Property Based Testing

---
name: property-based-testing
description: "Use when writing, maintaining, executing, and auditing property based testing test suites, assertions, mocks, and verification gates."
version: 6.0.0
last-updated: 2026-09-29
skills:
  - testing-patterns
  - tdd-workflow
  - clean-code
tools: Read, Grep, Glob, Bash, Edit, Write
scripts-binding:
  - .agent/scripts/test_runner.js
  - .agent/scripts/inner_loop_validator.js
  - .agent/scripts/lint_runner.js
  - .agent/scripts/verify_all.js
---

# Property-Based Testing — Invariant Verification

## Mandatory Pre-Flight Context Inspection
Before reading, generating, or refactoring code in the `property-based-testing` domain, inspect these 5 critical parameters:
1. **System Boundaries & Dependencies**: Verify that all required dependencies exist in target package manifests and environment paths.
2. **Runtime Context & Platform Invariants**: Confirm target platform constraints (Node.js, Browser, Mobile OS, Edge runtime) before applying APIs.
3. **Execution Guardrails**: Identify potential side-effects, state mutations, and unhandled asynchronous exceptions.
4. **Validation & Type Contracts**: Validate input data schemas and strict type constraints across all module interfaces.
5. **Observability & Proof of Execution**: Ensure execution produces tangible verification signals (terminal output, tests, metrics).


## Activation Boundaries
- **Activate when:** Use when writing, maintaining, executing, and auditing property based testing test suites, assertions, mocks, and verification gates.
- **DO NOT activate when:** The task falls outside the `property-based-testing` domain or is managed by a different dedicated specialist agent.


## 🔁 Multi-Pass Execution Protocol

| Pass | Phase | Core Action | Adaptive Depth |
|:---|:---|:---|:---|
| **Pass 1** | **Understand** | Deconstruct the user's explicit objective, implicit requirements, and platform constraints. | Fast / Standard / Deep |
| **Pass 2** | **Plan** | Decompose task into smallest logical steps; map dependencies, affected files, and tool calls. | Standard / Deep |
| **Pass 3** | **Execute** | Implement solution with production-grade craft, zero placeholders, and strict typing. | All Modes |
| **Pass 4** | **Verify** | Run linters, unit tests, or compiler checks to validate structural correctness. | All Modes |
| **Pass 5** | **Attack & Falsify** | Perform adversarial search for edge-case failures, counterexamples, race conditions, and traps. | Standard / Deep |
| **Pass 6** | **Harden** | Eliminate discovered friction, optimize performance, and harden error boundaries. | Standard / Deep |
| **Pass 7** | **Quality Gate** | Enforce Verification-Before-Completion (VBC) with concrete terminal proof before finalizing. | All Modes |


---

## 🛠️ Technical Architecture & Reference Recipes

## Fast-Check Arbitrary Generator & Vitest Invariant Test

```typescript
import fc from 'fast-check';
import { test, expect } from 'vitest';

function parseAmount(currencyStr: string): number | null {
  const cleaned = currencyStr.replace(/[^0-9.]/g, '');
  const num = parseFloat(cleaned);
  return isNaN(num) ? null : num;
}

test('currency parser invariant: non-negative parsed numbers', () => {
  fc.assert(
    fc.property(fc.tuple(fc.string(), fc.double({ min: 0, max: 1000000 })), ([prefix, val]) => {
      const input = `${prefix}$${val.toFixed(2)}`;
      const parsed = parseAmount(input);

      if (parsed !== null) {
        expect(parsed).toBeGreaterThanOrEqual(0);
        expect(Number.isFinite(parsed)).toBe(true);
      }
    }),
    { numRuns: 500 }, // Execute 500 generative iterations
  );
});
```

## 🚨 Edge-Case & Failure Mode Matrix

| Scenario | Risk | Production Mitigation |
|:---|:---|:---|
| **Empty or Null Inputs** | Unhandled exception or unexpected rendering collapse | Enforce fallback guards, optional chaining, and explicit empty state handlers |
| **Network Timeout / Latency** | Hanging operations or duplicate side-effects | Implement bounded abort controllers, exponential backoff, and idempotency keys |
| **Concurrency / Race Conditions** | Stale state overwrite or inconsistent data mutations | Use atomic transactions, mutex locking, or cancel-on-resubmit controls |
| **Invalid Schema / Malformed Payload** | Downstream runtime errors or security injection | Validate boundary payloads with Zod/Pydantic schemas prior to execution |
| **Resource / Memory Saturation** | OOM errors, frame drops, or memory leaks | Clean up listeners, cancel active timers, and enforce pagination/virtualization |


## 🤖 LLM-Specific Traps Table

| Anti-Pattern | What AI Commonly Does Wrong | What Is Actually Correct |
|:---|:---|:---|
| **Testing Implementation Details** | Asserting on private component state or internal helper functions | Assert on observable user behaviors, DOM roles, and network outcomes |
| **Flaky Async Assertion** | Using arbitrary setTimeout delays before asserting on asynchronous state | Use waitFor or condition-based queries that poll with timeout bounds |
| **Shared Mutable State** | Reusing database records across concurrent test runners | Isolate test databases per worker or execute in rolled-back transactions |


## 🏛️ Tribunal Verification & Guardrails

**Active Reviewers:** `test-engineer` · `qa-automation-engineer` · `logic-reviewer`
**Slash Command:** `/review` or `/tribunal-full`

### 🔬 Evidence Standard (Tri-State Verification)
Every finding, audit statement, or completion claim must classify its factual certainty:
- **`[OBSERVED]`**: Directly confirmed in the codebase or verified via executed terminal command.
- **`[INFERRED]`**: Logically deduced from code patterns, architectural data flow, or schema relations.
- **`[UNVERIFIED]`**: Speculative hypothesis or runtime possibility requiring active testing or measurement.

### ✅ Pre-Flight Self-Audit Checklist
```
✅ Do tests follow behavioral GIVEN/WHEN/THEN specifications?
✅ Are happy path, failure paths, and boundary conditions (0, null, max, unicode) covered?
✅ Are test mocks isolated and reset between successive test cases?
✅ Do E2E locators rely on stable ARIA attributes instead of brittle CSS selectors?
✅ Did I verify test suite passes deterministically without flaky race conditions?
```

### 🛑 Verification-Before-Completion (VBC) Protocol
**CRITICAL:** You must follow a strict "evidence-based closeout" state machine.
- ❌ **Forbidden:** Declaring a task complete because the output "looks correct."
- ✅ **Required:** You are explicitly forbidden from finalizing any task without providing **concrete evidence** (terminal output, passing test suites, compiler success, or equivalent operational proof) that your output works as intended.