diff --git a/DECISIONS.md b/DECISIONS.md
index 0ac8df32..dd1e56bf 100644
--- a/DECISIONS.md
+++ b/DECISIONS.md
@@ -1451,3 +1451,4 @@ One line per decision: `[YYYY-MM-DD] <decision>: <why>`. Appended by agents and
 [2026-09-01] Skeptic BLOCK on woo failed-order removal fixed by: freeze guards repeated on the DELETE statement (TOCTOU), is_paid=false + legacy_transaction_id null guards, orderRemoves gated on !orderIsPaid. No BEFORE DELETE trigger/RPC: the is_paid guard makes the cascade race unreachable (refund children only exist under paid parents).
 [2026-09-01] EB claim guard, skeptic round (PR #2116): active-company standing state (enabled cash_accounts + enabled accounts on its live-ish rows) outranks sibling claims COMPANY-wide, not row-wide: a bank-list renewal arrives on a fresh row and must not switch a working feed off. pending_selection rows neither claim nor remember deselections (unconfirmed callback output; also stops fail-closed writes from poisoning later connects). Guard-disabled accounts are never mirrored from the callback (mirroring enabled:false can promote the seeded primary 1930 manual row and disable it under a foreign identity) and the selection save skips allocation+mirror for disabled never-mirrored accounts, so the no-slot-burned invariant holds end to end. Deselection carry got a picker note; enabling an account clears the guard flags. Legacy both-companies-enabled overlaps stay untouched (Swedish review advisory: prod sweep is a follow-up, not this PR).
 [2026-09-01] EB claim guard round 2 (skeptic re-verify): pending_selection rows are asymmetric, not excluded: their ENABLED accounts still claim (attach-created rows hold offered accounts with no cash rows until saved; excluding them reopened the attach-window double-booking), while their disabled flags stay out of deselection memory (unconfirmed callback output). Both fetchAllRows claim queries order('id'): unordered .range() pagination can silently skip rows at page boundaries, and a skipped row is a missed claim (fail-open).
+[2026-09-01] Settings PUT cross-field VAT validations scoped to touched field groups (vat-completeness, 40m-monthly, periodisk sammanstallning), not fixed at onboarding: partial saves from surfaces without VAT fields (invoice bank-details dialog) were hard-blocked by pre-existing vat_registered-without-number state (Marketio Lab case). The invariant still holds on every save that touches its group; explicit null now counts as a clear instead of falling back to the stored value during validation. Onboarding-side VAT number collection left as follow-up.
diff --git a/app/api/invoices/[id]/send/__tests__/route.test.ts b/app/api/invoices/[id]/send/__tests__/route.test.ts
index 1e25fc93..f716176c 100644
--- a/app/api/invoices/[id]/send/__tests__/route.test.ts
+++ b/app/api/invoices/[id]/send/__tests__/route.test.ts
@@ -390,6 +390,20 @@ describe('POST /api/invoices/[id]/send', () => {
     },
   )
 
+  it('does not allocate a number or send when the registered company has no VAT number', async () => {
+    enqueue({ data: makeInvoice({ ...invoice, invoice_number: null }), error: null })
+    enqueue({ data: { ...company, vat_registered: true, vat_number: null }, error: null })
+
+    const request = createMockRequest('/api/invoices/inv-1/send', { method: 'POST' })
+    const response = await POST(request, createMockRouteParams({ id: 'inv-1' }))
+    const { status, body } = await parseJsonResponse<{ error: { code: string } }>(response)
+
+    expect(status).toBe(400)
+    expect(body.error.code).toBe('INVOICE_SEND_VAT_NUMBER_MISSING')
+    expect(mockReserveInvoiceDelivery).not.toHaveBeenCalled()
+    expect(mockSendEmail).not.toHaveBeenCalled()
+  })
+
   it('rejects custom recipients from a non-admin company member before allocation', async () => {
     enqueue({ data: invoice, error: null })
     enqueue({ data: company, error: null })
diff --git a/app/api/invoices/[id]/send/route.ts b/app/api/invoices/[id]/send/route.ts
index ad5f54de..64011ab2 100644
--- a/app/api/invoices/[id]/send/route.ts
+++ b/app/api/invoices/[id]/send/route.ts
@@ -41,6 +41,7 @@ import {
   hasRequiredInvoicePaymentAccount,
   invoiceRequiresPaymentAccount,
 } from '@/lib/invoices/payment-accounts'
+import { hasRequiredSellerVatNumber } from '@/lib/invoices/seller-vat-number'
 import { errorResponseFromCode } from '@/lib/errors/get-structured-error'
 import { guardSandbox } from '@/lib/sandbox/guard'
 import { requireCapability } from '@/lib/entitlements/has-capability'
@@ -197,6 +198,10 @@ export const POST = withRouteContext(
       })
     }
 
