/skills/tree/main/plugins/property-based-testing/skills/property-based-testing into .github/skills/property-based-testing/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "property-based-testing", then confirm the skill loads.GitHub Copilot copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.
$ npx skills add trailofbits/skills --skill property-based-testing -a opencodeOpenCode documents no install command of its own. Project install goes to .agents/skills/; add -g for ~/.config/opencode/skills/.
$ gh skill install trailofbits/skills property-based-testing --agent opencodeProject scope by default (.agents/skills/); add --scope user for a personal install.
$ git clone --depth 1 https://github.com/trailofbits/skills.git skills-src && mkdir -p .opencode/skills && cp -r skills-src/plugins/property-based-testing/skills/property-based-testing .opencode/skills/property-based-testing && rm -rf skills-srcUse ~/.config/opencode/skills/ instead of .opencode/skills for a personal install.
OpenCode skills documentation · loads skills from .opencode/skills/, .claude/skills/, .agents/skills/
Install the "property-based-testing" agent skill from https://github.com/trailofbits/skills/tree/main/plugins/property-based-testing/skills/property-based-testing into .opencode/skills/property-based-testing/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "property-based-testing", then confirm the skill loads.OpenCode copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.
Facts
- Skill name
property-based-testing- GitHub stars
- 7.4k
- Token cost
- ~1.1k tokens
- SKILL.md length
- 434 words
- Files
- 9 (incl. references, assets)
- Skills in repo
- 79
- Repo updated
- First seen
- Licence
- CC-BY-SA-4.0
At a glance
Writes, reviews, and debugs property-based tests — Hypothesis, fast-check, proptest, jqwik, rapid, and Echidna or Medusa for Solidity invariants.
- Tests should cover a whole input domain instead of a hand-picked list of examples: encode/decode and serialize/deserialize pairs
- SKILL.md covers Property catalog, The two ways a property test…, Where to look next and Introducing PBT to a project…
- Instructions only: no scripts, shell commands, URLs or credentials in SKILL.md
- Canonicalizers and normalizers
What it does
Property Based Testing is an agent skill from trailofbits/skills, published by the product's own GitHub organization. Writes, reviews, and debugs property-based tests — Hypothesis, fast-check, proptest, jqwik, rapid, and Echidna or Medusa for Solidity invariants. Use whenever tests should cover a whole input domain instead of a hand-picked list of examples: encode/decode and serialize/deserialize pairs, parsers, canonicalizers and normalizers, validators, numeric and Decimal types, comparators and sort order, data structures, and smart-contract state invariants. Also use when adding cases to an existing @given, fast-check, or…
Its SKILL.md is about 1.1k tokens, which your agent loads only when the skill is triggered. The skill folder holds 11 other files, including reference files and assets (for example `README.md`, `agents/openai.yaml` and `references/generating.md`).
It sits in Security, covering Fuzzing and Smart contracts. It works with Solidity. The repository describes itself as: Trail of Bits Claude Code skills for security research, vulnerability detection, and audit workflows. The licence is CC-BY-SA-4.0.
When your agent uses it
- Tests should cover a whole input domain instead of a hand-picked list of examples: encode/decode and serialize/deserialize pairs
- Canonicalizers and normalizers
- Numeric and Decimal types
- Comparators and sort order
Example prompts
- “/property-based-testing”
What it can do on your machine
Read from SKILL.md and the folder at commit 82fe822. It shows what the files ask for, not the result of running them.
Tool permissions
Pre-approves nothing: there is no allowed-tools line, so your agent's usual permission prompts apply.
From allowed-tools in the SKILL.md frontmatter.
Runs code
No scripts in the folder and no shell commands in SKILL.md.
From the folder's file list and the shell code blocks in SKILL.md.
Network
No URLs in SKILL.md.
From URLs in SKILL.md, links to its own repository left out.
Credentials
Names no API keys, tokens, secrets or passwords.
