fix(mcp): use canonical VAT deadlines (#1679)

* fix(mcp): use canonical VAT deadlines

* fix(mcp): handle incomplete VAT settings

* fix(mcp): block unknown VAT deadlines

* fix(mcp): tighten VAT tool description

* fix(mcp): fail closed on missing entity type

* ci: retry timed-out preview

* fix(vat): scope annual filing method requirement

DECISIONS.md

@@ -1052,6 +1052,9 @@ One line per decision: `[YYYY-MM-DD] <decision>: <why>`. Appended by agents and
 [2026-08-18] Shopify webshop_orders port: vat_breakdown is reconstructed from the ORDER-LEVEL taxLines (net = tax / rate, remainder as a 0%-bucket, refuse on missing rates or overshoot) instead of summing line items like the WooCommerce sync: Shopify's discountedTotalSet excludes cart-level discount allocations and lineItems is a paginated connection, so part-summing can silently produce a wrong per-rate net, while tax-per-rate and the charged total are authoritative order-level facts. Refund VAT is always prorated from the parent's mix (Shopify's Refund object exposes no per-rate tax without paging refundLineItems per refund).
 [2026-08-18] Shopify order feed keeps its paid-only qualification (PAID/PARTIALLY_REFUNDED/REFUNDED) after the webshop_orders port, unlike WooCommerce which also imports unpaid orders for the invoice flow: widening qualification is a product decision, out of scope for the port; unpaid orders re-surface via updatedAt when payment captures. The line-item snapshot is stored only when the parts reconstruct the charged total to the ore (else [] and the invoice conversion falls back to one aggregate line), and the bookkeeping-lock row filter was dropped: an Orders-page row behind the lock is an overview row, not permanent inbox noise, and booking is still blocked by the lock triggers (parity with WooCommerce).
 [2026-08-18] Skattekontoutdrag sum mismatch (opening + events != closing) demoted from a hard 400 to a preview confirm gate showing ingående/händelser/utgående/differens, mirroring the orgnr-mismatch gate: Sebastian's real export was refused on it (2026-08-18) with no way forward and no figures to diagnose; nothing is booked at import and dedup makes a later complete re-import safe, so refusing the file only blocked the rows that WERE readable. Parser also takes the earliest opening / latest closing across several marker pairs, reads a marker saldo from a trailing running-saldo column, and accepts U+2212 / plus-sign amounts; the route logs the figures (amounts and counts, never row text) so the next report is diagnosable from Vercel logs. Kept the hard reject only for zero readable rows.
+[2026-08-18] Issue #1659 exposes one canonical per-period VAT deadline resolver from deadline-config and makes both the MCP close check and VAT period default consume it: monthly, quarterly, annual, over-40M, and January/August rules must not drift across parallel formulas again; the MCP adapter alone applies the same banking-day adjustment as generated tax deadlines.
+[2026-08-18] PR #1679 reports deadline_unavailable instead of guessing when VAT settings are missing, and annual AB deadlines require the configured fiscal year to match the resolved report period: a missing or stale filing profile must not produce a plausible but legally wrong date.
+[2026-08-18] PR #1679 uses company_settings.entity_type as the sole annual VAT deadline source in MCP close checks: falling back to companies could mask missing or inconsistent filing settings, so annual deadlines now fail closed instead.
 
 [2026-08-18] Defect record (compliance-swarm PI1.3, no risk_register.csv exists in repo so this log is the equivalent): Swish QR on invoice PDFs encoded the pre-deduction total on ROT/RUT invoices with the amount locked (editmask 0), risking customer overpayment by the full skattereduktion; impact window = since Swish QR + ROT/RUT coexisted; remediation PR #1685 = QR now encodes getAmountToPay().toPay on every send/download surface (dashboard send/preview-of-send/pdf, v1 send/pdf, MCP pending-operations commit, recurring, issue-and-book; the editor preview is tracked separately in #1686), v1 pdf/send projections gained the amount-path columns (deduction_total, ore_rounding), pinned by lib/api/v1/__tests__/invoice-columns.test.ts.
 [2026-08-18] Generic CSV mapping (#1671): description guess now excludes clock-time columns (Time/Tid/Klockslag by label, HH:MM by values) and knows Lunar's Title/Titel label; Lunar detect() sniffs comma/semicolon/tab and matches header CELLS exactly (date, title|text, amount, balance) instead of substrings, aligned with what parse() resolves on. NOT changed: the 2026-08-13 generic_csv exemption from the parsed-0-rows auto-detect fallback stays; lifting it would route an explicit "Annan CSV" pick into a dedicated parser and remove the manual escape hatch. Not verified against the customer's actual file (Gmail thread not readable in-session): the semicolon/tab widening is the plausible detection miss, a Swedish-localized Lunar header is not confirmed to exist and was not special-cased in the Lunar detector (the generic path now maps it correctly anyway).