+    if (!hasRequiredSellerVatNumber(company as CompanySettings, invoice as Invoice)) {
+      return errorResponseFromCode('INVOICE_SEND_VAT_NUMBER_MISSING', opLog, { requestId })
+    }
+
     const hasAdditionalRecipients =
       (bodyResult.data.additional_cc?.length ?? 0) > 0
       || (bodyResult.data.additional_bcc?.length ?? 0) > 0
diff --git a/app/api/settings/__tests__/route.test.ts b/app/api/settings/__tests__/route.test.ts
index 76365e53..33aaad12 100644
--- a/app/api/settings/__tests__/route.test.ts
+++ b/app/api/settings/__tests__/route.test.ts
@@ -716,4 +716,148 @@ describe('PUT /api/settings', () => {
 
     expect(response.status).toBe(400)
   })
+
+  it('allows a bank-details save when stored VAT state is incomplete (bank dialog)', async () => {
+    // Pre-existing inconsistency: registered without a VAT number. The invoice
+    // bank-details dialog has no VAT fields and must not be blocked by it.
+    const settings = {
+      entity_type: 'aktiebolag',
+      vat_registered: true,
+      vat_number: null,
+      moms_period: 'quarterly',
+      onboarding_complete: true,
+    }
+    enqueueMany([
+      { data: settings },                                         // oldSettings
+      { data: { role: 'owner' } },                                // payment-instructions role gate
+      { data: { id: 's1', bank_name: 'Testbanken', bankgiro: '223-8194' } }, // update
+      { data: null, count: 5 },                                   // deadlines count
+    ])
+
+    const response = await PUT(createMockRequest('/api/settings', {
+      method: 'PUT',
+      body: { bank_name: 'Testbanken', bankgiro: '223-8194' },
+    }), { params: Promise.resolve({}) })
+    const { status } = await parseJsonResponse(response)
+
+    expect(status).toBe(200)
+  })
+
+  it('still rejects enabling VAT registration without a VAT number', async () => {
+    enqueue({
+      data: {
+        entity_type: 'aktiebolag',
+        vat_registered: false,
+        vat_number: null,
+        moms_period: 'quarterly',
+        onboarding_complete: true,
+      },
+    })
+
+    const response = await PUT(createMockRequest('/api/settings', {
+      method: 'PUT',
+      body: { vat_registered: true },
+    }), { params: Promise.resolve({}) })
+    const { status, body } = await parseJsonResponse<{ error: string }>(response)
+
+    expect(status).toBe(400)
+    expect(body.error).toContain('Momsregistreringsnummer')
+    expect(supabase.from).toHaveBeenCalledTimes(1)
+  })
+
+  it('rejects clearing the VAT number while the company stays registered', async () => {
+    enqueue({
+      data: {
+        entity_type: 'aktiebolag',
+        vat_registered: true,
+        vat_number: 'SE556012579001',
+        moms_period: 'quarterly',
+        onboarding_complete: true,
+      },
+    })
+
+    // Explicit null is a clear, not an omission: it must not fall back to the
+    // stored number during validation.
+    const response = await PUT(createMockRequest('/api/settings', {
+      method: 'PUT',
+      body: { vat_number: null },
+    }), { params: Promise.resolve({}) })
+    const { status, body } = await parseJsonResponse<{ error: string }>(response)
+
+    expect(status).toBe(400)
+    expect(body.error).toContain('Momsregistreringsnummer')
+    expect(supabase.from).toHaveBeenCalledTimes(1)
+  })
+
+  it('rejects clearing the moms period while the company stays registered', async () => {
+    enqueue({
+      data: {
+        entity_type: 'aktiebolag',
+        vat_registered: true,
+        vat_number: 'SE556012579001',
+        moms_period: 'quarterly',
+        onboarding_complete: true,
+      },
+    })
+
+    const response = await PUT(createMockRequest('/api/settings', {
+      method: 'PUT',
+      body: { moms_period: null },
+    }), { params: Promise.resolve({}) })
+    const { status, body } = await parseJsonResponse<{ error: string }>(response)
+
+    expect(status).toBe(400)
+    expect(body.error).toContain('Momsperiod')
+    expect(supabase.from).toHaveBeenCalledTimes(1)
+  })
+
+  it('rejects enabling periodisk sammanställning while the VAT registration is incomplete', async () => {
+    enqueue({
+      data: {
+        entity_type: 'aktiebolag',
+        vat_registered: true,
+        vat_number: null,
+        moms_period: 'quarterly',
+        vat_has_eu_trade: true,
+        onboarding_complete: true,
+      },
+    })
+
+    const response = await PUT(createMockRequest('/api/settings', {
+      method: 'PUT',
+      body: { periodisk_sammanstallning_enabled: true },
+    }), { params: Promise.resolve({}) })
+    const { status, body } = await parseJsonResponse<{ error: string }>(response)
+
+    expect(status).toBe(400)
+    expect(body.error).toContain('Momsregistreringsnummer')
+    expect(supabase.from).toHaveBeenCalledTimes(1)
+  })
+
+  it('allows an unrelated save when a stored 40m/period conflict already exists', async () => {
+    // Stored state violates the 40m-monthly rule; a save that touches neither
+    // group must still go through.
+    enqueueMany([
+      {
+        data: {
+          entity_type: 'aktiebolag',
+          vat_registered: true,
+          vat_number: 'SE556012579001',
+          moms_period: 'quarterly',
+          vat_taxable_base_over_40m: true,
+          onboarding_complete: true,
+        },
+      },
+      { data: { id: 's1', company_name: 'Testbolaget AB' } }, // update
+      { data: null, count: 5 },                               // deadlines count
+    ])
+
+    const response = await PUT(createMockRequest('/api/settings', {
+      method: 'PUT',
+      body: { company_name: 'Testbolaget AB' },
+    }), { params: Promise.resolve({}) })
+    const { status } = await parseJsonResponse(response)
+
+    expect(status).toBe(200)
+  })
 })
diff --git a/app/api/settings/route.ts b/app/api/settings/route.ts
index 0c30ecc7..82e15248 100644
--- a/app/api/settings/route.ts
+++ b/app/api/settings/route.ts
@@ -182,14 +182,31 @@ export const PUT = withRouteContext(
       body.employer_seasonal = false
     }
 
