feat(vat): oss momskod keeps unionsordningen sales out of the momsdeklaration (#1797)

A Fortnox user with OSS sales hit the SIE import mapping step and found no
way to map OSS accounts: the momskod picker had no OSS option, 3106-style
labels ("Försäljning varor till annat EU-land, momspliktig") were suggested
as EU-varor (ruta 35), and an OSS revenue account with a sats set leaked into
ruta 05. Skatteverket: "Den försäljning som du redovisar i OSS ska du inte
redovisa i den vanliga momsdeklarationen."

- add the 'oss' revenue treatment: allowed for class 3 only, mapped to no
  ruta, default rate null (destination-country rate is not a Swedish sats);
  explicit 'oss' also overrides static BAS mappings such as 3001
- REVENUE_RUTA becomes a partial map where null = allowed but off the
  declaration, so the class gate no longer conflates "no ruta" with
  "purchase-only"
- SIE label suggestion: OSS/unionsordningen labels suggest 'oss';
  momspliktig EU-varor labels are left for review instead of ruta 35
- AccountVatTreatmentSchema derives from ACCOUNT_VAT_TREATMENTS instead of a
  second literal list
- migration widens the class-aware CHECK with 'oss' for class 3 (superset;
  NOT VALID + VALIDATE like its predecessor); pg test extended
- sv/en labels; unit tests for resolver, suggestion, declaration exclusion

Per-country VAT rates on invoices and the quarterly EUR/ECB OSS underlag
remain unbuilt (DECISIONS.md).


Claude-Session: https://claude.ai/code/session_01E3QB8GxJ9tS217agHjLRk7

Co-authored-by: Jakob Wennberg <311770904+jakobwennberg-oss@users.noreply.github.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

lib/vat/__tests__/account-vat-treatment.test.ts

@@ -3,6 +3,7 @@ import {
   defaultRateForVatTreatment,
   resolveVatTreatmentRuta,
   suggestVatTreatment,
+  vatTreatmentsForAccountClass,
 } from '../account-vat-treatment'
 
 describe('resolveVatTreatmentRuta', () => {
@@ -28,6 +29,16 @@ describe('resolveVatTreatmentRuta', () => {
     expect(resolveVatTreatmentRuta('export_goods', 4)).toBeNull()
     expect(resolveVatTreatmentRuta('exempt', 4)).toBeNull()
   })
+
+  it('keeps OSS revenue off the declaration and offers it only for revenue accounts', () => {
+    // Unionsordningen: declared in the OSS declaration, never in a ruta.
+    expect(resolveVatTreatmentRuta('oss', 3)).toBeNull()
+    expect(resolveVatTreatmentRuta('oss', 4)).toBeNull()
+    expect(vatTreatmentsForAccountClass(3)).toContain('oss')
+    expect(vatTreatmentsForAccountClass(3)).not.toContain('reverse_charge_non_eu_services')
+    expect(vatTreatmentsForAccountClass(4)).not.toContain('oss')
+    expect(defaultRateForVatTreatment('oss', 3)).toBeNull()
+  })
 })
 
 describe('suggestVatTreatment', () => {
@@ -61,6 +72,20 @@ describe('suggestVatTreatment', () => {
     })
   })
 
+  it('recognises OSS labels and leaves momspliktig EU-försäljning for review', () => {
+    // Fortnox has no OSS accounts in its base chart; users name their own
+    // per country and rate ("Försäljning enl. OSS (Spanien 21%)").
+    expect(suggestVatTreatment('3111', 'Försäljning enl. OSS (Spanien 21%)')).toEqual({
+      treatment: 'oss', rate: null,
+    })
+    expect(suggestVatTreatment('3112', 'Försäljning varor unionsordningen Tyskland')).toEqual({
+      treatment: 'oss', rate: null,
+    })
+    // BAS 3106 is Swedish moms below the OSS threshold or OSS above it; a
+    // ruta 35 (momsfri EU-leverans) suggestion is wrong either way.
+    expect(suggestVatTreatment('3106', 'Försäljning varor till annat EU-land, momspliktig')).toBeNull()
+  })
+
   it('does not match EU inside an unrelated word', () => {
     expect(suggestVatTreatment('4010', 'Reumatologiska varor')).toBeNull()
   })

lib/vat/account-vat-treatment.ts

@@ -5,6 +5,7 @@ export const ACCOUNT_VAT_TREATMENTS = [
   'reverse_charge_domestic', 'reverse_charge_eu_goods',
   'reverse_charge_eu_services', 'reverse_charge_non_eu_services',
   'export_goods', 'export_services', 'vmb', 'rental_voluntary',
+  'oss',
 ] as const
 
 export type AccountVatTreatment = typeof ACCOUNT_VAT_TREATMENTS[number]
@@ -15,13 +16,22 @@ export interface AccountVatRutaMapping {
   side: 'credit' | 'debit'
 }
 
