Cross-border equity compensation test data

A grant of restricted stock units, incentive stock options, or non-qualified stock options is straightforward when the employee is a single-country resident throughout the grant-vest-exercise-sale lifecycle. The standard tax treatment applies: ordinary income at vest (RSUs), AMT preference at exercise (ISOs held), capital gain at sale. The same instrument issued to an internationally-mobile employee — common at every multinational tech employer — fragments into a per-country sourcing problem that domestic equity-comp engines silently underhandle.

This article is the deep dive on the data shapes a cross-border equity-comp engine needs: residency-day-counting for sourcing, per-country tax-treatment matrices, the trailing-liability problem when an employee leaves a country, treaty interaction, and the specific edge cases a synthetic test corpus has to model.

Why the simple model breaks

Domestic equity-comp tax treatment makes one big simplifying assumption: the grant, the vest, the exercise, and the sale all happen while the employee is a tax resident of one country (the US, in the domestic-only case). The taxable events are calculated under that one country's rules. Forms are filed with that one country's revenue authority.

Cross-border equity comp invalidates the assumption. A typical internationally-mobile pattern:

• Grant date: January 2022. Employee is a US tax resident.
• Cliff vest: January 2023. Employee has been in the UK on assignment since July 2022.
• Subsequent vests: every 3 months 2023-2026. Employee in UK through 2024, Singapore through 2025, US through 2026.
• Exercise (for options): March 2026. Employee in US.
• Sale: October 2026. Employee in Switzerland for a project.

The tax events fragment. The income at vest is sourced based on where the employee was during the grant-to-vest "earning period" — which means the cliff vest in January 2023 has roughly 6 months of US-source earning and 6 months of UK-source earning, with each portion taxable under the corresponding country's rules. The 2024 quarterly vests are (mostly) UK-source. The 2025 quarterly vests are Singapore-source. The 2026 vests are US-source. Each per-country sourcing produces its own tax computation, its own withholding obligation, its own filing requirement.

The per-country tax-treatment matrix

Different countries treat the same equity-comp event differently. A simplified matrix for RSU vesting:

For ISOs, the matrix is even more fragmented because the favorable US treatment (deferred ordinary income, AMT preference at exercise, LTCG at sale if holding-period requirements met) generally has no analog in other countries — most jurisdictions treat ISO exercise as a taxable ordinary-income event regardless of US treatment. An ISO held by an internationally-mobile employee can produce US AMT exposure and current-period ordinary income in the country of residence simultaneously.

Residency-day-counting and sourcing rules

The sourcing of equity-comp income to a particular country typically follows a "days worked" formula across the relevant earning period. For a US grant that vests in 2023 with 50% of the earning period in the US and 50% in the UK, 50% of the vest income is US-source and 50% UK-source — each subject to the corresponding country's tax under that country's rules.

The day-counting is the data-shape problem. The earning period for an RSU is typically grant-to-vest; for an option it can be grant-to-exercise (for the spread portion) and exercise-to-sale (for the appreciation). Each day in the earning period needs to be assigned to a country of work (not necessarily the country of residence — they can differ). The country-day allocation drives the sourcing.

// Internationally-mobile employee with day-counting
{
  "employee_id": "E-2022-MULTI-001",
  "grant_id": "G-2022-01-15-RSU-001",
  "grant_date": "2022-01-15",
  "shares_granted": 4000,
  "vesting_schedule": {
    "type": "graded_with_cliff",
    "cliff_months": 12,
    "total_months": 48,
    "tranche_size": 1
  },
  "country_residency_history": [
    { "from": "2022-01-15", "to": "2022-06-30", "country": "USA", "work_days": 121 },
    { "from": "2022-07-01", "to": "2024-12-31", "country": "GBR", "work_days": 657 },
    { "from": "2025-01-01", "to": "2025-12-31", "country": "SGP", "work_days": 261 },
    { "from": "2026-01-01", "to": "2026-12-31", "country": "USA", "work_days": 261 }
  ],
  "vest_events": [
    {
      "vest_date": "2023-01-15",
      "shares_vested": 1000,
      "fmv_at_vest": 18.42,
      "earning_period_days": 365,
      "country_allocation": {
        "USA": { "days": 168, "fraction": 0.46, "income_sourced": 8473.20 },
        "GBR": { "days": 197, "fraction": 0.54, "income_sourced": 9946.80 }
      }
    }
    /* ... more vest events with progressively different country mixes */
  ]
}

The platform's tax engine needs the country-residency history at the day grain (or at minimum at the month grain), and the per-vest country-allocation calculation has to produce per-country income amounts that map to per-country withholding obligations and per-country tax-form line items.

Treaty interaction

Tax treaties between the source and residence countries affect the per-country tax computation in two ways:

1. Double-tax relief. A treaty typically gives the residence country the primary tax claim on equity-comp income, with the source country withholding at a reduced rate (sometimes zero) and the residence country providing a credit for the foreign tax paid. The interaction with the foreign tax credit machinery is direct.

2. Tie-breaker rules. When an employee is potentially a tax resident of two countries simultaneously, treaty tie-breaker rules (under the OECD model, typically: permanent home, center of vital interests, habitual abode, citizenship) determine which country has primary residency for treaty purposes. The tie-breaker affects which country's tax rules apply to the equity-comp event.

Treaty article numbers vary by treaty, but the equity-comp-relevant articles are typically: Article 14 (Income from Employment), Article 21 (Other Income), Article 23 (Relief from Double Taxation). A platform handling cross-border equity comp has to consume both the source-country and residence-country treaty positions for any given employee.

What synthetic test data has to model

A realistic cross-border equity-comp test corpus needs:

How this shows up in our catalog

The cross-border bundles in the WealthSynth catalog include households with internationally-mobile employees holding cross-border equity comp. The country-residency history is generated at the month grain with realistic patterns (US → UK → Singapore → US is a typical multinational path; US → Switzerland → US is another). Per-vest country-allocation is computed at generation time and stored in the longitudinal data, so the platform's tax engine can either consume the pre-computed allocations or recompute against the residency history.

For the broader cross-border context, see Cross-Border & Multi-Currency Wealth. For the FTC machinery that pairs with cross-border equity comp, see Treaty-tier withholding and foreign tax credit modeling. For the domestic equity-comp foundation this builds on, see Building an equity compensation platform and the Equity Compensation theme.