app/api/sandbox/seed/__tests__/route.test.ts

@@ -0,0 +1,197 @@
+import { beforeEach, afterAll, describe, expect, it, vi } from 'vitest'
+import { NextResponse } from 'next/server'
+
+const mocks = vi.hoisted(() => ({
+  requireAuth: vi.fn(),
+  getActiveCompanyId: vi.fn(),
+  checkRateLimit: vi.fn(),
+  ensureSandboxAgentProfile: vi.fn(),
+  markEntriesNoDocRequired: vi.fn(),
+}))
+
+vi.mock('@/lib/auth/require-auth', () => ({ requireAuth: mocks.requireAuth }))
+vi.mock('@/lib/company/context', () => ({ getActiveCompanyId: mocks.getActiveCompanyId }))
+vi.mock('@/lib/auth/rate-limit-http', () => ({ checkRateLimit: mocks.checkRateLimit }))
+vi.mock('@/lib/sandbox/ensure-agent', () => ({
+  ensureSandboxAgentProfile: mocks.ensureSandboxAgentProfile,
+}))
+vi.mock('@/lib/bookkeeping/no-doc-required', () => ({
+  markEntriesNoDocRequired: mocks.markEntriesNoDocRequired,
+}))
+vi.mock('@/lib/logger', () => ({
+  createLogger: () => ({ info: vi.fn(), warn: vi.fn(), error: vi.fn() }),
+}))
+vi.mock('@/lib/bookkeeping/bas-reference', () => ({
+  getBASReference: (accountNumber: string) => ({
+    account_name: `Account ${accountNumber}`,
+    account_class: Number(accountNumber[0]),
+    account_group: accountNumber.slice(0, 2),
+    account_type: 'asset',
+    normal_balance: 'debit',
+    sru_code: null,
+    k2_excluded: false,
+  }),
+}))
+vi.mock('@/lib/salary/personnummer', () => ({
+  encryptPersonnummer: (value: string) => `encrypted:${value}`,
+}))
+vi.mock('../customers', () => ({
+  buildSandboxCustomers: () => [
+    { name: 'Björk & Partner AB' },
+    { name: 'Schmidt GmbH' },
+    { name: 'Anna Lindström' },
+  ],
+}))
+vi.mock('../pending-operations', () => ({ buildSandboxPendingOperations: () => [] }))
+vi.mock('../articles', () => ({ buildSandboxArticles: () => [] }))
+vi.mock('../ledger-history', () => ({
+  SANDBOX_LEDGER_ACCOUNT_NUMBERS: [],
+  buildSandboxLedgerHistory: () => ({ entries: [], linesByEntryIndex: [] }),
+}))
+vi.mock('../salary-vouchers', () => ({
+  SANDBOX_SALARY_ACCOUNT_NUMBERS: [],
+  buildSandboxSalaryVouchers: () => [],
+}))
+vi.mock('../salary', () => ({
+  SANDBOX_RUN_TOTALS: {
+    total_gross: 0,
+    total_tax: 0,
+    total_net: 0,
+    total_avgifter: 0,
+    total_vacation_accrual: 0,
+  },
+  SANDBOX_TOTAL_VACATION_ACCRUAL_AVGIFTER: 0,
+  buildSandboxEmployees: () => [
+    { last_name: 'Andersson' },
+    { last_name: 'Berg' },
+  ],
+  mapSandboxEmployeeIds: (rows: Array<{ id: string }>) => ({
+    annaEmployeeId: rows[0].id,
+    erikEmployeeId: rows[1].id,
+  }),
+  buildSandboxSalaryRuns: () => [
+    { status: 'booked' },
+    { status: 'draft' },
+  ],
+  buildSandboxSalaryRunEmployees: ({ annaEmployeeId, erikEmployeeId }: {
+    annaEmployeeId: string
+    erikEmployeeId: string
+  }) => [
+    { employee_id: annaEmployeeId },
+    { employee_id: erikEmployeeId },
+  ],
+  buildSandboxSalaryLineItems: () => [],
+  resolveSandboxSalaryPeriods: () => ({
+    booked: { paymentDate: '2026-04-25', year: 2026, month: 4 },
+  }),
+}))
+
+import { POST } from '../route'
+
+interface MockSupabaseResult {
+  supabase: Record<string, unknown>
+  deadlineInserts: unknown[][]
+}
+
+function createMockSupabase(): MockSupabaseResult {
+  let rowId = 0
+  let voucherNumber = 0
+  const deadlineInserts: unknown[][] = []
+
+  const from = vi.fn((table: string) => {
+    let insertPayload: unknown = undefined
+    const chain: Record<string, unknown> = {}
+
+    const insertedRows = () => {
+      if (insertPayload === undefined) return []
+      const rows = Array.isArray(insertPayload) ? insertPayload : [insertPayload]
+      return rows.map(row => ({
+        ...(row as Record<string, unknown>),
+        id: `${table}-${++rowId}`,
+      }))
+    }
+
+    chain.insert = (payload: unknown) => {
+      insertPayload = payload
+      if (table === 'deadlines') deadlineInserts.push(payload as unknown[])
+      return chain
+    }
+    chain.update = () => chain
+    chain.select = () => chain
+    for (const method of ['eq', 'in', 'is', 'order', 'limit', 'gte', 'lte', 'not', 'or']) {
+      chain[method] = () => chain
+    }
+    chain.maybeSingle = async () => ({ data: null, error: null })
+    chain.single = async () => ({ data: insertedRows()[0] ?? { id: `${table}-${++rowId}` }, error: null })
+    chain.then = (resolve: (value: unknown) => void) => resolve({
+      data: insertedRows(),
+      error: null,
+    })
+
+    return chain
+  })
+
+  const supabase = {
+    from,
+    rpc: vi.fn(async (fn: string) => ({
+      data: fn === 'next_voucher_number' ? ++voucherNumber : null,
+      error: null,
+    })),
+  }
+
+  return { supabase, deadlineInserts }
+}
+
+function request(): Request {
+  return new Request('http://localhost:3000/api/sandbox/seed', {
+    method: 'POST',
+    headers: { 'x-forwarded-for': '192.0.2.15' },
+  })
+}
+
+describe('POST /api/sandbox/seed', () => {
+  beforeEach(() => {
+    vi.clearAllMocks()
+    vi.useFakeTimers()
+    vi.setSystemTime(new Date(2026, 4, 15, 12, 0, 0))
+    mocks.checkRateLimit.mockResolvedValue({ ok: true })
+    mocks.getActiveCompanyId.mockResolvedValue('company-1')
+    mocks.ensureSandboxAgentProfile.mockResolvedValue(undefined)
+    mocks.markEntriesNoDocRequired.mockResolvedValue(undefined)
+  })
+
+  afterAll(() => {
+    vi.useRealTimers()
+  })
+
+  it('returns 401 when authentication fails', async () => {
+    mocks.requireAuth.mockResolvedValue({
+      error: NextResponse.json({ error: 'Unauthorized' }, { status: 401 }),
+    })
+
+    const response = await POST(request())
+
+    expect(response.status).toBe(401)
+  })
+
+  it('inserts the canonical quarterly VAT deadline during a successful seed', async () => {
+    const { supabase, deadlineInserts } = createMockSupabase()
+    mocks.requireAuth.mockResolvedValue({
+      error: null,
+      user: { id: 'user-1', is_anonymous: true },
+      supabase,
+    })
+
+    const response = await POST(request())
+
+    expect(response.status).toBe(200)
+    await expect(response.json()).resolves.toEqual({ seeded: true })
+    expect(deadlineInserts).toHaveLength(1)
+    expect(deadlineInserts[0][0]).toMatchObject({
+      title: 'Momsdeklaration Q2 2026',
+      due_date: '2026-08-17',
+      tax_deadline_type: 'moms_quarterly',
+      tax_period: '2026-Q2',
+    })
+  })
+})

