Map reverse charge to the supplier regime's category

lines.go

@@ -196,10 +196,25 @@ func FromInvoiceTaxAmountToTaxCombo(taxAmount *stripe.InvoiceTotalTaxAmount, reg
 	// Instead of the percentage we can also look at the taxability_reason field.
 	// There are different types defined and we could map them to the tax categories in GOBL.
 
+	// A reverse charge is issued from the supplier's side: the supplier charges no
+	// tax and the customer self-accounts. We therefore express it with the supplier
+	// regime's own category and country, not the customer's. Stripe reports the
+	// customer's tax category (e.g. Australian GST for an AU customer), which an EU
+	// supplier's regime (e.g. Poland) doesn't define, so in that case we fall back to
+	// the regime's VAT category. An actual customer-country rate is not a reverse
+	// charge and is handled as a foreign tax below.
 	if taxAmount.TaxabilityReason == stripe.InvoiceTotalTaxAmountTaxabilityReasonReverseCharge {
-		tc.Country = regimeDef.Country
-		tc.Key = tax.KeyReverseCharge
-		return tc
+		cat := reverseChargeCategory(tc.Category, regimeDef)
+		if cat != "" {
+			tc.Category = cat
+			tc.Country = regimeDef.Country
+			tc.Key = tax.KeyReverseCharge
+			return tc
+		}
+		// The regime defines no reverse-charge-capable category (no VAT), so a valid
+		// reverse charge can't be expressed here: GOBL only accepts the reverse-charge
+		// key on VAT. Record the Stripe tax as-is below, without the key — forcing the
+		// key would make GOBL reject the line.
 	}
 
 	taxDate := newDateFromTS(taxAmount.TaxRate.Created, regimeDef.TimeLocation())
@@ -361,10 +376,22 @@ func FromCreditNoteTaxAmountToTaxCombo(taxAmount *stripe.CreditNoteTaxAmount, re
 	// Instead of the percentage we can also look at the taxability_reason field.
 	// There are different types defined and we could map them to the tax categories in GOBL.
 
+	// A reverse charge is issued from the supplier's side (see the invoice version of
+	// this function for the full rationale): express it with the supplier regime's own
+	// category and country, falling back to VAT when Stripe reports a foreign category
+	// the regime doesn't define.
 	if taxAmount.TaxabilityReason == stripe.CreditNoteTaxAmountTaxabilityReasonReverseCharge {
-		tc.Country = regimeDef.Country
-		tc.Key = tax.KeyReverseCharge
-		return tc
+		cat := reverseChargeCategory(tc.Category, regimeDef)
+		if cat != "" {
+			tc.Category = cat
+			tc.Country = regimeDef.Country
+			tc.Key = tax.KeyReverseCharge
+			return tc
+		}
+		// The regime defines no reverse-charge-capable category (no VAT), so a valid
+		// reverse charge can't be expressed here: GOBL only accepts the reverse-charge
+		// key on VAT. Record the Stripe tax as-is below, without the key — forcing the
+		// key would make GOBL reject the line.
 	}
 
 	taxDate := newDateFromTS(taxAmount.TaxRate.Created, regimeDef.TimeLocation())
@@ -395,6 +422,24 @@ func FromCreditNoteTaxAmountToTaxCombo(taxAmount *stripe.CreditNoteTaxAmount, re
 
 //Useful functions
 
+// reverseChargeCategory picks the category to use for a supplier-side reverse
+// charge. Stripe reports the category from the customer's perspective (e.g.
+// Australian GST for an AU customer), but a reverse charge is the supplier's
+// statement that it charges no tax, so it must sit in the supplier's regime. If
+// the regime already defines the reported category (e.g. VAT for an EU supplier)
+// we keep it; otherwise we fall back to the regime's VAT category. It returns an
+// empty code when the regime defines neither, signalling the caller to record the
+// tax as-is instead.
+func reverseChargeCategory(cat cbc.Code, regimeDef *tax.RegimeDef) cbc.Code {
+	if regimeDef.CategoryDef(cat) != nil {
+		return cat
+	}
+	if regimeDef.CategoryDef(tax.CategoryVAT) != nil {
+		return tax.CategoryVAT
+	}
+	return ""
+}
+
 // lookupRateValue looks up a tax rate and value from a regime definition.
 func lookupRateValue(sRate float64, country l10n.Code, cat cbc.Code, date *cal.Date) (rate *tax.RateDef, val *tax.RateValueDef) {
 	regimeDef := tax.RegimeDefFor(country)