From names ending in _API_KEY, _TOKEN, _SECRET, _KEY or _PASSWORD in SKILL.md.
Context cost
Property Based Testing loads about 1.1k tokens when it runs, and up to ~5.3k if it reads all its reference files. Until then it costs about 217 tokens; SKILL.md has 434 words of instructions outside code blocks.
Estimates: characters ÷ 4, the usual rule of thumb; real counts depend on the model's tokenizer. Scripts and assets cost tokens only if the agent reads them.
Safety
Auto-check passed
The automated check found no risky patterns in SKILL.md.
Automated static check — not a guarantee. Review scripts before installing. It scans the text of SKILL.md for risky patterns (piping downloads into a shell, reading credential files, hidden Unicode, destructive commands); files beside SKILL.md are not scanned.
SKILL.md
The full file from trailofbits/skills at commit 82fe822, republished under its CC-BY-SA-4.0 licence (© trailofbits). 434 words, ~1,071 tokens.
Download SKILL.mdSave it as
.claude/skills/property-based-testing/SKILL.md (or your agent's skills folder). This skill also uses 8 other files; get the full folder from GitHub.- name
- property-based-testing
- description
- Writes, reviews, and debugs property-based tests — Hypothesis, fast-check, proptest, jqwik, rapid, and Echidna or Medusa for Solidity invariants. Use whenever tests should cover a whole input domain instead of a hand-picked list of examples: encode/decode and serialize/deserialize pairs, parsers, canonicalizers and normalizers, validators, numeric and Decimal types, comparators and sort order, data structures, and smart-contract state invariants. Also use when adding cases to an existing @given, fast-check, or proptest suite, when judging whether existing property tests assert anything real, and when a generator has shrunk a counterexample and you need to tell a wrong property from a genuine bug. Not for coverage-guided binary fuzzing (libFuzzer, AFL), mutation-testing campaigns, static analysis, benchmarking, or end-to-end UI tests.
- effort
- low
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.
Show full SKILL.md (194 more words)Show less
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.
Where to look next
Load the one that matches the task in front of you:
| Task | File |
|---|---|
| Writing new tests, designing strategies | references/generating.md |
| The code has no property to assert yet | references/refactoring.md |
| Reviewing existing property tests | references/reviewing.md |
| A property test just failed | references/interpreting-failures.md |
| Library choice, Echidna and Medusa | references/libraries.md |
Introducing PBT to a project that lacks it
If the project already uses a PBT library, just write the tests in it. If it does not, adding one is a dependency decision that belongs to the user — offer it once with the specific property you would write, and take the answer either way.
© trailofbits, CC-BY-SA-4.0. Rendered from Markdown: HTML in the file is shown as text, images as links, and headings moved down two levels. Raw file
Files
SKILL.md and 8 other files (references, assets) in plugins/property-based-testing/skills/property-based-testing of trailofbits/skills.
- SKILL.md
- README.md
- agents/openai.yaml
- assets/trail-of-bits-mark.svg
- references/generating.md
- references/interpreting-failures.md
- references/libraries.md
- references/refactoring.md
- references/reviewing.md
Open the folder on GitHubat commit 82fe822
Compare with similar skills
Property Based Testing next to the 5 skills that share the most tags, products or categories with it. Stars are the repository's; “used in” counts other GitHub owners with a copy.
| Skill | Stars | Used in | Tokens | Auto-check | Licence | Repo updated |
|---|---|---|---|---|---|---|
| Property Based Testing this skilltrailofbits/skills | 7.4k | — | ~1.1k | Automated safety check: Pass | CC-BY-SA-4.0 | |
| Stateful Invariant Testingaviggiano/security | 144 | — | ~2.8k | Automated safety check: Pass | MIT | |
| Fizzpashov/skills | 1.2k | 2 repos | ~11k | Automated safety check: Pass | MIT | |
| Fizz Convertpashov/skills | 1.2k | 2 repos | ~3.7k | Automated safety check: Pass | MIT | |
| Fuzz Generatoralt-research2/SolidityGuard | 104 | — | ~763 | Automated safety check: Notes | Custom licence | |
| Fuzzing Patternsccashwell/evm-cortex | 131 | — | ~1.6k | Automated safety check: Pass | MIT |
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Related
Works with
Categories
Questions about Property Based Testing
What does Property Based Testing do?
