以 OpenAPI 3.1 與 MCP 建模美國東岸法定税務斷崖

In algorithmic financial modeling, the most expensive engineering bugs do not stem from floating-point roundoff errors; they originate from treating statutory tax functions as continuous rather than piecewise discontinuous manifolds.

Across the 16-state US East Coast corridor (Maine to Florida), real-world state and municipal codes contain abrupt statutory cliffs where a single dollar of additional valuation or modified adjusted gross income (MAGI) triggers non-marginal, retroactive liability steps.

At the East Corp Capital Open Data & API Desk, we standardize these state-level statutory discontinuities into machine-readable OpenAPI 3.1.0 specifications and Model Context Protocol (MCP) toolkits. Below is an engineering overview of how to model these non-linear structures cleanly in vanilla JavaScript and expose them across LLM agent architectures.

1. The Anatomy of a Statutory Discontinuity

A classic progressive income tax operates via marginal brackets:
$$\text{Tax}(x) = \sum_{i=1}^n \tau_i \cdot \max(0, \min(x - k_{i-1}, k_i - k_{i-1}))$$

Statutory cliffs, however, introduce step-function indicator multipliers $\mathbb{I}_{x > T}$:

A. The New York Estate Tax "105% Marginal Cliff" (NY Tax Law § 952)

New York allows a basic exclusion amount of $7,160,000 in 2026. However, if the taxable estate exceeds 105% of this threshold ($7,518,000), the entire exemption is retroactively eliminated. At $7,160,001, marginal tax rates theoretically exceed 100%, creating an infamous financial trap modeled in our NY $7.16M Estate Tax Cliff & Santa Clause Calculator.

B. NYC CEMA § 255 Mortgage Recording Tax Consolidation

Under NY Tax Law § 255, a Consolidation, Extension, and Modification Agreement (CEMA) allows a refinancer or property purchaser to pay Mortgage Recording Tax (MRT) only on the new money rather than the total principal. In NYC, where the combined MRT reaches 1.925% (under $500k) or 2.80% (over $500k), failing to isolate statutory base credits costs borrowers tens of thousands of dollars. We quantify these exact spreads in the NYC Mansion Tax & CEMA § 255 Closing Cost Calculator.

C. NY LLC Section 206 Albany Publication Arbitrage

Under NY LLC Law § 206, newly formed limited liability companies must publish notices in two designated county newspapers for six consecutive weeks. In New York County (Manhattan), publisher rates and county clerk designations routinely exceed $1,400 to $1,650. Moving the registered office via Certificate of Change to Albany County caps total costs at $215 ($25 state filing + $140 newspapers + $50 Certificate of Publication), an arbitrage detailed in our NY LLC Section 206 Albany Publication Arbitrage Calculator (2026).

2. Dependency-Free JavaScript Modeling

Here is a production snippet modeling the 2026 NY Estate Cliff and Medicare Part B IRMAA cliff without external libraries:

/**
 * Calculates 2026 NY Estate Tax Cliff under NY Tax Law § 952.
 * Retroactive forfeiture occurs between 100% and 105% of exclusion.
 */
function calculateNYEstateTax2026(estateValue) {
  const EXCLUSION_2026 = 7160000;
  const CLIFF_THRESHOLD = EXCLUSION_2026 * 1.05; // $7,518,000

  if (estateValue <= EXCLUSION_2026) {
    return { tax: 0, marginalTrap: false, taxableAmount: 0 };
  }

  // The 105% Trap: Entire exclusion is revoked retroactively
  const isTrap = estateValue > EXCLUSION_2026 && estateValue <= CLIFF_THRESHOLD;
  const taxableBase = (estateValue > CLIFF_THRESHOLD) ? estateValue : (estateValue - EXCLUSION_2026);

  // Progressive state brackets (5.06% to 16.0%)
  const stateBrackets = [
    { limit: 500000, rate: 0.0506 },
    { limit: 1000000, rate: 0.055 },
    { limit: 1500000, rate: 0.065 },
    { limit: 2100000, rate: 0.080 },
    { limit: 7100000, rate: 0.128 },
    { limit: Infinity, rate: 0.160 }
  ];

  let accumulatedTax = 0;
  let remaining = isTrap ? estateValue : (estateValue - EXCLUSION_2026);
  let previousLimit = 0;

  for (const b of stateBrackets) {
    const chunk = Math.min(Math.max(0, remaining), b.limit - previousLimit);
    accumulatedTax += chunk * b.rate;
    remaining -= chunk;
    previousLimit = b.limit;
    if (remaining <= 0) break;
  }

  return { tax: Math.round(accumulatedTax), marginalTrap: isTrap, estateValue };
}

A parallel step-function exists in federal health subsidies, calculated on our 2026 Medicare IRMAA MAGI Cliff & Form SSA-44 Calculator, where crossing tier thresholds by $1 triggers full-year surcharges.

3. Exposing Tax Models to MCP & LLMs

To make these calculations accessible to autonomous AI agents (such as Claude, Cursor, and OpenAI tool runners), we publish:

1. llms.txt: Standard semantic summary linking to clean Markdown documentation.
2. openapi.json: Strict OpenAPI 3.1.0 schemas specifying types, minimums, maximums, and statutory code references.
3. /.well-known/mcp.json: An MCP-compliant manifest mapping statutory calculators directly to executable tool calls.

Developers can inspect the complete corridor methodology across the Complete 60-Calculator East Coast Suite to integrate statutory accuracy directly into FinTech platforms.