Prototype. Microcosm-BE: synthetic Belgian population calibrated to Belgian administrative and national-accounts targets (sums of Chronicle facts; surveys validation-only). Support records: US survey donor pool, reweighted; a Belgian donor pool is the planned upgrade. Cross-engine agreement is evidence about the encodings. Reform cells computed 24 August on
Microcosm-BE v0.5: 155 engine-computed cells across 28 statute parameters, verified by isolated re-execution.
The population, checked
Microcosm-BE v0.5 aggregates against administrative truth, computed by both engines on the same population. Errors are relative to the Chronicle truth level; the misses are shown, not hidden — each carries a named mechanism in the cross-engine ledger, which classifies every one of EUROMOD BE_2025's 558 output columns as matched, explained, or a gap with an owner.
| Concept | Truth (2025 level) | Axiom rules engine | EUROMOD BE_2025 |
|---|
| Net taxable income | €293.7bn | +1.6% | +1.6% |
| Pensions (old-age + survivor) | €71.2bn | +0.1% | +0.1% |
| Pension recipients | 2,674.5k | +2.9% | +2.9% |
| Self-employment income | €43.1bn | +0.5% | +0.5% |
| Movable income | €40.2bn | −0.1% | −0.1% |
| Unemployment recipients | 990.1k | +0.2% | +0.2% |
| Unemployment benefits | €6.5bn | +84.5% | +0.3% |
| Social assistance (RIS) | €2.2bn | +113.5% | +379.9% |
| GRAPA (income guarantee, elderly) | €1.0bn | −83.6% | −100.0% |
| Child benefits | €8.6bn | −46.2% | −15.5% |
Child-benefit composition (regional cohort routing, rank, age and social supplements) was encoded on 24 August and matched to EUROMOD across a 432-case grid; it enters the population once the corpus release lands. An overnight feasibility audit proved which remaining misses are donor-pool structure — the specification for the Belgian donor upgrade.