app/api/sandbox/seed/__tests__/vat-deadline.test.ts

@@ -0,0 +1,20 @@
+import { describe, expect, it } from 'vitest'
+import { buildSandboxVatDeadline } from '../vat-deadline'
+
+describe('buildSandboxVatDeadline', () => {
+  it('uses the August 17 deadline for a Q2 sandbox', () => {
+    expect(buildSandboxVatDeadline(new Date(2026, 4, 15))).toEqual({
+      title: 'Momsdeklaration Q2 2026',
+      dueDate: '2026-08-17',
+      period: '2026-Q2',
+    })
+  })
+
+  it('rolls the Q4 deadline into February of the following year', () => {
+    expect(buildSandboxVatDeadline(new Date(2026, 10, 15))).toEqual({
+      title: 'Momsdeklaration Q4 2026',
+      dueDate: '2027-02-12',
+      period: '2026-Q4',
+    })
+  })
+})

app/api/sandbox/seed/route.ts

@@ -13,6 +13,7 @@ import { markEntriesNoDocRequired } from '@/lib/bookkeeping/no-doc-required'
 import { buildSandboxCustomers } from './customers'
 import { buildSandboxPendingOperations } from './pending-operations'
 import { buildSandboxArticles } from './articles'
+import { buildSandboxVatDeadline } from './vat-deadline'
 import {
   buildSandboxLedgerHistory,
   SANDBOX_LEDGER_ACCOUNT_NUMBERS,
@@ -823,9 +824,7 @@ export async function POST(request: Request) {
     )
 
     // 12. Create deadlines
-    const momsDeadline = new Date(today)
-    momsDeadline.setMonth(momsDeadline.getMonth() + 2)
-    momsDeadline.setDate(12)
+    const momsDeadline = buildSandboxVatDeadline(today)
 
     const { error: dlError } = await supabase
       .from('deadlines')
@@ -833,15 +832,15 @@ export async function POST(request: Request) {
         {
           user_id: userId,
           company_id: companyId,
-          title: `Momsdeklaration Q1 ${currentYear}`,
-          due_date: toDateStr(momsDeadline),
+          title: momsDeadline.title,
+          due_date: momsDeadline.dueDate,
           deadline_type: 'tax',
           priority: 'important',
           // Current generator types: the bare 'moms'/'inkomstdeklaration'
           // types were retired and seeding them recreates legacy rows the
           // cleanup migration removed.
           tax_deadline_type: 'moms_quarterly',
-          tax_period: `${currentYear}-Q1`,
+          tax_period: momsDeadline.period,
           source: 'system',
           status: 'upcoming',
           linked_report_type: 'vat',

app/api/sandbox/seed/vat-deadline.ts

@@ -0,0 +1,33 @@
+import { getVatDeadlineForPeriod } from '@/lib/tax/deadline-config'
+import { adjustDeadlineToNextBankingDay } from '@/lib/tax/swedish-holidays'
+
+export interface SandboxVatDeadline {
+  title: string
+  dueDate: string
+  period: string
+}
+
+/** Build the current quarter's canonical, banking-day-adjusted sandbox deadline. */
+export function buildSandboxVatDeadline(today: Date): SandboxVatDeadline {
+  const year = today.getFullYear()
+  const quarter = Math.floor(today.getMonth() / 3) + 1
+  const rawDeadline = getVatDeadlineForPeriod('quarterly', year, quarter, {
+    vat_taxable_base_over_40m: false,
+  })
+  if (!rawDeadline) throw new Error('Could not resolve sandbox VAT deadline')
+
+  const deadline = adjustDeadlineToNextBankingDay(
+    new Date(rawDeadline.year, rawDeadline.month, rawDeadline.day),
+  )
+  const dueDate = [
+    deadline.getFullYear(),
+    String(deadline.getMonth() + 1).padStart(2, '0'),
+    String(deadline.getDate()).padStart(2, '0'),
+  ].join('-')
+
+  return {
+    title: `Momsdeklaration Q${quarter} ${year}`,
+    dueDate,
+    period: `${year}-Q${quarter}`,
+  }
+}

extensions/general/mcp-server/__tests__/vat-close-check-completeness.test.ts

@@ -51,7 +51,15 @@ interface MockChartAccount {
  * fixture; everything else (transactions, supplier_invoices, company_settings)
  * comes back empty so no unrelated blocker fires.
  */
-function mockSupabase(lines: MockLine[], chartAccounts: MockChartAccount[] = []) {
+function mockSupabase(
+  lines: MockLine[],
+  chartAccounts: MockChartAccount[] = [],
+  companySettings: Record<string, unknown> | null = {
+    moms_period: 'monthly',
+    vat_taxable_base_over_40m: false,
+  },
+  companyEntityType: 'aktiebolag' | 'enskild_firma' | null = null,
+) {
   const entries = [
     ...new Map(
       lines.map((l, i) => {
@@ -83,7 +91,7 @@ function mockSupabase(lines: MockLine[], chartAccounts: MockChartAccount[] = [])
     const chain: Record<string, unknown> = {}
     const settled = { data: rows, error: null, count: rows.length }
     chain.range = () => settled
-    chain.single = async () => ({ data: null, error: null })
+    chain.single = async () => ({ data: rows[0] ?? null, error: null })
     chain.maybeSingle = async () => ({ data: null, error: null })
     chain.then = (resolve: (v: unknown) => void) => resolve(settled)
     for (const m of [
@@ -100,6 +108,12 @@ function mockSupabase(lines: MockLine[], chartAccounts: MockChartAccount[] = [])
       if (table === 'journal_entries') return makeChain(entries)
       if (table === 'journal_entry_lines') return makeChain(bareLines)
       if (table === 'chart_of_accounts') return makeChain(chartAccounts)
+      if (table === 'company_settings') {
+        return makeChain(companySettings ? [companySettings] : [])
+      }
+      if (table === 'companies') {
+        return makeChain(companyEntityType ? [{ entity_type: companyEntityType }] : [])
+      }
       return makeChain([])
     },
     // The missing-underlag blocker reads the verifikat_without_documents RPC,
@@ -116,6 +130,104 @@ function mockSupabase(lines: MockLine[], chartAccounts: MockChartAccount[] = [])
 const PERIOD = { period_type: 'monthly', year: 2026, period: 1 }
 
