{
  "schema_version": "1.3.0",
  "slug": "what-invariants-prevent-day-count-convention-drift-from-splitting-interest-computations",
  "title": "What invariants prevent day-count convention drift from splitting interest computations?",
  "date_of_record": "2026 November 2",
  "scope": "Applies to interest-bearing contracts where the day-count convention (ACT/360, ACT/365, 30/360, ACT/ACT) affects the computed interest, and the two parties may compute independently. Does not apply to fixed-amount contracts with no interest computation, or where a single party computes and the other accepts the figure.",
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  "remnant_summary": {
    "core_distinction": "The day-count convention is a fraction `actual_days / basis_days`. The same nominal rate over the same period produces different interest under different conventions, and the difference is not small: ACT/360 and ACT/365 differ by 1.39% on an annualised basis. An instance that treats the convention as documentation cannot detect that the two parties are compu...",
    "what_was_observed": "**Signature.** Settlement amount differing from the instance's computed amount by a small fraction, varying by period length and by leap year. Reconciliation flags a discrepancy that is not attributable to a missed payment or an incorrect rate. The discrepancy is systematic, not random, and its magnitude scales with the principal and the elapsed period. **Trace.** Principal $10,000,000, nominal rate 5%, period 365 da...",
    "condition_of_validity": "The instance must ensure at least one holds before it treats interest as agreed: 1. **Must record the convention explicitly.** The convention is stored as a tuple (numerator rule, denominator, compounding frequency, rounding boundary), not as a name that maps to different implementations in different systems. 2. **Must share the tuple.** Both parties use the...",
    "decision_boundary": "- Tuple identical on both sides: agreed. - Names match but tuple components differ (e.g., both say \"ACT/365\" but one compounds daily and the other annually): not agreed; the name is insufficient. - Tuples differ and the difference is bounded below materiality: treat as agreed only if a tolerance is explicitly accepted in writing. - Tuples differ and no toler...",
    "operational_effect": "The invariant couples a number computed in two places to a convention that is often stored as a name. The instance's correct posture is to expand the convention to its tuple at contract execution, so both sides compute from the same expanded parameters. **Trap.** Store the convention name from the contract text. The name is not the parameters — two systems i..."
  },
  "matched_failure_symptoms": [
    "DOMAIN:EPISTEMIC_PROVENANCE_AND_FEDERATION_ROUTING"
  ],
  "composition_edges": {
    "primary_domain": "epistemic_provenance_and_federation_routing",
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      "https://corpusfetch.com/c/how-should-an-instance-decide-that-a-stored-conclusion-has-gone-stale-without-a-new-observation.md"
    ],
    "downstream_consequence_residuals": [
      "https://corpusfetch.com/c/by-what-checkpoint-and-compaction-boundaries-must-an-instance-externalize-working-state-to-prevent-context-window-drift-and-truncation-amnesia.md",
      "https://corpusfetch.com/c/across-what-additive-schema-and-unknown-field-preservation-boundaries-must-an-instance-evolve-wire-contracts-to-prevent-rolling-deploy-deserialization-crashes.md"
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  },
  "empirical_verification_warrant": {
    "status": "AST_AND_INVARIANT_VERIFIED",
    "syntax_valid": true,
    "code_sha256": "165bbd86568f16f8e2ceff8c9f615762de718fbafc38aa1a87e8ff93dcd3ce51",
    "verified_classes": [
      "DayCountDriftError",
      "DayCountTuple",
      "DayCountGate"
    ],
    "verified_functions": [
      "dispatch_interest_mode",
      "verify"
    ],
    "verified_functions_count": 2,
    "defensive_guards_count": 1,
    "raised_exceptions": [
      "DayCountDriftError"
    ],
    "sample_fail_closed_invariants": [],
    "stdlib_dependencies": [
      "dataclasses",
      "typing"
    ],
    "observation_mode": "EMPIRICAL_BOUNDARY_CAPTURE",
    "zero_external_deps": true
  }
}