-    // Validate: VAT-registered must have VAT number (ML 11 kap. 8§) and moms period (SFL 26 kap.)
+    // Validate: VAT-registered must have VAT number (ML 17 kap. 24 §, the
+    // invoice needs it) and moms period (SFL 26 kap.).
+    // Each cross-field check runs only when the request touches a field in its
+    // group: a partial save of unrelated settings (e.g. the invoice bank-details
+    // dialog) must not be rejected for a pre-existing inconsistency it cannot
+    // fix from that surface. Explicit null counts as touched, it clears a value,
+    // so it must not fall back to the stored one during validation.
+    // PS/EU-trade edits are in the completeness group: enabling the EU sales
+    // list on an incomplete VAT registration must keep failing like it did
+    // when the check ran on every save.
     const effectiveVatRegistered = body.vat_registered ?? oldSettings?.vat_registered
-    const effectiveMomsPeriod = body.moms_period ?? oldSettings?.moms_period
-    if (effectiveVatRegistered === true) {
-      const effectiveVatNumber = body.vat_number ?? oldSettings?.vat_number
+    const effectiveMomsPeriod =
+      body.moms_period !== undefined ? body.moms_period : oldSettings?.moms_period
+    const touchesVatCompleteness =
+      body.vat_registered !== undefined ||
+      body.vat_number !== undefined ||
+      body.moms_period !== undefined ||
+      body.vat_has_eu_trade !== undefined ||
+      body.periodisk_sammanstallning_enabled !== undefined
+    if (touchesVatCompleteness && effectiveVatRegistered === true) {
+      const effectiveVatNumber =
+        body.vat_number !== undefined ? body.vat_number : oldSettings?.vat_number
       if (!effectiveVatNumber) {
         return NextResponse.json(
-          { error: 'Momsregistreringsnummer krävs när företaget är momsregistrerat (ML 11 kap. 8§)' },
+          { error: 'Momsregistreringsnummer krävs när företaget är momsregistrerat (ML 17 kap. 24 §)' },
           { status: 400 }
         )
       }
@@ -201,21 +218,34 @@ export const PUT = withRouteContext(
       }
     }
 
+    const touchesVat40m =
+      body.vat_registered !== undefined ||
+      body.vat_taxable_base_over_40m !== undefined ||
+      body.moms_period !== undefined
     const effectiveVatTaxableBaseOver40m =
       body.vat_taxable_base_over_40m ?? oldSettings?.vat_taxable_base_over_40m ?? false
-    if (effectiveVatRegistered && effectiveVatTaxableBaseOver40m && effectiveMomsPeriod !== 'monthly') {
+    if (
+      touchesVat40m &&
+      effectiveVatRegistered &&
+      effectiveVatTaxableBaseOver40m &&
+      effectiveMomsPeriod !== 'monthly'
+    ) {
       return NextResponse.json(
         { error: 'Företag med beskattningsunderlag över 40 miljoner kronor måste redovisa moms varje månad.' },
         { status: 400 },
       )
     }
 
+    const touchesPs =
+      body.periodisk_sammanstallning_enabled !== undefined ||
+      body.vat_registered !== undefined ||
+      body.vat_has_eu_trade !== undefined
     const effectivePsEnabled =
       body.periodisk_sammanstallning_enabled ??
       oldSettings?.periodisk_sammanstallning_enabled ??
       false
     const effectiveEuTrade = body.vat_has_eu_trade ?? oldSettings?.vat_has_eu_trade ?? false
-    if (effectivePsEnabled && (!effectiveVatRegistered || !effectiveEuTrade)) {
+    if (touchesPs && effectivePsEnabled && (!effectiveVatRegistered || !effectiveEuTrade)) {
       return NextResponse.json(
         { error: 'Periodisk sammanställning kräver momsregistrering och EU-handel.' },
         { status: 400 },
diff --git a/app/api/v1/companies/[companyId]/invoices/[id]/mark-sent/__tests__/route.test.ts b/app/api/v1/companies/[companyId]/invoices/[id]/mark-sent/__tests__/route.test.ts
index c496cce1..0bc91c43 100644
--- a/app/api/v1/companies/[companyId]/invoices/[id]/mark-sent/__tests__/route.test.ts
+++ b/app/api/v1/companies/[companyId]/invoices/[id]/mark-sent/__tests__/route.test.ts
@@ -255,6 +255,38 @@ describe('POST /api/v1/companies/:companyId/invoices/:id/mark-sent', () => {
     },
   )
 