 describe('gnubok_vat_close_check: declaration completeness', () => {
+  it('reads the over-40M setting and returns the following-month 26th deadline', async () => {
+    const result = await computeVatCloseCheck(
+      PERIOD,
+      'company-1',
+      mockSupabase([], [], {
+        moms_period: 'monthly',
+        vat_taxable_base_over_40m: true,
+      }),
+    )
+
+    expect(result.payment.deadline).toBe('2026-02-26')
+    expect(result.payment.deadline_label).toBe('26 februari 2026')
+  })
+
+  it('surfaces an unavailable deadline instead of guessing when settings are missing', async () => {
+    const result = await computeVatCloseCheck(
+      PERIOD,
+      'company-1',
+      mockSupabase([], [], null),
+    )
+
+    expect(result.payment.deadline).toBeNull()
+    expect(result.blockers).toContainEqual(expect.objectContaining({
+      kind: 'deadline_unavailable',
+      severity: 'high',
+    }))
+    expect(result.ready_to_close).toBe(false)
+  })
+
+  it('does not fall back to the company row when annual settings omit entity type', async () => {
+    const result = await computeVatCloseCheck(
+      { period_type: 'yearly', year: 2026, period: 1 },
+      'company-1',
+      mockSupabase([], [], {
+        moms_period: 'yearly',
+        vat_taxable_base_over_40m: false,
+        fiscal_year_start_month: 1,
+        vat_has_eu_trade: false,
+        vat_filing_method: 'electronic',
+      }, 'aktiebolag'),
+    )
+
+    expect(result.payment.deadline).toBeNull()
+    expect(result.blockers).toContainEqual(expect.objectContaining({
+      kind: 'deadline_unavailable',
+      severity: 'high',
+    }))
+    expect(result.ready_to_close).toBe(false)
+  })
+
+  it.each([
+    ['enskild_firma', false, '2027-05-12'],
+    ['aktiebolag', true, '2027-02-26'],
+  ] as const)('does not require an annual filing method for %s with EU trade %s', async (
+    entityType,
+    vatHasEuTrade,
+    expectedDeadline,
+  ) => {
+    const result = await computeVatCloseCheck(
+      { period_type: 'yearly', year: 2026, period: 1 },
+      'company-1',
+      mockSupabase([], [], {
+        moms_period: 'yearly',
+        vat_taxable_base_over_40m: false,
+        entity_type: entityType,
+        fiscal_year_start_month: 1,
+        vat_has_eu_trade: vatHasEuTrade,
+        vat_filing_method: null,
+      }),
+    )
+
+    expect(result.payment.deadline).toBe(expectedDeadline)
+    expect(result.blockers).not.toContainEqual(expect.objectContaining({
+      kind: 'deadline_unavailable',
+    }))
+  })
+
+  it('requires an annual filing method for an AB without EU trade', async () => {
+    const result = await computeVatCloseCheck(
+      { period_type: 'yearly', year: 2026, period: 1 },
+      'company-1',
+      mockSupabase([], [], {
+        moms_period: 'yearly',
+        vat_taxable_base_over_40m: false,
+        entity_type: 'aktiebolag',
+        fiscal_year_start_month: 1,
+        vat_has_eu_trade: false,
+        vat_filing_method: null,
+      }),
+    )
+
+    expect(result.payment.deadline).toBeNull()
+    expect(result.blockers).toContainEqual(expect.objectContaining({
+      kind: 'deadline_unavailable',
+      severity: 'high',
+    }))
+  })
+
   it('includes a null-rate 3011 with matching domestic VAT evidence (#1289)', async () => {
     const result = await computeVatCloseCheck(
       PERIOD,

extensions/general/mcp-server/__tests__/vat-close-check-missing-underlag.test.ts

@@ -45,7 +45,7 @@ function mockSupabase(rpcResults: Record<string, unknown>) {
     const chain: Record<string, unknown> = {}
     const settled = { data: rows, error: null, count: rows.length }
     chain.range = () => settled
-    chain.single = async () => ({ data: null, error: null })
+    chain.single = async () => ({ data: rows[0] ?? null, error: null })
     chain.maybeSingle = async () => ({ data: null, error: null })
     chain.then = (resolve: (v: unknown) => void) => resolve(settled)
     for (const m of [
@@ -66,7 +66,12 @@ function mockSupabase(rpcResults: Record<string, unknown>) {
   })
 
   return {
-    supabase: { from: () => makeChain([]), rpc } as never,
+    supabase: {
+      from: (table: string) => makeChain(table === 'company_settings'
+        ? [{ moms_period: 'monthly', vat_taxable_base_over_40m: false }]
+        : []),
+      rpc,
+    } as never,
     rpc,
   }
 }

extensions/general/mcp-server/__tests__/vat-close-check.test.ts

@@ -55,29 +55,97 @@ describe('gnubok_vat_close_check', () => {
 })
 
 describe('computeMomsDeadline', () => {
-  it('monthly: March 2026 → 12 April 2026', () => {
-    const d = computeMomsDeadline('monthly', 2026, 3)
-    expect(d?.date).toBe('2026-04-12')
-    expect(d?.label).toBe('12 april 2026')
+  const standardSettings = {
+    vat_taxable_base_over_40m: false,
+    entity_type: 'aktiebolag' as const,
+    fiscal_year_start_month: 1,
+    vat_has_eu_trade: false,
+    vat_filing_method: 'electronic' as const,
+  }
+
+  it('monthly: June 2026 is due 17 August 2026', () => {
+    const d = computeMomsDeadline('monthly', 2026, 6, standardSettings)
+    expect(d?.date).toBe('2026-08-17')
+    expect(d?.label).toBe('17 augusti 2026')
   })
 
