diff --git a/DECISIONS.md b/DECISIONS.md
index 026419c0..598dc73e 100644
--- a/DECISIONS.md
+++ b/DECISIONS.md
@@ -52,3 +52,4 @@ One line per decision: `[YYYY-MM-DD] <decision>: <why>`. Appended by agents and
 [2026-07-09] Issue #916 (disconnect orphans ledger accounts): release claims by demoting cash_accounts rows to manual (bank_connection_id = null), never deleting: transactions.cash_account_id and ledger history reference the rows, and upsertFromPsd2 promotes a manual holder in place on reconnect so the bank lands back on its original BAS slot. Orphans predating the fix self-heal via a revoked-status filter in the allocator + collision guard (not data repair). When a promote collides with a duplicate row for the same connection+uid (callback mirrored onto an overflow slot pre-fix), the duplicate is deleted only if it has zero linked transactions, otherwise demoted: preserves FK links while freeing the slot. Picker-save rejections now render inline in the picker instead of routing to the sync-progress modal, whose parent-unmount-on-close made every save outcome invisible.
 [2026-07-09] #917 fix scoped to the current-year suggestion: "Sedan räkenskapsårets början" now resolves from the fiscal_periods row containing today, but the "Föregående räkenskapsårets start" custom option still derives from the recurring fiscal_year_start_month: the issue only covers the current-year date and a first-year company has no previous period row to resolve against.
 [2026-07-09] Issue #919 (duplicate guard should steer to matching): the match action lives INSIDE DuplicateBookingDialog (fetch to /api/reconciliation/bank/link + account resolution via /api/cash-accounts + resolveAccount, exactly the MatchVoucherDialog path) rather than in each call site or a new endpoint: both call sites (transactions page runCategorize + TransactionBookingDialog/JournalEntryForm) share one implementation and pass only the transaction context + an onMatched callback mirroring onLinked. Match is primary ONLY for ledger-only candidates (transaction_id null, the SIE-import case); sibling-transaction candidates keep "Bokför ändå" primary since N:1 matching is the edge case. No lib change: the candidate already carries the transaction_id discriminator, covered by existing tests.
+[2026-07-09] Demo/sandbox users could reach Stripe: an anonymous user on a sandbox company hit POST /api/billing/checkout and created a live Stripe customer (no subscription = no charge; exact tenant/customer IDs kept out of source control, see the incident PR). Root cause: neither billing/checkout nor billing/portal checked is_anonymous or is_sandbox, and withRouteContext lets anonymous users through (they are authenticated, just anonymously). Fix guards BOTH conditions in both routes (is_anonymous is the identity truth; guardSandbox matches the existing lib/sandbox/guard.ts "never charge a token" doctrine), belt-and-suspenders since anon and sandbox happen to co-occur today but are orthogonal. Anon check runs first (in-memory, no DB round trip). Also surfaced isDemo on GET /api/billing/status so the client hides the upgrade CTA instead of showing a button that 403s. Blast radius = exactly one company (no other sandbox/anon tenant had a stripe_customer_id). Left the stray company_subscriptions row + orphan Stripe customer for manual cleanup (prod write / external destructive action, not done unilaterally).
diff --git a/app/api/billing/__tests__/checkout.test.ts b/app/api/billing/__tests__/checkout.test.ts
index f0cdfbcc..1c76ef02 100644
--- a/app/api/billing/__tests__/checkout.test.ts
+++ b/app/api/billing/__tests__/checkout.test.ts
@@ -24,6 +24,14 @@ vi.mock('@/lib/supabase/server', () => ({
   createServiceClient: () => serviceSupabase,
 }))
 
+// Keep sandboxBlockedResponse real; stub only the DB-backed guardSandbox so the
+// route's company_settings read doesn't need a live supabase mock.
+const guardSandboxMock = vi.fn()
+vi.mock('@/lib/sandbox/guard', async (importOriginal) => {
+  const actual = await importOriginal<typeof import('@/lib/sandbox/guard')>()
+  return { ...actual, guardSandbox: (...args: unknown[]) => guardSandboxMock(...args) }
+})
+
 const customersCreate = vi.fn()
 const sessionsCreate = vi.fn()
 vi.mock('@/lib/stripe/client', () => ({
@@ -41,8 +49,9 @@ const routeParams = { params: Promise.resolve({}) }
 beforeEach(() => {
   vi.clearAllMocks()
   reset()
+  guardSandboxMock.mockResolvedValue(null)
   requireAuthMock.mockResolvedValue({
-    user: { id: 'user-1', email: 'u@example.com' },
+    user: { id: 'user-1', email: 'u@example.com', is_anonymous: false },
     supabase: {},
     error: null,
   })
@@ -61,6 +70,42 @@ describe('POST /api/billing/checkout', () => {
     expect(res.status).toBe(401)
   })
 