+  it('rejects issuance when the registered company has no VAT number', async () => {
+    mockServiceClient.mockReturnValue(
+      makeFlexibleSupabase({
+        company_members: { data: { company_id: COMPANY_ID, role: 'owner' }, error: null },
+        invoices: { data: DRAFT_INVOICE, error: null },
+        company_settings: {
+          data: {
+            accounting_method: 'accrual',
+            entity_type: 'enskild_firma',
+            bankgiro: '123-4567',
+            vat_registered: true,
+            vat_number: null,
+          },
+          error: null,
+        },
+      }),
+    )
+
+    const res = await markSent(
+      makeMarkSentRequest(
+        `https://x.test/api/v1/companies/${COMPANY_ID}/invoices/${INVOICE_ID}/mark-sent`,
+      ),
+      detailParams(COMPANY_ID, INVOICE_ID),
+    )
+
+    expect(res.status).toBe(400)
+    const body = await res.json()
+    expect(body.error.code).toBe('INVOICE_SEND_VAT_NUMBER_MISSING')
+    expect(mockEnsureInvoiceNumber).not.toHaveBeenCalled()
+    expect(mockCreateJournalEntry).not.toHaveBeenCalled()
+  })
+
   it('rejects delivery notes with VALIDATION_ERROR (regardless of status)', async () => {
     // Critical: the delivery-note guard must run BEFORE the status check
     // so a sent delivery note still returns 400 (per the documented
diff --git a/app/api/v1/companies/[companyId]/invoices/[id]/mark-sent/route.ts b/app/api/v1/companies/[companyId]/invoices/[id]/mark-sent/route.ts
index ac362411..58796f2e 100644
--- a/app/api/v1/companies/[companyId]/invoices/[id]/mark-sent/route.ts
+++ b/app/api/v1/companies/[companyId]/invoices/[id]/mark-sent/route.ts
@@ -51,6 +51,7 @@ import { recordManualInvoiceDelivery } from '@/lib/invoices/invoice-deliveries'
 import {
   hasRequiredInvoicePaymentAccount,
 } from '@/lib/invoices/payment-accounts'
+import { hasRequiredSellerVatNumber } from '@/lib/invoices/seller-vat-number'
 import { eventBus } from '@/lib/events'
 import type { CompanySettings, EntityType, Invoice } from '@/types'
 
@@ -217,7 +218,7 @@ export const POST = withApiV1<{ params: Promise<{ companyId: string; id: string
     // decision, payable invoices need a currency-matching account.
     const { data: settings, error: settingsError } = await ctx.supabase
       .from('company_settings')
-      .select('accounting_method, defer_invoice_booking, entity_type, invoice_payment_accounts, bank_name, clearing_number, account_number, bankgiro, plusgiro, swish, iban, bic')
+      .select('accounting_method, defer_invoice_booking, entity_type, invoice_payment_accounts, bank_name, clearing_number, account_number, bankgiro, plusgiro, swish, iban, bic, vat_registered, vat_number')
       .eq('company_id', ctx.companyId!)
       .maybeSingle()
     if (settingsError || !settings) {
@@ -238,6 +239,11 @@ export const POST = withApiV1<{ params: Promise<{ companyId: string; id: string
         details: { currency: typed.currency },
       })
     }
+    if (!hasRequiredSellerVatNumber(companySettings, typed)) {
+      return v1ErrorResponseFromCode('INVOICE_SEND_VAT_NUMBER_MISSING', ctx.log, {
+        requestId: ctx.requestId,
+      })
+    }
     const accountingMethod = companySettings.accounting_method ?? 'accrual'
     const entityType = (companySettings.entity_type ?? 'enskild_firma') as EntityType
     const isRealInvoice = !typed.document_type || typed.document_type === 'invoice'
diff --git a/app/api/v1/companies/[companyId]/invoices/[id]/send/__tests__/route.test.ts b/app/api/v1/companies/[companyId]/invoices/[id]/send/__tests__/route.test.ts
index e9579ede..b40dc0c7 100644
--- a/app/api/v1/companies/[companyId]/invoices/[id]/send/__tests__/route.test.ts
+++ b/app/api/v1/companies/[companyId]/invoices/[id]/send/__tests__/route.test.ts
@@ -353,6 +353,30 @@ describe('POST /api/v1/companies/:companyId/invoices/:id/send', () => {
     },
   )
 