-  it('monthly: December rolls into next year', () => {
-    const d = computeMomsDeadline('monthly', 2026, 12)
-    expect(d?.date).toBe('2027-01-12')
+  it('monthly: December 2026 is due 12 February 2027', () => {
+    const d = computeMomsDeadline('monthly', 2026, 12, standardSettings)
+    expect(d?.date).toBe('2027-02-12')
   })
 
-  it('quarterly: Q1 2026 → 26 April 2026', () => {
-    const d = computeMomsDeadline('quarterly', 2026, 1)
-    expect(d?.date).toBe('2026-04-26')
+  it('monthly: filers above SEK 40 million use the 26th of the following month', () => {
+    const d = computeMomsDeadline('monthly', 2026, 1, {
+      ...standardSettings,
+      vat_taxable_base_over_40m: true,
+    })
+    expect(d?.date).toBe('2026-02-26')
+    expect(d?.label).toBe('26 februari 2026')
   })
 
-  it('quarterly: Q4 2026 → 26 January 2027', () => {
-    const d = computeMomsDeadline('quarterly', 2026, 4)
-    expect(d?.date).toBe('2027-01-26')
+  it('adjusts a raw 26 December deadline to the next banking day', () => {
+    const d = computeMomsDeadline('monthly', 2026, 11, {
+      ...standardSettings,
+      vat_taxable_base_over_40m: true,
+    })
+    expect(d?.date).toBe('2026-12-28')
   })
 
-  it('yearly: 2026 → 26 February 2027', () => {
-    const d = computeMomsDeadline('yearly', 2026, 1)
+  it.each([
+    [1, '2026-05-12'],
+    [2, '2026-08-17'],
+    [3, '2026-11-12'],
+    [4, '2027-02-12'],
+  ])('quarterly: Q%s uses the canonical table', (quarter, expected) => {
+    expect(computeMomsDeadline('quarterly', 2026, quarter, standardSettings)?.date)
+      .toBe(expected)
+  })
+
+  it('yearly: an enskild firma with EU trade is due 26 February', () => {
+    const d = computeMomsDeadline('yearly', 2026, 1, {
+      ...standardSettings,
+      entity_type: 'enskild_firma',
+      vat_has_eu_trade: true,
+    })
     expect(d?.date).toBe('2027-02-26')
   })
+
+  it('yearly: an enskild firma does not require a filing method', () => {
+    const d = computeMomsDeadline('yearly', 2026, 1, {
+      ...standardSettings,
+      entity_type: 'enskild_firma',
+      vat_filing_method: null,
+    })
+    expect(d?.date).toBe('2027-05-12')
+  })
+
+  it('yearly: an AB with EU trade does not require a filing method', () => {
+    const d = computeMomsDeadline('yearly', 2026, 1, {
+      ...standardSettings,
+      vat_has_eu_trade: true,
+      vat_filing_method: null,
+    })
+    expect(d?.date).toBe('2027-02-26')
+  })
+
+  it('yearly: an aktiebolag uses its canonical electronic filing deadline', () => {
+    const d = computeMomsDeadline('yearly', 2026, 1, standardSettings)
+    expect(d?.date).toBe('2027-08-17')
+  })
+
+  it('yearly: an AB without EU trade still requires a filing method', () => {
+    const d = computeMomsDeadline('yearly', 2026, 1, {
+      ...standardSettings,
+      vat_filing_method: null,
+    })
+    expect(d).toBeNull()
+  })
+
+  it('yearly: does not guess a calendar year for an AB without fiscal settings', () => {
+    const d = computeMomsDeadline('yearly', 2026, 1, {
+      ...standardSettings,
+      fiscal_year_start_month: null,
+    })
+    expect(d).toBeNull()
+  })
 })

extensions/general/mcp-server/__tests__/vat-report-compute.test.ts

@@ -17,7 +17,10 @@ interface MockLine {
   journal_entries?: { source_type: string | null }
 }
 
-function mockSupabaseWithLines(lines: MockLine[]) {
+function mockSupabaseWithLines(
+  lines: MockLine[],
+  fiscalPeriod?: { period_start: string; period_end: string },
+) {
   // computeVatReport uses the two-step entry-lines fetch
   // (lib/bookkeeping/entry-lines.ts): journal_entries is queried first, then
   // journal_entry_lines by parent id, and the parent is reattached under
@@ -46,6 +49,7 @@ function mockSupabaseWithLines(lines: MockLine[]) {
   const makeChain = (rows: unknown[]) => {
     const chain: Record<string, () => unknown> = {}
     chain.range = () => ({ data: rows, error: null })
+    chain.maybeSingle = async () => ({ data: rows[0] ?? null, error: null })
     for (const m of ['order', 'lte', 'gte', 'neq', 'in', 'not', 'eq', 'select', 'limit', 'contains', 'filter']) {
       chain[m] = () => chain
     }
@@ -59,6 +63,7 @@ function mockSupabaseWithLines(lines: MockLine[]) {
     from: (table: string) => {
       if (table === 'journal_entries') return makeChain(entries)
       if (table === 'chart_of_accounts') return makeChain([])
+      if (table === 'fiscal_periods') return makeChain(fiscalPeriod ? [fiscalPeriod] : [])
       return makeChain(bareLines)
     },
   } as never
@@ -175,10 +180,15 @@ describe('computeVatReport', () => {
     const result = await computeVatReport(
       { period_type: 'yearly', year: 2026, period: 1 },
       'company-1',
-      mockSupabaseWithLines(lines)
+      mockSupabaseWithLines(lines, {
+        period_start: '2025-07-01',
+        period_end: '2026-06-30',
+      })
     )
 
