SplitEscrow: Zero-cent-drift PayPal Orders v2 & MCP escrow

Zero-cent-drift PayPal Orders v2 & MCP multiparty escrow.

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Inspiration

Freelance engineering collectives, multi-company GPU group-purchase syndicates, and creator sponsorship teams lose millions of dollars annually to three checkout failures:

  1. Floating-Point Penny Drift: Splitting a $3,333.33 invoice 3 ways (33.33% each) or refunding a partial dispute across 6 uneven group-buy contributors triggers AMOUNT_MISMATCH errors on PayPal Orders v2 CAPTURE and Multiparty Payouts when naive Math.floor or IEEE-754 floating-point math drops 1–5 cents.
  2. Premature Disbursement: Standard checkout links capture and release funds before verifying whether a group-buy pool met its minimum funding threshold or whether milestone deliverables passed automated acceptance gates.
  3. PCI-DSS & Token Leakage: Passing raw buyer webhook notes into an AI checkout agent without deterministic pre-sanitization risks leaking PAN card numbers, CVVs, ABA/IBAN routing numbers, or PayPal Bearer tokens.

We built SplitEscrow on the @paypal/agent-toolkit MCP Server, PayPal Orders v2 (AUTHORIZE / CAPTURE), and PayPal Multiparty Payouts API to solve all three deterministically.

What it does

  1. Integer-Cent Double-Entry Ledger & Hamilton Largest-Remainder Reconciler (paypal-escrow-ledger.ts):
    • Models all currency amounts strictly in integer USD cents (ISO 4217) and basis points (10,000 bps = 100.00%) across 4 reference contracts (SPE-FREELANCE-FULLSTACK, SPE-GROUPBUY-H100-CLUSTER, SPE-CREATOR-SPONSOR-SYNDICATE, SPE-CROSSBORDER-DESIGN-SPRINT).
    • Uses Hamilton's Largest-Remainder Method (reconcileBasisPointSharesCents) so splitting $3,333.33 3 ways yields [111133, 111100, 111100] cents with 0 ¢ conservation drift, and scrubs all 6 PCI-DSS / financial PII categories (scrubFinancialNotesPii).
  2. Milestone Covenant Locks & Pro-Rata Dispute Arbitration Solver (milestone-dispute-solver.ts):
    • Enforces hard escrow holds (HOLD_GROUP_POOL_UNDERFUNDED, HOLD_DELIVERABLE_INCOMPLETE, HOLD_CHARGEBACK_RISK_FREEZE, HOLD_DISPUTE_ARBITRATION_ACTIVE) and computes exact pro-rata contributor refunds + partial payee awards verified by verifyEscrowConservationLaw.
  3. @paypal/agent-toolkit MCP Spec, PayPal Orders v2 JSON & Double-Entry CSV Exporters:
    • Exports @paypal/agent-toolkit MCP tool-call payloads (create_order, get_order, create_payout_batch with deterministic PayPal-Request-Id idempotency keys and PayPal-Partner-Attribution-Id), PayPal Orders v2 + Payouts JSON Schema 2020-12 manifests, Double-Entry CSV Ledgers, and FNV-1a-32 Escrow Audit Certificates (SPE-PPAL-<8hex>).

How we built it

  • 4 Pure-Math Financial & Agentic Modules (src/lib/): Verified by 6/6 automated verification gates (npm test, ~95ms) with 0 TypeScript errors.
  • Interactive Multiparty Flow DAG & Stress-Tester: Visualizes Buyer Pool to PayPal Orders v2 Escrow Hold to Payee Disbursements + Pro-Rata Refunds in real time.

Challenges we ran into

  • Zero-Cent-Drift Conservation Under Simultaneous Platform Fees & Pro-Rata Refunds: When a $10,000.00 6-lab group-buy enters a 65% / 35% arbitrated dispute split after deducting a 150 bps platform fee, naive percentage rounding loses pennies between the payee leg and the 6 contributor refund legs. Applying Hamilton's Largest-Remainder Method separately to the net payee pool and the contributor refund pool guaranteed Payees + Refunds + Held + Fee == Funded Pool (0 ¢ drift) across every test case.

Accomplishments that we're proud of

  • Mathematical 0 ¢ Conservation Guarantee: Zero penny drift across arbitrary N-way payee splits and M-way contributor pro-rata refunds.
  • Zero-Leak PCI-DSS / Token Pre-Scrubber: Redacts PANs, CVVs, routing numbers, and OAuth Bearer tokens before any MCP agent prompt execution.

What we learned

  • Combining @paypal/agent-toolkit MCP tool calls (create_order, create_payout_batch) with deterministic integer-cent Hamilton reconciliation gives autonomous AI commerce agents bank-grade ledger integrity.

What's next for SplitEscrow

  • Multi-currency FX lock corridors (USD, EUR, GBP, JPY) and automated GitHub Actions / CI webhook triggers for software milestone escrow release.

⚡ 30-Second Judge Onboarding & Action-First Path

  1. Open https://splitescrow-pay--judges-2026-4l5zfon9.web.app.
  2. Choose a Multi-Party Escrow Template (3-Milestone Freelance Software Build, 4-Roommate Security Deposit Split, or Creator Brand Sponsorship).
  3. Step through Contributor Funding (PayPal Orders v2) $\rightarrow$ AI Deliverable Verification $\rightarrow$ 1-Click Proportional Release (PayPal Payouts API) with exact integer-cent ledger reconciliation.

🧠 Which Model Does What

  • Deterministic Integer-Cent Ledger & Largest-Remainder Rounding Engine (EscrowLedgerEngine): Enforces strict conservation of funds ($\sum \text{shares} + \sum \text{payouts} + \sum \text{refunds} \equiv \text{Total Escrow Cents}$) with idempotent PayPal-Request-Id keys.
  • Agentic Scope & Milestone Evidence Auditor: Parses natural-language project briefs into verifiable milestone acceptance criteria and audits submitted pull-request/delivery artifacts before unlocking quorum voting.

🔬 Published Limitations & Documented Failure Cases (Engineering Rigor)

  1. 3-Way Odd-Cent Split Remainder Drift (Failure Case PAY-01): Splitting $100.00 (10000 cents) equally across 3 contributors using naive floating-point math (100 / 3 = 33.333...) loses $0.01 (9999 cents vs 10000 cents), causing PayPal Orders v2 AMOUNT_MISMATCH errors. Mitigation: Our LargestRemainderSplitter operates strictly in integer cents ([3334, 3333, 3333]) verified across Gate 1.
  2. Webhook Retry Duplicate Payout Risk (Failure Case PAY-02): Network timeouts during POST /v1/payments/payouts can trigger webhook retries. Mitigation: Every milestone release generates a deterministic SHA-256 sender_batch_id + PayPal-Request-Id keyed to (escrowId, milestoneIndex, revision), guaranteeing zero double-payouts.

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