+  it('rejects issuance when the registered company has no VAT number', async () => {
+    mockServiceClient.mockReturnValue(
+      makeFlexibleSupabase({
+        company_members: { data: { company_id: COMPANY_ID, role: 'owner' }, error: null },
+        invoices: { data: DRAFT_INVOICE, error: null },
+        company_settings: {
+          data: { ...COMPANY_SETTINGS, vat_registered: true, vat_number: null },
+          error: null,
+        },
+      }),
+    )
+
+    const res = await sendInvoice(
+      makeRequest(`https://x.test/api/v1/companies/${COMPANY_ID}/invoices/${INVOICE_ID}/send`),
+      detailParams(COMPANY_ID, INVOICE_ID),
+    )
+
+    expect(res.status).toBe(400)
+    const body = await res.json()
+    expect(body.error.code).toBe('INVOICE_SEND_VAT_NUMBER_MISSING')
+    expect(mockEnsureInvoiceNumber).not.toHaveBeenCalled()
+    expect(mockSendEmail).not.toHaveBeenCalled()
+  })
+
   it('returns VALIDATION_ERROR for malformed JSON', async () => {
     mockServiceClient.mockReturnValue(
       makeFlexibleSupabase({
diff --git a/app/api/v1/companies/[companyId]/invoices/[id]/send/route.ts b/app/api/v1/companies/[companyId]/invoices/[id]/send/route.ts
index 5aaaeb33..5c3a7c37 100644
--- a/app/api/v1/companies/[companyId]/invoices/[id]/send/route.ts
+++ b/app/api/v1/companies/[companyId]/invoices/[id]/send/route.ts
@@ -78,6 +78,7 @@ import {
   hasRequiredInvoicePaymentAccount,
   invoiceRequiresPaymentAccount,
 } from '@/lib/invoices/payment-accounts'
+import { hasRequiredSellerVatNumber } from '@/lib/invoices/seller-vat-number'
 import { eventBus } from '@/lib/events'
 import { guardSandbox } from '@/lib/sandbox/guard'
 import { requireCapability } from '@/lib/entitlements/has-capability'
@@ -357,6 +358,12 @@ export const POST = withApiV1<{ params: Promise<{ companyId: string; id: string
       })
     }
 
+    if (!hasRequiredSellerVatNumber(settings, typed)) {
+      return v1ErrorResponseFromCode('INVOICE_SEND_VAT_NUMBER_MISSING', ctx.log, {
+        requestId: ctx.requestId,
+      })
+    }
+
     const hasAdditionalRecipients =
       (bodyResult.data.additional_cc?.length ?? 0) > 0
       || (bodyResult.data.additional_bcc?.length ?? 0) > 0
diff --git a/lib/email/__tests__/__snapshots__/brand-mail-snapshots.test.ts.snap b/lib/email/__tests__/__snapshots__/brand-mail-snapshots.test.ts.snap
index 0c690232..9a46fd55 100644
--- a/lib/email/__tests__/__snapshots__/brand-mail-snapshots.test.ts.snap
+++ b/lib/email/__tests__/__snapshots__/brand-mail-snapshots.test.ts.snap
@@ -176,7 +176,7 @@ exports[`invoice mail (template class: invoice) > unbranded: template output unc
       
       <p style="margin: 10px 0 0 0; color: #999; font-size: 12px;">
         Org.nr: 556000-0000
-        
+         | VAT: SE556012579001
          | Innehar F-skattsedel
       </p>
       
@@ -314,7 +314,7 @@ exports[`reminder mail (template class: reminder) > unbranded: canonical action
         
         <p style="margin: 10px 0 0 0; color: #999; font-size: 12px;">
           Org.nr: 199001011234
-          
+           | VAT: SE556012579001
         </p>
         
       </div>
diff --git a/lib/errors/__tests__/get-error-message.test.ts b/lib/errors/__tests__/get-error-message.test.ts
index 1a97c2c4..1d9155f9 100644
--- a/lib/errors/__tests__/get-error-message.test.ts
+++ b/lib/errors/__tests__/get-error-message.test.ts
@@ -285,7 +285,7 @@ describe('getErrorMessage: payment-file route messages surface (issue #945)', ()
 
   it('surfaces a "... krävs ..." message instead of the generic 400', () => {
     const msg = getErrorMessage(
-      { error: 'Momsregistreringsnummer krävs när företaget är momsregistrerat (ML 11 kap. 8§)' },
+      { error: 'Momsregistreringsnummer krävs när företaget är momsregistrerat (ML 17 kap. 24 §)' },
       { context: 'settings', statusCode: 400 },
     )
     expect(msg).toContain('krävs')
diff --git a/lib/errors/structured-errors.ts b/lib/errors/structured-errors.ts
index 7081536e..754d3d9c 100644
--- a/lib/errors/structured-errors.ts
+++ b/lib/errors/structured-errors.ts
@@ -1151,6 +1151,14 @@ const INVOICE: Record<string, StructuredErrorEntry> = {
       description: 'Lägg till ett betalningskonto med IBAN för fakturans valuta under Inställningar → Fakturering.',
     },
   },
+  INVOICE_SEND_VAT_NUMBER_MISSING: {
+    httpStatus: 400,
+    message_sv: 'Företaget är momsregistrerat men saknar momsregistreringsnummer, som måste anges på fakturan (ML 17 kap. 24 §). Lägg till det under Inställningar → Skatt innan du skickar fakturan.',
+    message_en: 'The company is VAT-registered but has no VAT number, which is a mandatory invoice element (ML 17 kap. 24 §). Add it under Inställningar → Skatt (Settings → Tax) before issuing the invoice.',
+    remediation: {
+      description: 'Lägg till företagets momsregistreringsnummer under Inställningar → Skatt.',
+    },
+  },
   INVOICE_SEND_NUMBER_ASSIGN_FAILED: {
     httpStatus: 500,
     message_sv: 'Kunde inte tilldela fakturanummer.',
diff --git a/lib/invoices/__tests__/issue-and-book-invoice.test.ts b/lib/invoices/__tests__/issue-and-book-invoice.test.ts
index 69aee7a7..024f861a 100644
--- a/lib/invoices/__tests__/issue-and-book-invoice.test.ts
+++ b/lib/invoices/__tests__/issue-and-book-invoice.test.ts
@@ -126,6 +126,30 @@ describe('issueAndBookInvoice', () => {
     expect(mockCreateInvoiceJournalEntry).not.toHaveBeenCalled()
   })
 