     expect(result.rutor.ruta05).toBe(2100)
+    expect(result.period.start).toBe('2025-07-01')
+    expect(result.period.end).toBe('2026-06-30')
   })
 
   it('excludes 3004 (momsfri) from ruta05: exempt sales must NOT be in the taxable base', async () => {

extensions/general/mcp-server/server.ts

@@ -16,6 +16,11 @@ import {
 } from '@/lib/auth/api-keys'
 import { createLogger } from '@/lib/logger'
 import { roundOre, sumOre } from '@/lib/money'
+import {
+  getVatDeadlineForPeriod,
+  type VatDeadlineCalculationSettings,
+} from '@/lib/tax/deadline-config'
+import { adjustDeadlineToNextBankingDay } from '@/lib/tax/swedish-holidays'
 import type { SupabaseClient } from '@supabase/supabase-js'
 import { buildMappingResultFromCategory } from '@/lib/bookkeeping/category-mapping'
 import { applyAccountOverride } from '@/lib/bookkeeping/account-override'
@@ -47,6 +52,7 @@ import {
   rutorFromTotals,
   rcInputTotalsFromDeclaration,
   calculateVatDeclaration,
+  resolvePeriodDates,
 } from '@/lib/reports/vat-declaration'
 import { fetchDynamicVatAccounts } from '@/lib/reports/vat-revenue-accounts'
 // The momsdeklaration completeness checks live in core (lib/reports) and are
@@ -1529,23 +1535,13 @@ export async function computeVatReportWithRutor(
   if (periodType === 'monthly' && (period < 1 || period > 12)) throw new Error('period must be 1-12 for monthly')
   if (periodType === 'quarterly' && (period < 1 || period > 4)) throw new Error('period must be 1-4 for quarterly')
 
-  let startDate: string
-  let endDate: string
-
-  if (periodType === 'monthly') {
-    startDate = `${year}-${String(period).padStart(2, '0')}-01`
-    const lastDay = new Date(year, period, 0).getDate()
-    endDate = `${year}-${String(period).padStart(2, '0')}-${String(lastDay).padStart(2, '0')}`
-  } else if (periodType === 'quarterly') {
-    const startMonth = (period - 1) * 3 + 1
-    const endMonth = period * 3
-    startDate = `${year}-${String(startMonth).padStart(2, '0')}-01`
-    const lastDay = new Date(year, endMonth, 0).getDate()
-    endDate = `${year}-${String(endMonth).padStart(2, '0')}-${String(lastDay).padStart(2, '0')}`
-  } else {
-    startDate = `${year}-01-01`
-    endDate = `${year}-12-31`
-  }
+  const { start: startDate, end: endDate } = await resolvePeriodDates(
+    supabase,
+    companyId,
+    periodType as 'monthly' | 'quarterly' | 'yearly',
+    year,
+    period,
+  )
 
   // Two-step fetch (lib/bookkeeping/entry-lines.ts) rather than a
   // `journal_entries!inner` embed: PostgREST compiles that embed into a
@@ -1795,6 +1791,7 @@ interface VatCloseBlocker {
     | 'missing_high_value_receipts'
     | 'reverse_charge_input_missing'
     | 'declaration_incomplete'
+    | 'deadline_unavailable'
   severity: 'high' | 'medium' | 'low'
   count: number
   message: string
@@ -1924,47 +1921,27 @@ interface VatCloseCheckResult {
   summary: string
 }
 
-/** Compute the Skatteverket momsdeklaration deadline for a period.
- *  - monthly: due on the 12th of (period-end-month + 1)
- *  - quarterly: 26th of the month after quarter-end (Q4 → 26 Jan next year)
- *  - yearly: 26 Feb of next year
- */
+/** Adapt the canonical VAT deadline configuration to the MCP wire shape. */
 export function computeMomsDeadline(
   periodType: 'monthly' | 'quarterly' | 'yearly',
   year: number,
-  period: number
+  period: number,
+  settings: VatDeadlineCalculationSettings,
 ): { date: string; label: string } | null {
-  if (periodType === 'monthly') {
-    // period 1-12; deadline = 12th of next month
-    const deadlineMonth = period === 12 ? 1 : period + 1
-    const deadlineYear = period === 12 ? year + 1 : year
-    return {
-      date: `${deadlineYear}-${String(deadlineMonth).padStart(2, '0')}-12`,
-      label: `12 ${monthName(deadlineMonth)} ${deadlineYear}`,
-    }
+  const instance = getVatDeadlineForPeriod(periodType, year, period, settings)
+  if (!instance) return null
+
+  const adjusted = adjustDeadlineToNextBankingDay(
+    new Date(instance.year, instance.month, instance.day),
+  )
+  const deadlineYear = adjusted.getFullYear()
+  const deadlineMonth = adjusted.getMonth() + 1
+  const deadlineDay = adjusted.getDate()
+
+  return {
+    date: `${deadlineYear}-${String(deadlineMonth).padStart(2, '0')}-${String(deadlineDay).padStart(2, '0')}`,
+    label: `${deadlineDay} ${monthName(deadlineMonth)} ${deadlineYear}`,
   }
-  if (periodType === 'quarterly') {
-    // Q1→26 apr, Q2→26 jul, Q3→26 okt, Q4→26 jan next year
-    const monthByQuarter: Record<number, { m: number; yOffset: number }> = {
-      1: { m: 4, yOffset: 0 },
-      2: { m: 7, yOffset: 0 },
-      3: { m: 10, yOffset: 0 },
-      4: { m: 1, yOffset: 1 },
-    }
-    const cfg = monthByQuarter[period]
-    if (!cfg) return null
-    return {
-      date: `${year + cfg.yOffset}-${String(cfg.m).padStart(2, '0')}-26`,
-      label: `26 ${monthName(cfg.m)} ${year + cfg.yOffset}`,
-    }
-  }
-  if (periodType === 'yearly') {
-    return {
-      date: `${year + 1}-02-26`,
-      label: `26 februari ${year + 1}`,
-    }
-  }
-  return null
 }
 
 function monthName(m: number): string {
@@ -2142,21 +2119,72 @@ export async function computeVatCloseCheck(
     await computeVatReportWithRutor(args, companyId, supabase)
   const { start, end, type: periodType, year, period } = vatReport.period
 