lines_test.go

@@ -754,6 +754,32 @@ func TestFromInvoiceTaxAmountsReverseCharge(t *testing.T) {
 				},
 			},
 		},
+		{
+			// Stripe reports the customer's category (AU GST), which the supplier's
+			// regime (PL) doesn't define. A reverse charge is issued from the supplier's
+			// side, so it must fall back to the supplier regime's VAT category and country.
+			name: "reverse charge with foreign category not in supplier regime",
+			input: []*stripe.InvoiceTotalTaxAmount{
+				{
+					TaxRate: &stripe.TaxRate{
+						Country:             "AU",
+						TaxType:             stripe.TaxRateTaxTypeGST,
+						EffectivePercentage: 0.0,
+						Percentage:          10.0,
+						Created:             time.Date(2024, 1, 1, 0, 0, 0, 0, time.UTC).Unix(),
+					},
+					TaxabilityReason: stripe.InvoiceTotalTaxAmountTaxabilityReasonReverseCharge,
+				},
+			},
+			regime: l10n.PL, // Polish supplier billing an Australian customer
+			expected: tax.Set{
+				{
+					Category: tax.CategoryVAT,
+					Country:  l10n.PL.Tax(),
+					Key:      tax.KeyReverseCharge,
+				},
+			},
+		},
 	}
 
 	for _, tt := range tests {
@@ -794,6 +820,31 @@ func TestFromCreditNoteTaxAmountsReverseCharge(t *testing.T) {
 				},
 			},
 		},
+		{
+			// Foreign category (AU GST) not defined in the supplier regime (PL):
+			// falls back to the supplier regime's VAT category and country.
+			name: "credit note reverse charge with foreign category not in supplier regime",
+			input: []*stripe.CreditNoteTaxAmount{
+				{
+					TaxRate: &stripe.TaxRate{
+						Country:             "AU",
+						TaxType:             stripe.TaxRateTaxTypeGST,
+						EffectivePercentage: 0.0,
+						Percentage:          10.0,
+						Created:             time.Date(2024, 1, 1, 0, 0, 0, 0, time.UTC).Unix(),
+					},
+					TaxabilityReason: stripe.CreditNoteTaxAmountTaxabilityReasonReverseCharge,
+				},
+			},
+			regime: l10n.PL, // Polish supplier billing an Australian customer
+			expected: tax.Set{
+				{
+					Category: tax.CategoryVAT,
+					Country:  l10n.PL.Tax(),
+					Key:      tax.KeyReverseCharge,
+				},
+			},
+		},
 	}
 
 	for _, tt := range tests {
@@ -804,6 +855,30 @@ func TestFromCreditNoteTaxAmountsReverseCharge(t *testing.T) {
 	}
 }
 
+// TestReverseChargeFallbackWithoutVATRegime documents the fall-through: when the
+// supplier regime defines no reverse-charge-capable category (no VAT), the Stripe
+// tax is recorded as-is and the reverse-charge key is NOT forced onto the combo.
+// Forcing it would be invalid, as GOBL only accepts the reverse-charge key on VAT.
+func TestReverseChargeFallbackWithoutVATRegime(t *testing.T) {
+	input := []*stripe.InvoiceTotalTaxAmount{
+		{
+			TaxRate: &stripe.TaxRate{
+				Country:    "AU",
+				TaxType:    stripe.TaxRateTaxTypeGST,
+				Percentage: 10.0,
+				Created:    time.Date(2024, 1, 1, 0, 0, 0, 0, time.UTC).Unix(),
+			},
+			TaxabilityReason: stripe.InvoiceTotalTaxAmountTaxabilityReasonReverseCharge,
+		},
+	}
+
+	// US has no VAT category, so the reverse charge cannot be placed in the regime.
+	result := goblstripe.FromInvoiceTaxAmountsToTaxSet(input, tax.RegimeDefFor(l10n.US))
+	assert.Len(t, result, 1)
+	assert.Equal(t, tax.CategoryGST, result[0].Category)
+	assert.Empty(t, result[0].Key, "reverse-charge key must not be forced onto a combo the regime can't validate")
+}
+
 // Credit Notes
 
 func validCreditNoteLine() *stripe.CreditNoteLineItem {