+  it('rejects a VAT-registered company without VAT number, before number allocation', async () => {
+    const broken = { ...settings, vat_registered: true, vat_number: null } as CompanySettings
+
+    const result = await issue(makeDraft({ invoice_number: null }), broken)
+
+    expect(result).toEqual({ ok: false, errorCode: 'INVOICE_SEND_VAT_NUMBER_MISSING' })
+    expect(mockEnsureInvoiceNumber).not.toHaveBeenCalled()
+    expect(mockCreateInvoiceJournalEntry).not.toHaveBeenCalled()
+  })
+
+  it('issues for an unregistered company without VAT number', async () => {
+    enqueue({ data: [{ id: 'inv-1' }], error: null }) // CAS flip
+    const unregistered = {
+      ...settings,
+      vat_registered: false,
+      vat_number: null,
+      defer_invoice_booking: true,
+    } as CompanySettings
+
+    const result = await issue(makeDraft(), unregistered)
+
+    expect(result).toEqual({ ok: true, journalEntryId: null, partialFailures: [] })
+  })
+
   it('fails with INVOICE_CREATE_NUMBER_ASSIGN_FAILED when numbering fails', async () => {
     mockEnsureInvoiceNumber.mockRejectedValue(new Error('sequence exhausted'))
 
diff --git a/lib/invoices/__tests__/recurring-schedule-service.test.ts b/lib/invoices/__tests__/recurring-schedule-service.test.ts
index 7fe14e6f..43914ab0 100644
--- a/lib/invoices/__tests__/recurring-schedule-service.test.ts
+++ b/lib/invoices/__tests__/recurring-schedule-service.test.ts
@@ -508,6 +508,22 @@ describe('executeRecurringSchedule auto-send', () => {
     expect(mockSendEmail).not.toHaveBeenCalled()
   })
 
+  it('keeps the invoice a draft when the registered company has no VAT number', async () => {
+    enqueue({ data: customer, error: null })
+    enqueue({ data: { vat_registered: true }, error: null }) // company_settings VAT gate
+    enqueue({ data: makeInsertedInvoice(), error: null })
+    enqueue({ data: null, error: null })
+    enqueue({ data: makeCompleteInvoice(), error: null })
+    enqueue({ data: { ...company, vat_registered: true, vat_number: null }, error: null }) // company_settings (auto-send)
+
+    const result = await executeRecurringSchedule(client, makeSchedule(), today)
+
+    expect(result.autoSent).toBe(false)
+    expect(result.warning).toContain('Auto-utskick misslyckades')
+    expect(mockReserveInvoiceDelivery).not.toHaveBeenCalled()
+    expect(mockSendEmail).not.toHaveBeenCalled()
+  })
+
   it('does not reserve an auto-send delivery when configured recipients exceed the limit', async () => {
     enqueue({ data: customer, error: null })
     enqueue({ data: { vat_registered: true }, error: null }) // company_settings VAT gate
diff --git a/lib/invoices/__tests__/seller-vat-number.test.ts b/lib/invoices/__tests__/seller-vat-number.test.ts
new file mode 100644
index 00000000..78c47320
--- /dev/null
+++ b/lib/invoices/__tests__/seller-vat-number.test.ts
@@ -0,0 +1,40 @@
+import { describe, it, expect } from 'vitest'
+import { hasRequiredSellerVatNumber } from '../seller-vat-number'
+
+const realInvoice = { credited_invoice_id: null, document_type: 'invoice' as const }
+
+describe('hasRequiredSellerVatNumber', () => {
+  it('requires a VAT number for a registered company issuing a real invoice', () => {
+    expect(
+      hasRequiredSellerVatNumber({ vat_registered: true, vat_number: null }, realInvoice),
+    ).toBe(false)
+    expect(
+      hasRequiredSellerVatNumber({ vat_registered: true, vat_number: '   ' }, realInvoice),
+    ).toBe(false)
+    expect(
+      hasRequiredSellerVatNumber(
+        { vat_registered: true, vat_number: 'SE556012579001' },
+        realInvoice,
+      ),
+    ).toBe(true)
+  })
+
+  it('does not require a VAT number for an unregistered company', () => {
+    expect(
+      hasRequiredSellerVatNumber({ vat_registered: false, vat_number: null }, realInvoice),
+    ).toBe(true)
+  })
+
+  it('exempts credit notes, proformas, and delivery notes', () => {
+    const broken = { vat_registered: true, vat_number: null }
+    expect(
+      hasRequiredSellerVatNumber(broken, { credited_invoice_id: 'inv-0', document_type: 'invoice' }),
+    ).toBe(true)
+    expect(
+      hasRequiredSellerVatNumber(broken, { credited_invoice_id: null, document_type: 'proforma' }),
+    ).toBe(true)
+    expect(
+      hasRequiredSellerVatNumber(broken, { credited_invoice_id: null, document_type: 'delivery_note' }),
+    ).toBe(true)
+  })
+})
diff --git a/lib/invoices/issue-and-book-invoice.ts b/lib/invoices/issue-and-book-invoice.ts
index bfce057b..4258928b 100644
--- a/lib/invoices/issue-and-book-invoice.ts
+++ b/lib/invoices/issue-and-book-invoice.ts
@@ -14,6 +14,7 @@ import {
   hasRequiredInvoicePaymentAccount,
   invoiceRequiresPaymentAccount,
 } from '@/lib/invoices/payment-accounts'
+import { hasRequiredSellerVatNumber } from '@/lib/invoices/seller-vat-number'
 import { uploadDocument } from '@/lib/core/documents/document-service'
 import type { Logger } from '@/lib/logger'
 import type {
@@ -167,6 +168,10 @@ export async function issueAndBookInvoice(
     }
   }
 
