diff --git a/.gitignore b/.gitignore
index 6cdca95..eda6e4d 100644
--- a/.gitignore
+++ b/.gitignore
@@ -4,6 +4,9 @@ node_modules
 .venv
 
 .codex
+.claude
+AGENTS.md
+CLAUDE.md
 
 # test artifacts
 data/
diff --git a/Makefile b/Makefile
index 5df1b8c..2a2a605 100644
--- a/Makefile
+++ b/Makefile
@@ -37,6 +37,7 @@ test: teststatic
 
 teststatic:
 	node tests/check_static_load.js
+	node tests/test_tax.js
 
 test-e2e:
 	"$(abspath ../../../node_modules/.bin/playwright)" test --config "$(CURDIR)/tests/e2e/playwright.config.ts"
diff --git a/static/js/tpos.js b/static/js/tpos.js
index 1a8a77b..de6783f 100644
--- a/static/js/tpos.js
+++ b/static/js/tpos.js
@@ -631,7 +631,11 @@ window.app = Vue.createApp({
       for (let item of this.cart.values()) {
         let tax = item.tax || this.taxDefault
         if (tax > 0) {
-          total += item.price * item.quantity * (tax * 0.01)
+          const gross = item.price * item.quantity
+          const taxRate = tax * 0.01
+          total += this.taxInclusive
+            ? (gross * taxRate) / (1 + taxRate)
+            : gross * taxRate
         }
       }
       this.cartTax = roundTposCurrencyAmount(total, this.currency)
diff --git a/tests/test_api.py b/tests/test_api.py
index 3df8632..6dce278 100644
--- a/tests/test_api.py
+++ b/tests/test_api.py
@@ -154,6 +154,35 @@ async def test_tpos_crud_settings_and_wrapper_token(client: AsyncClient):
     assert await get_tpos(tpos["id"]) is None
 
 
+@pytest.mark.asyncio
+async def test_manual_invoice_tax_value_extracts_inclusive_tax(
+    client: AsyncClient,
+):
+    _user, wallet = await _user_with_tabs("manualtaxtest")
+    headers = {"X-API-KEY": wallet.adminkey}
+    create = await client.post(
+        "/tpos/api/v1/tposs",
+        json=_tpos_payload(currency="EUR", tax_default=21),
+        headers=headers,
+    )
+    assert create.status_code == 201
+    tpos = create.json()
+
+    response = await client.post(
+        f"/tpos/api/v1/tposs/{tpos['id']}/invoices",
+        json={"amount": 350, "exchange_rate": 100},
+    )
+    assert response.status_code == 201
+
+    payment = await get_standalone_payment(
+        response.json()["payment_hash"], incoming=True
+    )
+    assert payment is not None
+    details = payment.extra["details"]
+    assert details["taxIncluded"] is True
+    assert details["taxValue"] == pytest.approx(3.5 * 0.21 / 1.21)
+
+
 @pytest.mark.asyncio
 async def test_tabs_endpoints_use_real_tabs_api(client: AsyncClient):
     _user, wallet = await _user_with_tabs("tabsuser")
diff --git a/tests/test_tax.js b/tests/test_tax.js
new file mode 100644
index 0000000..95f318c
--- /dev/null
+++ b/tests/test_tax.js
@@ -0,0 +1,100 @@
+const assert = require('node:assert/strict')
+const fs = require('node:fs')
+const path = require('node:path')
+const vm = require('node:vm')
+
+const root = path.resolve(__dirname, '..')
+const sandbox = {
+  Intl,
+  Math,
+  Number,
+  Object,
+  Array,
+  Date,
+  JSON,
+  Map,
+  Set,
+  Promise,
+  console,
+  windowMixin: {},
+  tpos: {},
+  i18n: {global: {locale: 'en-US'}},
+  Vue: {
+    createApp(config) {
+      return config
+    }
+  }
+}
+sandbox.window = sandbox
+sandbox.globalThis = sandbox
+
+const context = vm.createContext(sandbox)
+for (const file of ['static/js/tpos-utils.js', 'static/js/tpos.js']) {
+  vm.runInContext(fs.readFileSync(path.join(root, file), 'utf8'), context, {
+    filename: file
+  })
+}
+
+const cartTaxTotal = sandbox.app.methods.cartTaxTotal
+
+function taxTotal({items, taxDefault = 0, taxInclusive}) {
+  const state = {
+    cart: new Map(items.map((item, index) => [item.id || index, item])),
+    taxDefault,
+    taxInclusive,
+    currency: 'EUR',
+    cartTax: 0
+  }
+  cartTaxTotal.call(state)
+  return state.cartTax
+}
+
+assert.equal(
+  taxTotal({
+    items: [{price: 3.5, quantity: 1, tax: 21}],
+    taxInclusive: true
+  }),
+  0.61
+)
+assert.equal(
+  taxTotal({
+    items: [{price: 3.5, quantity: 1, tax: 21}],
+    taxInclusive: false
+  }),
+  0.74
+)
+assert.equal(
+  taxTotal({
+    items: [{price: 3.5, quantity: 2, tax: 21}],
+    taxInclusive: true
+  }),
+  1.21
+)
+assert.equal(
+  taxTotal({
+    items: [
+      {price: 10, quantity: 1, tax: 10},
+      {price: 5, quantity: 2, tax: 20}
+    ],
+    taxInclusive: true
+  }),
+  2.58
+)
+assert.equal(
+  taxTotal({
+    items: [{price: 3.5, quantity: 1, tax: 0}],
+    taxDefault: 0,
+    taxInclusive: true
+  }),
+  0
+)
+assert.equal(
+  taxTotal({
+    items: [{price: 3.5, quantity: 1}],
+    taxDefault: 21,
+    taxInclusive: true
+  }),
+  0.61
+)
+
+console.log('Tax calculation tests passed')
diff --git a/views_payments.py b/views_payments.py
index fb2abe4..ff2af05 100644
--- a/views_payments.py
+++ b/views_payments.py
@@ -81,12 +81,10 @@ async def api_tpos_create_invoice(
 
     if not data.details:
         tax_value = 0.0
-        if tpos.tax_default:
-            tax_value = (
-                (data.amount / data.exchange_rate) * (tpos.tax_default * 0.01)
-                if data.exchange_rate
-                else 0.0
-            )
+        if tpos.tax_default and data.exchange_rate:
+            gross_amount = data.amount / data.exchange_rate
+            tax_rate = tpos.tax_default * 0.01
+            tax_value = (gross_amount * tax_rate) / (1 + tax_rate)
         data.details = {
             "currency": tpos.currency,
             "exchangeRate": data.exchange_rate,