-  // 2) Company settings: moms_period drives deadline labelling
-  const { data: settings } = await supabase
+  // 2) Company settings: deadline inputs come from the same fields used by
+  //    lib/tax/deadline-config.ts. The over-40M flag changes monthly filers
+  //    from the 12th/17th M+2 schedule to the 26th M+1 schedule.
+  const { data: settings, error: settingsError } = await supabase
     .from('company_settings')
-    .select('moms_period')
+    .select('moms_period, vat_taxable_base_over_40m, entity_type, fiscal_year_start_month, vat_has_eu_trade, vat_filing_method')
     .eq('company_id', companyId)
     .single()
   const momsPeriod = (settings?.moms_period as 'monthly' | 'quarterly' | 'yearly' | null) ?? null
-
+  const entityType = settings?.entity_type === 'aktiebolag' || settings?.entity_type === 'enskild_firma'
+    ? settings.entity_type
+    : null
   // 3) Deadline: based on the *requested* period type, not company setting,
   //    so the model gets the right deadline even when querying ad-hoc periods.
-  const deadline = computeMomsDeadline(
-    periodType as 'monthly' | 'quarterly' | 'yearly',
-    Number(year),
-    Number(period)
-  )
+  //    Never turn missing settings into a plausible statutory date. Monthly
+  //    deadlines need the turnover threshold; annual deadlines additionally
+  //    need the filing profile and a configured fiscal year matching the
+  //    report range. Quarterly dates are independent of company settings.
+  let deadline: { date: string; label: string } | null = null
+  if (periodType === 'quarterly') {
+    deadline = computeMomsDeadline('quarterly', Number(year), Number(period), {
+      vat_taxable_base_over_40m: false,
+    })
+  } else if (
+    periodType === 'monthly'
+    && typeof settings?.vat_taxable_base_over_40m === 'boolean'
+  ) {
+    deadline = computeMomsDeadline('monthly', Number(year), Number(period), {
+      vat_taxable_base_over_40m: settings.vat_taxable_base_over_40m,
+    })
+  } else if (periodType === 'yearly' && settings && entityType) {
+    const configuredStartMonth = typeof settings.fiscal_year_start_month === 'number'
+      && settings.fiscal_year_start_month >= 1
+      && settings.fiscal_year_start_month <= 12
+      ? settings.fiscal_year_start_month
+      : null
+    const reportEndMonth = Number(end.slice(5, 7))
+    const reportStartMonth = reportEndMonth === 12 ? 1 : reportEndMonth + 1
+    const fiscalYearMatches = entityType === 'enskild_firma'
+      ? reportEndMonth === 12
+      : configuredStartMonth === reportStartMonth
+    const filingMethodRequired = entityType === 'aktiebolag' && settings.vat_has_eu_trade === false
+    const filingProfileComplete = typeof settings.vat_has_eu_trade === 'boolean'
+      && (!filingMethodRequired
+        || settings.vat_filing_method === 'electronic'
+        || settings.vat_filing_method === 'paper')
+
+    if (fiscalYearMatches && filingProfileComplete) {
+      const deadlineSettings: VatDeadlineCalculationSettings = {
+        vat_taxable_base_over_40m: settings.vat_taxable_base_over_40m === true,
+        entity_type: entityType,
+        fiscal_year_start_month: reportStartMonth,
+        vat_has_eu_trade: settings.vat_has_eu_trade,
+        vat_filing_method: settings.vat_filing_method,
+      }
+      deadline = computeMomsDeadline('yearly', Number(year), Number(period), deadlineSettings)
+    }
+  }
+  if (!deadline) {
+    log.warn('VAT deadline unavailable', {
+      companyId,
+      periodType,
+      hasSettings: Boolean(settings),
+      settingsErrorCode: settingsError?.code,
+    })
+  }
 
   // 4) Blocker scans: run in parallel
   const [uncategorizedRes, unapprovedRes, recon, missingUnderlag] = await Promise.all([
@@ -2184,6 +2212,15 @@ export async function computeVatCloseCheck(
   ])
 
   const blockers: VatCloseBlocker[] = []
+  if (!deadline) {
+    blockers.push({
+      kind: 'deadline_unavailable',
+      severity: 'high',
+      count: 1,
+      message: 'Momsens inlämningsdatum kunde inte fastställas säkert',
+      hint: 'Kontrollera momsinställningar, deklarationssätt och räkenskapsperiod innan deklarationen lämnas in.',
+    })
+  }
   const uncategorizedCount = uncategorizedRes.count ?? 0
   if (uncategorizedCount > 0) {
     blockers.push({
@@ -5464,7 +5501,7 @@ export const tools: McpTool[] = [
   {
     name: 'gnubok_vat_close_check',
     title: 'VAT Close Check (Momsdeklaration)',
-    description: "Answer 'can I close VAT?' in one call. Returns SKV 4700 rutor, blockers (uncategorized, unapproved supplier invoices, reconciliation diff, missing receipts) plus declaration_checks: the momsdeklaration completeness gate the web filing UI uses. ready_to_close covers both.",
+    description: "Answer 'can I close VAT?' in one call. Returns SKV 4700 rutor, bookkeeping blockers (including unavailable deadlines), and declaration_checks from the same momsdeklaration completeness gate used by the web filing UI. ready_to_close covers both.",
     inputSchema: {
       type: 'object',
       additionalProperties: false,

lib/tax/deadline-config.ts

@@ -83,6 +83,21 @@ export interface DeadlineInstance {
   taxAssessmentNoticeId?: string
 }
 