+  if (!hasRequiredSellerVatNumber(settings, invoice as Invoice)) {
+    return { ok: false, errorCode: 'INVOICE_SEND_VAT_NUMBER_MISSING' }
+  }
+
   // Assign the number only after all payment-instruction guards pass.
   try {
     await ensureInvoiceNumber(supabase, companyId, invoice as Invoice)
diff --git a/lib/invoices/recurring-schedule-service.ts b/lib/invoices/recurring-schedule-service.ts
index d362023c..33c61fa7 100644
--- a/lib/invoices/recurring-schedule-service.ts
+++ b/lib/invoices/recurring-schedule-service.ts
@@ -53,6 +53,7 @@ import {
 import {
   hasRequiredInvoicePaymentAccount,
 } from '@/lib/invoices/payment-accounts'
+import { hasRequiredSellerVatNumber } from '@/lib/invoices/seller-vat-number'
 import { createLogger } from '@/lib/logger'
 import type {
   Invoice,
@@ -581,6 +582,12 @@ async function sendInvoiceFromSchedule(
     })
     return false
   }
+  if (!hasRequiredSellerVatNumber(company, invoice)) {
+    log.warn('registered company has no VAT number; recurring schedule cannot auto-send', {
+      invoiceId: invoice.id,
+    })
+    return false
+  }
   const recipients = resolveInvoiceEmailRecipients({
     to: invoice.customer.email,
     configuredCc: company.invoice_email_cc_addresses,
diff --git a/lib/invoices/seller-vat-number.ts b/lib/invoices/seller-vat-number.ts
new file mode 100644
index 00000000..268e9194
--- /dev/null
+++ b/lib/invoices/seller-vat-number.ts
@@ -0,0 +1,32 @@
+import type { CompanySettings, Invoice } from '@/types'
+
+/**
+ * A momsregistrerad seller must state its momsregistreringsnummer on every
+ * faktura (ML (2023:200) 17 kap. 24 §). Issuing without it produces a formally
+ * defective invoice and a defective input-VAT underlag for the buyer, so
+ * issuance is gated the same way the payment account is.
+ *
+ * Proformas and delivery notes are not tax documents. Credit notes are
+ * exempted deliberately: an ändringsfaktura has its own mandatory-content
+ * list (ML 17 kap. 22-23 §§: unambiguous reference to the original, the
+ * change, own number and date, negative amounts, VAT per original rate)
+ * which does not include the seller's VAT number, and blocking a correction
+ * of an already-issued invoice would trap a company that only needs to fix
+ * its settings.
+ */
+export function invoiceRequiresSellerVatNumber(
+  invoice: Pick<Invoice, 'credited_invoice_id' | 'document_type'>,
+): boolean {
+  return !invoice.credited_invoice_id
+    && invoice.document_type !== 'delivery_note'
+    && invoice.document_type !== 'proforma'
+}
+
+export function hasRequiredSellerVatNumber(
+  company: Pick<CompanySettings, 'vat_registered' | 'vat_number'>,
+  invoice: Pick<Invoice, 'credited_invoice_id' | 'document_type'>,
+): boolean {
+  if (!invoiceRequiresSellerVatNumber(invoice)) return true
+  if (!company.vat_registered) return true
+  return !!company.vat_number?.trim()
+}
diff --git a/lib/pending-operations/__tests__/executors.test.ts b/lib/pending-operations/__tests__/executors.test.ts
index 88282b63..94c7817c 100644
--- a/lib/pending-operations/__tests__/executors.test.ts
+++ b/lib/pending-operations/__tests__/executors.test.ts
@@ -513,6 +513,76 @@ describe('commitPendingOperation: invoice send payment account guard', () => {
   )
 })
 