+  it('blocks an anonymous (demo) user with 403 and never touches Stripe', async () => {
+    requireAuthMock.mockResolvedValue({
+      user: { id: 'anon-1', email: null, is_anonymous: true },
+      supabase: {},
+      error: null,
+    })
+
+    const req = createMockRequest('/api/billing/checkout', { method: 'POST', body: {} })
+    const { status, body } = await parseJsonResponse<{ sandbox_blocked?: boolean }>(
+      await POST(req, routeParams),
+    )
+
+    expect(status).toBe(403)
+    expect(body.sandbox_blocked).toBe(true)
+    expect(customersCreate).not.toHaveBeenCalled()
+    expect(sessionsCreate).not.toHaveBeenCalled()
+    // The cheap identity check short-circuits before the DB-backed guard.
+    expect(guardSandboxMock).not.toHaveBeenCalled()
+  })
+
+  it('blocks a sandbox company with 403 and never touches Stripe', async () => {
+    const { sandboxBlockedResponse } = await import('@/lib/sandbox/guard')
+    guardSandboxMock.mockResolvedValue(sandboxBlockedResponse())
+
+    const req = createMockRequest('/api/billing/checkout', { method: 'POST', body: { plan: 'monthly' } })
+    const { status, body } = await parseJsonResponse<{ sandbox_blocked?: boolean }>(
+      await POST(req, routeParams),
+    )
+
+    expect(status).toBe(403)
+    expect(body.sandbox_blocked).toBe(true)
+    expect(guardSandboxMock).toHaveBeenCalledWith(expect.anything(), 'company-1')
+    expect(customersCreate).not.toHaveBeenCalled()
+    expect(sessionsCreate).not.toHaveBeenCalled()
+  })
+
   it('rejects an unknown plan with 400', async () => {
     const req = createMockRequest('/api/billing/checkout', {
       method: 'POST',
diff --git a/app/api/billing/__tests__/portal.test.ts b/app/api/billing/__tests__/portal.test.ts
index 4ed9678e..a081c7b5 100644
--- a/app/api/billing/__tests__/portal.test.ts
+++ b/app/api/billing/__tests__/portal.test.ts
@@ -21,6 +21,13 @@ vi.mock('@/lib/supabase/server', () => ({
   createServiceClient: () => serviceSupabase,
 }))
 
+// Keep sandboxBlockedResponse real; stub only the DB-backed guardSandbox.
+const guardSandboxMock = vi.fn()
+vi.mock('@/lib/sandbox/guard', async (importOriginal) => {
+  const actual = await importOriginal<typeof import('@/lib/sandbox/guard')>()
+  return { ...actual, guardSandbox: (...args: unknown[]) => guardSandboxMock(...args) }
+})
+
 const portalCreate = vi.fn()
 vi.mock('@/lib/stripe/client', () => ({
   getStripe: () => ({
@@ -35,7 +42,8 @@ const routeParams = { params: Promise.resolve({}) }
 beforeEach(() => {
   vi.clearAllMocks()
   reset()
-  requireAuthMock.mockResolvedValue({ user: { id: 'user-1' }, supabase: {}, error: null })
+  guardSandboxMock.mockResolvedValue(null)
+  requireAuthMock.mockResolvedValue({ user: { id: 'user-1', is_anonymous: false }, supabase: {}, error: null })
 })
 
 describe('POST /api/billing/portal', () => {
@@ -51,6 +59,39 @@ describe('POST /api/billing/portal', () => {
     expect(res.status).toBe(401)
   })
 