Writes, reviews, and debugs property-based tests — Hypothesis, fast-check, proptest, jqwik, rapid, and Echidna or Medusa for Solidity invariants. Property Based Testing is an agent skill from trailofbits/skills, published by the product's own GitHub organization. Writes, reviews, and debugs property-based tests — Hypothesis, fast-check, proptest, jqwik, rapid, and Echidna or Medusa for Solidity invariants.
When should I use Property Based Testing?
Property Based Testing fits situations like: tests should cover a whole input domain instead of a hand-picked list of examples: encode/decode and serialize/deserialize pairs; canonicalizers and normalizers; numeric and Decimal types; comparators and sort order.
How do I install Property Based Testing in Claude Code?
Run `npx skills add trailofbits/skills --skill property-based-testing -a claude-code`. Or copy the skill folder (plugins/property-based-testing/skills/property-based-testing in trailofbits/skills) into .claude/skills/property-based-testing in your project. Claude Code loads it when a task matches its description.
How do I install Property Based Testing in Codex?
Run `npx skills add trailofbits/skills --skill property-based-testing -a codex`. Or copy the skill folder (plugins/property-based-testing/skills/property-based-testing in trailofbits/skills) into .agents/skills/property-based-testing in your project. Codex loads it when a task matches its description.
Can I use Property Based Testing in Cursor, Gemini CLI or GitHub Copilot?
Cursor, Gemini CLI, GitHub Copilot and OpenCode also load SKILL.md folders. With the skills CLI, run `npx skills add trailofbits/skills --skill property-based-testing -a cursor` (or -a gemini-cli, github-copilot or opencode for the others). To copy it by hand, put the folder in .cursor/skills/property-based-testing, .gemini/skills/property-based-testing, .github/skills/property-based-testing and .opencode/skills/property-based-testing in your project.
What does Property Based Testing need to run?
SKILL.md names no scripts, command-line tools or credentials: Property Based Testing is instructions for the agent only.
Does Property Based Testing access the network?
SKILL.md contains no URLs. Any network use would come from the scripts or tools the agent runs. This is read from the text; nothing was executed.
Is Property Based Testing safe to install?
Our automated static check of SKILL.md found no risky patterns, such as piping downloads into a shell, reading credential files or hidden Unicode. It is not a guarantee. Review the folder before installing.
What licence does Property Based Testing use?
Property Based Testing is published under the CC-BY-SA-4.0 licence (the repository's licence). It allows redistribution, so the full SKILL.md is shown on this page.
How many tokens does Property Based Testing use?
About 1.1k tokens (SKILL.md is roughly 4.3k characters). Agents keep only the skill's name and description in context until a task matches; then they load SKILL.md in full. Its references folder adds about 4.2k tokens, read only when the agent opens those files.
What are the alternatives to Property Based Testing?
Skills that share tags, products or a category with Property Based Testing: Stateful Invariant Testing (aviggiano/security, 144 stars), Fizz (pashov/skills, 1.2k stars), Fizz Convert (pashov/skills, 1.2k stars) and Fuzz Generator (alt-research2/SolidityGuard, 104 stars). The comparison table on this page puts their stars, adoption, token cost, safety result and licence side by side.
Who maintains Property Based Testing?
trailofbits (a GitHub organization, an official publisher) maintains it in trailofbits/skills, which has 7,400 GitHub stars. The repository holds 79 skills in this directory. The repository was last updated on October 2, 2026.
Source: trailofbits/skills on GitHub. Facts on this page come from the repository at the commit we read; the author's words are quoted as theirs.