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Rules Engine Architecture

FairProp's core value proposition is its deterministic explainability combined with probabilistic AI safety nets. This document explains how the rules engine processes text.

🏗️ The 3-Layer "Swiss Cheese" Model

We use a layered approach to balance performance (latency) with accuracy (recall).

Layer 1: Deterministic Rules (Regex & Fuzzy)

~1ms latency

  • Mechanism: Aho-Corasick string matching and optimized RegEx patterns.
  • Purpose: Catch "Low Hanging Fruit" (e.g., "No kids", "White only").
  • Config: Defined in JSON (e.g., rules/federal/fha.json).
  • Explainability: 100%. We can point to the exact word and the specific law section.

Layer 2: Semantic Vector Search (Contextual)

~15ms latency

  • Mechanism: SentenceTransformers (all-MiniLM-L6-v2) embedding input text + ChromaDB query.
  • Purpose: Catch paraphrases that avoid keywords.
    • Example: "Perfect for active lifestyles" (Implies ableism/ageism).
  • Thresholding: Cosine similarity > 0.75 (tunable).

Layer 3: Neural Guardrail (Intent Classifier)

~100ms latency

  • Mechanism: Zero-shot classification (BART-large-mnli) or fine-tuned BERT.
  • Purpose: Disambiguation.
    • Example: "Walking distance to Catholic church" (Location description vs. Religious steering).
  • Logic: If Layer 1 & 2 are unsure (score 0.4-0.6), Layer 3 assists.

🔄 Rule Loading & Precedence

  1. Global Rules: Loaded first (Base FHA).
  2. Jurisdiction Overrides: Loaded second. Specific state rules can add to or modify sensitivity.
    • Example: "Source of Income" is NOT protected federally but IS protected in CA/NY.
    • If jurisdictions=['US-CA'], the engine enables source_of_income checks.

🛠️ Data Structure

Rules are stored as standardized JSON objects:

{
  "id": "FHA-FAM-001",
  "category": "Familial Status",
  "severity": "High",
  "patterns": [
    "no children",
    "adults only",
    "singles only"
  ],
  "associated_laws": ["Fair Housing Act Sec. 804(c)"],
  "suggestion": "Describe the property, not the tenant."
}

This structure ensures that every flag is traceable to a specific rule definition, essential for audit trails.