+  it('blocks an anonymous (demo) user with 403 and never touches Stripe', async () => {
+    requireAuthMock.mockResolvedValue({
+      user: { id: 'anon-1', is_anonymous: true },
+      supabase: {},
+      error: null,
+    })
+
+    const req = createMockRequest('/api/billing/portal', { method: 'POST', body: {} })
+    const { status, body } = await parseJsonResponse<{ sandbox_blocked?: boolean }>(
+      await POST(req, routeParams),
+    )
+
+    expect(status).toBe(403)
+    expect(body.sandbox_blocked).toBe(true)
+    expect(portalCreate).not.toHaveBeenCalled()
+    expect(guardSandboxMock).not.toHaveBeenCalled()
+  })
+
+  it('blocks a sandbox company with 403 and never touches Stripe', async () => {
+    const { sandboxBlockedResponse } = await import('@/lib/sandbox/guard')
+    guardSandboxMock.mockResolvedValue(sandboxBlockedResponse())
+
+    const req = createMockRequest('/api/billing/portal', { method: 'POST', body: {} })
+    const { status, body } = await parseJsonResponse<{ sandbox_blocked?: boolean }>(
+      await POST(req, routeParams),
+    )
+
+    expect(status).toBe(403)
+    expect(body.sandbox_blocked).toBe(true)
+    expect(guardSandboxMock).toHaveBeenCalledWith(expect.anything(), 'company-1')
+    expect(portalCreate).not.toHaveBeenCalled()
+  })
+
   it('returns 400 with NO_SUBSCRIPTION when the company has no Stripe customer', async () => {
     enqueue({ data: null })
 
diff --git a/app/api/billing/checkout/route.ts b/app/api/billing/checkout/route.ts
index a9009470..09339433 100644
--- a/app/api/billing/checkout/route.ts
+++ b/app/api/billing/checkout/route.ts
@@ -4,6 +4,7 @@ import { withRouteContext } from '@/lib/api/with-route-context'
 import { validateBody } from '@/lib/api/validate'
 import { createServiceClient } from '@/lib/supabase/server'
 import { getStripe, priceIdForPlan } from '@/lib/stripe/client'
+import { guardSandbox, sandboxBlockedResponse } from '@/lib/sandbox/guard'
 
 const CheckoutSchema = z.object({
   plan: z.enum(['monthly', 'yearly']).default('monthly'),
@@ -19,7 +20,16 @@ const CheckoutSchema = z.object({
  * still filters by the membership-validated companyId.
  */
 export const POST = withRouteContext('billing.checkout', async (request, ctx) => {
-  const { user, companyId, log } = ctx
+  const { user, supabase, companyId, log } = ctx
+
+  // Demo accounts must never reach Stripe. An anonymous user has no real
+  // identity to bill, and a sandbox company must never charge a token (same
+  // doctrine as lib/sandbox/guard.ts: no real external side effects). Both
+  // checks run before any Stripe call: this is the gap that let an anonymous
+  // demo user create a live Stripe customer.
+  if (user.is_anonymous) return sandboxBlockedResponse()
+  const blocked = await guardSandbox(supabase, companyId)
+  if (blocked) return blocked
 
   const validation = await validateBody(request, CheckoutSchema, {
     log,
diff --git a/app/api/billing/portal/route.ts b/app/api/billing/portal/route.ts
index b3914220..4560d595 100644
--- a/app/api/billing/portal/route.ts
+++ b/app/api/billing/portal/route.ts
@@ -2,6 +2,7 @@ import { NextResponse } from 'next/server'
 import { withRouteContext } from '@/lib/api/with-route-context'
 import { createServiceClient } from '@/lib/supabase/server'
 import { getStripe } from '@/lib/stripe/client'
+import { guardSandbox, sandboxBlockedResponse } from '@/lib/sandbox/guard'
 
 /**
  * Create a Stripe Billing Customer Portal session so the user can manage,
@@ -13,7 +14,14 @@ import { getStripe } from '@/lib/stripe/client'
  * by the membership-validated companyId.
  */
 export const POST = withRouteContext('billing.portal', async (_request, ctx) => {
-  const { companyId } = ctx
+  const { user, supabase, companyId } = ctx
+
+  // Demo accounts must never reach Stripe (see billing/checkout for the full
+  // rationale). Defense in depth: a demo tenant should never own a portal
+  // session even if a stray customer row exists.
+  if (user.is_anonymous) return sandboxBlockedResponse()
+  const blocked = await guardSandbox(supabase, companyId)
+  if (blocked) return blocked
 