-const REVENUE_RUTA: Record<AccountVatTreatment, keyof VatDeclarationRutor | null> = {
+/**
+ * Revenue (class 3) treatments and the momsdeklaration box they feed. A key
+ * mapped to `null` is a valid revenue treatment whose amounts are deliberately
+ * kept out of every ruta: OSS sales (unionsordningen) are declared only in the
+ * quarterly OSS declaration, never in the Swedish momsdeklaration
+ * (Skatteverket: "Den försäljning som du redovisar i OSS ska du inte redovisa
+ * i den vanliga momsdeklarationen"). Treatments missing from the map are
+ * purchase-only.
+ */
+const REVENUE_RUTA: Partial<Record<AccountVatTreatment, keyof VatDeclarationRutor | null>> = {
   standard_25: 'ruta05', reduced_12: 'ruta05', reduced_6: 'ruta05',
   exempt: 'ruta42', reverse_charge_domestic: 'ruta41',
   reverse_charge_eu_goods: 'ruta35', reverse_charge_eu_services: 'ruta39',
-  reverse_charge_non_eu_services: null,
   export_goods: 'ruta36', export_services: 'ruta40', vmb: 'ruta07',
   rental_voluntary: 'ruta08',
+  oss: null,
 }
 
 export function resolveVatTreatmentRuta(
@@ -48,6 +58,7 @@ export function isVatTreatmentAllowedForAccountClass(
   treatment: AccountVatTreatment,
   accountClass: number,
 ): boolean {
+  if (accountClass === 3) return treatment in REVENUE_RUTA
   return resolveVatTreatmentRuta(treatment, accountClass) !== null
 }
 
@@ -66,7 +77,9 @@ export function defaultRateForVatTreatment(
   if (treatment === 'reduced_12') return 0.12
   if (treatment === 'reduced_6') return 0.06
   if (treatment === 'exempt') return 0
-  if (treatment === 'vmb') return null
+  // VMB has no single sats; OSS accounts carry the destination country's
+  // rate, which is not a Swedish sats and never drives ruta 05 arithmetic.
+  if (treatment === 'vmb' || treatment === 'oss') return null
   if (treatment === 'rental_voluntary') return 0.25
   if (treatment === 'export_goods' || treatment === 'export_services') return 0
   return accountClass >= 4 && accountClass <= 6 ? 0.25 : 0
@@ -98,12 +111,20 @@ export function suggestVatTreatment(
   const rate = percent ? Number(percent[1]) / 100 : 0.25
 
   if (accountClass === 3) {
+    if (/\boss\b|one stop shop|unionsordning/.test(name)) return { treatment: 'oss', rate: null }
     if (/vmb|vinstmarginal/.test(name)) return { treatment: 'vmb', rate: null }
     if (/hyra|uthyrning/.test(name) && /frivillig/.test(name)) return { treatment: 'rental_voluntary', rate }
     if (/omvänd/.test(name)) return { treatment: 'reverse_charge_domestic', rate: 0 }
     if (/export|utanför eu/.test(name) && /var/.test(name)) return { treatment: 'export_goods', rate: 0 }
     if (/export|utanför eu/.test(name) && /tjänst|tjanst/.test(name)) return { treatment: 'export_services', rate: 0 }
-    if (/\beu\b/.test(name) && /var/.test(name)) return { treatment: 'reverse_charge_eu_goods', rate: 0 }
+    if (/\beu\b/.test(name) && /var/.test(name)) {
+      // BAS 3106 "Försäljning varor till annat EU-land, momspliktig" carries
+      // Swedish moms below the OSS threshold and destination-country moms
+      // (OSS) above it. The label cannot tell which, so leave the row for
+      // review instead of suggesting the momsfri ruta 35 treatment.
+      if (/momspliktig/.test(name)) return null
+      return { treatment: 'reverse_charge_eu_goods', rate: 0 }
+    }
     if (/\beu\b/.test(name) && /tjänst|tjanst/.test(name)) return { treatment: 'reverse_charge_eu_services', rate: 0 }
     if (/momsfri|utan moms/.test(name)) return { treatment: 'exempt', rate: 0 }
     if (/försälj|forsalj|intäkt|intakt/.test(name) && percent) {