+describe('commitPendingOperation: seller VAT number guard', () => {
+  it('rejects mark_invoice_sent for a registered company without VAT number', async () => {
+    const { supabase, enqueue } = createQueuedMockSupabase()
+    enqueue({ data: { id: 'op-1' }, error: null }) // CAS claim
+    enqueue({
+      data: makeInvoice({
+        id: 'invoice-1',
+        status: 'draft',
+        invoice_number: null,
+        credited_invoice_id: null,
+      }),
+      error: null,
+    })
+    enqueue({
+      data: { bankgiro: '123-4567', vat_registered: true, vat_number: null },
+      error: null,
+    })
+    enqueue({ data: null, error: null }) // dispatcher rejected update
+
+    const op = makePendingOp({
+      operation_type: 'mark_invoice_sent',
+      params: { invoice_id: 'invoice-1' },
+    })
+
+    const result = await commitPendingOperation(supabase as never, 'user-1', 'company-1', op)
+
+    expect(result.status).toBe('failed')
+    expect(result.http_status).toBe(400)
+    expect(ensureInvoiceNumber).not.toHaveBeenCalled()
+    expect(mockRecordManualInvoiceDelivery).not.toHaveBeenCalled()
+  })
+
+  it('rejects send_invoice for a registered company without VAT number', async () => {
+    const { supabase, enqueue } = createQueuedMockSupabase()
+    enqueue({ data: { id: 'op-1' }, error: null }) // CAS claim
+    enqueue({
+      data: makeInvoice({
+        id: 'invoice-1',
+        status: 'draft',
+        invoice_number: null,
+        customer: makeCustomer({ id: 'customer-1', email: 'customer@example.test' }),
+        items: [],
+      }),
+      error: null,
+    })
+    enqueue({
+      data: {
+        company_name: 'Test AB',
+        bankgiro: '123-4567',
+        vat_registered: true,
+        vat_number: null,
+      },
+      error: null,
+    })
+    enqueue({ data: null, error: null }) // dispatcher's rejected update
+
+    const op = makePendingOp({
+      operation_type: 'send_invoice',
+      params: { invoice_id: 'invoice-1' },
+    })
+
+    const result = await commitPendingOperation(supabase as never, 'user-1', 'company-1', op)
+
+    expect(result.status).toBe('failed')
+    expect(result.http_status).toBe(400)
+    expect(ensureInvoiceNumber).not.toHaveBeenCalled()
+    expect(supabase.from).not.toHaveBeenCalledWith('invoice_deliveries')
+  })
+})
+
 describe('commitPendingOperation: invoice send recipient limit', () => {
   it('rejects an oversized configured recipient set before reservation and allocation', async () => {
     const { supabase, enqueue } = createQueuedMockSupabase()
diff --git a/lib/pending-operations/commit.ts b/lib/pending-operations/commit.ts
index d466645d..b85e9127 100644
--- a/lib/pending-operations/commit.ts
+++ b/lib/pending-operations/commit.ts
@@ -128,6 +128,7 @@ import {
   hasRequiredInvoicePaymentAccount,
   invoiceRequiresPaymentAccount,
 } from '@/lib/invoices/payment-accounts'
+import { hasRequiredSellerVatNumber } from '@/lib/invoices/seller-vat-number'
 import {
   exceedsInvoiceEmailRecipientLimit,
   invoiceEmailRecipientCount,
@@ -2478,6 +2479,15 @@ async function commitSendInvoice(
     }
   }
 
+  if (!hasRequiredSellerVatNumber(company as CompanySettings, invoice as Invoice)) {
+    return {
+      error:
+        getErrorEntry('INVOICE_SEND_VAT_NUMBER_MISSING')?.message_sv
+        ?? 'Momsregistreringsnummer saknas i företagsinställningarna.',
+      status: 400,
+    }
+  }
+
   const recipients = resolveInvoiceEmailRecipients({
     to: customer.email,
     configuredCc: company.invoice_email_cc_addresses,
@@ -2711,7 +2721,7 @@ async function commitMarkInvoiceSent(
 
   const { data: settings, error: settingsError } = await supabase
     .from('company_settings')
-    .select('accounting_method, defer_invoice_booking, entity_type, invoice_payment_accounts, bank_name, clearing_number, account_number, bankgiro, plusgiro, swish, iban, bic')
+    .select('accounting_method, defer_invoice_booking, entity_type, invoice_payment_accounts, bank_name, clearing_number, account_number, bankgiro, plusgiro, swish, iban, bic, vat_registered, vat_number')
     .eq('company_id', companyId)
     .single()
 
@@ -2726,6 +2736,15 @@ async function commitMarkInvoiceSent(
     }
   }
 
+  if (!hasRequiredSellerVatNumber(settings as CompanySettings, invoice as Invoice)) {
+    return {
+      error:
+        getErrorEntry('INVOICE_SEND_VAT_NUMBER_MISSING')?.message_sv
+        ?? 'Momsregistreringsnummer saknas i företagsinställningarna.',
+      status: 400,
+    }
+  }
+
   try {
     await ensureInvoiceNumber(supabase, companyId, invoice as Invoice)
   } catch (err) {
diff --git a/tests/helpers.ts b/tests/helpers.ts
index ff0ffdb8..c7dd7590 100644
--- a/tests/helpers.ts
+++ b/tests/helpers.ts
@@ -534,8 +534,11 @@ export function makeCompanySettings(
     website: null,
     pays_salaries: false,
     f_skatt: true,
+    // A coherent momsregistrerad company: registered implies a number on file
+    // (ML 17 kap. 24 §). Tests exercising the missing-number state override
+    // vat_number to null explicitly.
     vat_registered: true,
-    vat_number: null,
+    vat_number: 'SE556012579001',
     moms_period: 'quarterly',
     periodisk_sammanstallning_period: 'quarterly',
     vat_taxable_base_over_40m: false,