   const service = createServiceClient()
   const { data: sub } = await service
diff --git a/app/api/billing/status/route.ts b/app/api/billing/status/route.ts
index 8aa2b8de..10a651da 100644
--- a/app/api/billing/status/route.ts
+++ b/app/api/billing/status/route.ts
@@ -2,6 +2,7 @@ import { NextResponse } from 'next/server'
 import { requireAuth } from '@/lib/auth/require-auth'
 import { requireCompanyId } from '@/lib/company/context'
 import { isStripeConfigured } from '@/lib/stripe/client'
+import { isSandboxCompany } from '@/lib/sandbox/guard'
 
 /**
  * Billing status for the client-rendered billing section (which lives inside the
@@ -20,6 +21,13 @@ export async function GET() {
     companyId = null
   }
 
+  // Demo accounts (anonymous user or sandbox company) can't check out, so the
+  // client hides the upgrade CTA rather than showing a button that only errors.
+  let isDemo = user.is_anonymous === true
+  if (companyId && !isDemo) {
+    isDemo = await isSandboxCompany(supabase, companyId)
+  }
+
   let isPaying = false
   let trialEndsAt: string | null = null
   if (companyId) {
@@ -44,5 +52,5 @@ export async function GET() {
     trialEndsAt = (trial as { expires_at: string | null } | null)?.expires_at ?? null
   }
 
-  return NextResponse.json({ isPaying, configured: isStripeConfigured(), trialEndsAt })
+  return NextResponse.json({ isPaying, configured: isStripeConfigured(), trialEndsAt, isDemo })
 }
diff --git a/components/settings/sections/BillingSettingsContent.tsx b/components/settings/sections/BillingSettingsContent.tsx
index ab78e14e..09159bf3 100644
--- a/components/settings/sections/BillingSettingsContent.tsx
+++ b/components/settings/sections/BillingSettingsContent.tsx
@@ -23,6 +23,7 @@ interface BillingView {
   configured: boolean
   trialEndsAt: string | null
   daysLeft: number | null
+  isDemo: boolean
 }
 
 /**
@@ -37,16 +38,16 @@ export function BillingSettingsContent() {
     let active = true
     fetch('/api/billing/status')
       .then((r) => r.json())
-      .then((d: { isPaying: boolean; configured: boolean; trialEndsAt: string | null }) => {
+      .then((d: { isPaying: boolean; configured: boolean; trialEndsAt: string | null; isDemo?: boolean }) => {
         if (!active) return
         // Compute days-left here (effect), not during render, to keep render pure.
         const daysLeft = d.trialEndsAt
           ? Math.max(0, Math.ceil((new Date(d.trialEndsAt).getTime() - Date.now()) / 86_400_000))
           : null
-        setView({ ...d, daysLeft })
+        setView({ ...d, daysLeft, isDemo: d.isDemo ?? false })
       })
       .catch(() => {
-        if (active) setView({ isPaying: false, configured: false, trialEndsAt: null, daysLeft: null })
+        if (active) setView({ isPaying: false, configured: false, trialEndsAt: null, daysLeft: null, isDemo: false })
       })
     return () => { active = false }
   }, [])
@@ -60,6 +61,32 @@ export function BillingSettingsContent() {
     )
   }
 
+  // Demo / sandbox account → can't check out. Show the value prop but point
+  // them to creating a real account instead of a pay button that would 403.
+  if (view.isDemo) {
+    return (
+      <Card>
+        <CardHeader>
+          <CardTitle className="text-base">Abonnemang</CardTitle>
+        </CardHeader>
+        <CardContent className="space-y-4">
+          <p className="text-sm text-muted-foreground">
+            Du provkör Accounted i en demo. Skapa ett riktigt konto för att aktivera
+            abonnemang, AI-assistent, bankkoppling och inlämning till Skatteverket.
+          </p>
+          <ul className="space-y-2">
+            {INCLUDED.map((item) => (
+              <li key={item} className="flex items-start gap-2 text-sm">
+                <Check className="h-4 w-4 mt-1 shrink-0 text-foreground" />
+                <span>{item}</span>
+              </li>
+            ))}
+          </ul>
+        </CardContent>
+      </Card>
+    )
+  }
+
   // Paying company → manage view.
   if (view.isPaying) {
     return (