Fair Lending Disparity Audit

Gated two-step chain for fair lending disparity analysis. Step 1 computes HMDA rate-spread reportability using the FFIEC APOR methodology (1.5 pp and 3.5 pp thresholds) to establish context for the audit population. Step 2 computes disparity metrics from aggregate counts: the 4/5ths (80%) adverse-impact ratio, two-proportion z-statistic, odds ratio, and standardized mean difference. A §21.4 gate exits with remediation required when AIR falls below 0.8; the default path completes as a clean audit pass. All computation operates on aggregate counts only — zero PII by construction.

OpenChainGraph · 2 Steps compliance mandate · §21.4 Gate Hash-Anchored §4 chain_depth:2 Zero PII · Aggregate Only

Chain Stages · 2 Steps · 1 Decision Gate (§21.4)

1 ROOT node

HMDA Rate Spread art-230-compute-hmda-rate-spread

HMDA rate-spread reportability screen using FFIEC APOR methodology. First-lien threshold 1.5 pp; subordinate-lien threshold 3.5 pp; HELOC first-lien 6.5 pp. Establishes whether the loan population warrants HMDA reporting context for the disparity audit in Step 2.

MCP Call · compute_hmda_rate_spread

{
  "jsonrpc": "2.0",
  "method": "tools/call",
  "params": {
    "name": "compute_hmda_rate_spread",
    "arguments": {
      "loan_apr_pct": 7.2,
      "apor_pct": 6.1,
      "lien_type": "first_lien",
      "product_type": "fixed",
      "rate_lock_date": "2026-07-04"
    }
  },
  "id": 1
}

↓

2 GATE & TERMINAL node §21.4 gate

Disparity Metrics art-229-compute-disparity-metrics

Computes adverse-impact ratio (4/5ths rule per 29 CFR §1607.4(D)), two-proportion z-statistic (one-tail and two-tail), odds ratio, and standardized mean difference from aggregate group counts. Gate: adverse_impact_ratio < 0.8 exits requiring remediation. Default (AIR ≥ 0.8) completes the audit as a pass. Aggregate counts only — zero PII.

△ Gate: if /adverse_impact_ratio < 0.8 → END (remediation required) | default → END (pass)

MCP Call · compute_disparity_metrics

{
  "jsonrpc": "2.0",
  "method": "tools/call",
  "params": {
    "name": "compute_disparity_metrics",
    "arguments": {
      "reference_group_label": "White Non-Hispanic",
      "comparison_group_label": "Black or African American",
      "reference_approved": 90,
      "reference_total": 100,
      "comparison_approved": 70,
      "comparison_total": 100
    }
  },
  "id": 2
}

Fair Lending Disparity Audit