+export interface VatDeadlineCalculationSettings {
+  vat_taxable_base_over_40m: boolean
+  entity_type?: EntityType | null
+  fiscal_year_start_month?: number | null
+  vat_has_eu_trade?: boolean | null
+  vat_filing_method?: TaxFilingMethod | null
+}
+
+interface AnnualVatDeadlineSettings {
+  entity_type: 'aktiebolag' | 'enskild_firma'
+  fiscal_year_start_month: number
+  vat_has_eu_trade: boolean
+  vat_filing_method?: TaxFilingMethod | null
+}
+
 /**
  * Day-of-month of the nth Swedish banking day in a month (1-based n).
  * Used for Intrastat, whose SCB reporting dates follow the ~10th working
@@ -111,7 +126,7 @@ function getFiscalYearLabel(fiscalYearEndMonth: number, fiscalYearEndYear: numbe
 function getAnnualVatDeadline(
   fiscalYearEndMonth: number,
   fiscalYearEndYear: number,
-  settings: CompanySettingsForDeadlines,
+  settings: AnnualVatDeadlineSettings,
 ): { day: number; month: number; year: number } {
   // Enskild firma (calendar year only, BFL 3 kap.): without EU trade the
   // annual momsdeklaration follows the income tax return (12 May); with EU
@@ -146,25 +161,100 @@ function getAnnualVatDeadline(
   return { day: paper ? 12 : 17, month: paper ? 6 : 7, year: fiscalYearEndYear + 1 }
 }
 
+/**
+ * Resolve the raw statutory VAT filing date for one reporting period.
+ * Banking-day adjustment is deliberately left to callers, matching the other
+ * deadline configs whose generated instances are adjusted by the generator.
+ */
+export function getVatDeadlineForPeriod(
+  periodType: 'monthly' | 'quarterly' | 'yearly',
+  year: number,
+  period: number,
+  settings: VatDeadlineCalculationSettings,
+): DeadlineInstance | null {
+  if (periodType === 'monthly') {
+    if (period < 1 || period > 12) return null
+
+    const month = period - 1
+    const monthOffset = settings.vat_taxable_base_over_40m ? 1 : 2
+    const deadlineMonth = (month + monthOffset) % 12
+    const deadlineYear = year + Math.floor((month + monthOffset) / 12)
+    const day = settings.vat_taxable_base_over_40m
+      ? 26
+      : (deadlineMonth === 0 || deadlineMonth === 7 ? 17 : 12)
+
+    return {
+      day,
+      month: deadlineMonth,
+      year: deadlineYear,
+      period: `${year}-${String(period).padStart(2, '0')}`,
+      periodLabel: getMonthLabel(month, year),
+    }
+  }
+
+  if (periodType === 'quarterly') {
+    if (period < 1 || period > 4) return null
+
+    return [
+      { day: 12, month: 4, year, period: `${year}-Q1`, periodLabel: `Q1 ${year}` },
+      { day: 17, month: 7, year, period: `${year}-Q2`, periodLabel: `Q2 ${year}` },
+      { day: 12, month: 10, year, period: `${year}-Q3`, periodLabel: `Q3 ${year}` },
+      { day: 12, month: 1, year: year + 1, period: `${year}-Q4`, periodLabel: `Q4 ${year}` },
+    ][period - 1]
+  }
+
+  if (period !== 1) return null
+  if (settings.entity_type !== 'aktiebolag' && settings.entity_type !== 'enskild_firma') {
+    return null
+  }
+  if (typeof settings.vat_has_eu_trade !== 'boolean') return null
+  if (
+    settings.entity_type === 'aktiebolag'
+    && settings.vat_has_eu_trade === false
+    && settings.vat_filing_method !== 'electronic'
+    && settings.vat_filing_method !== 'paper'
+  ) {
+    return null
+  }
+
+  const configuredFiscalYearStartMonth = settings.fiscal_year_start_month != null
+    && settings.fiscal_year_start_month >= 1
+    && settings.fiscal_year_start_month <= 12
+    ? settings.fiscal_year_start_month
+    : null
+  if (settings.entity_type === 'aktiebolag' && configuredFiscalYearStartMonth === null) {
+    return null
+  }
+  const fiscalYearStartMonth = settings.entity_type === 'enskild_firma'
+    ? 1
+    : configuredFiscalYearStartMonth!
+  const fiscalYearEndMonth = settings.entity_type === 'enskild_firma'
+    ? 12
+    : (fiscalYearStartMonth === 1 ? 12 : fiscalYearStartMonth - 1)
+  const deadline = getAnnualVatDeadline(fiscalYearEndMonth, year, {
+    entity_type: settings.entity_type,
+    fiscal_year_start_month: fiscalYearStartMonth,
+    vat_has_eu_trade: settings.vat_has_eu_trade,
+    vat_filing_method: settings.vat_filing_method,
+  })
+  const fiscalYearLabel = getFiscalYearLabel(fiscalYearEndMonth, year)
+
+  return {
+    ...deadline,
+    period: fiscalYearLabel,
+    periodLabel: fiscalYearLabel,
+  }
+}
+
 function generateAnnualVatDates(
   deadlineYear: number,
   settings: CompanySettingsForDeadlines,
 ): DeadlineInstance[] {
-  const fiscalYearEndMonth = settings.entity_type === 'enskild_firma'
-    ? 12
-    : (settings.fiscal_year_start_month === 1 ? 12 : settings.fiscal_year_start_month - 1)
   const results: DeadlineInstance[] = []
 
   for (const fiscalYearEndYear of [deadlineYear - 1, deadlineYear]) {
-    const deadline = getAnnualVatDeadline(fiscalYearEndMonth, fiscalYearEndYear, settings)
-    if (deadline.year !== deadlineYear) continue
-
-    const period = getFiscalYearLabel(fiscalYearEndMonth, fiscalYearEndYear)
-    results.push({
-      ...deadline,
-      period,
-      periodLabel: period,
-    })
+    const instance = getVatDeadlineForPeriod('yearly', fiscalYearEndYear, 1, settings)
+    if (instance?.year === deadlineYear) results.push(instance)
   }
 
   return results
@@ -182,27 +272,10 @@ export const TAX_DEADLINE_CONFIGS: TaxDeadlineConfig[] = [
     condition: (s) => s.vat_registered && s.moms_period === 'monthly',
     priority: 'important',
     linkedReportType: 'vat',
-    generateDates: (year, settings) => {
-      const instances: DeadlineInstance[] = []
-      for (let month = 0; month < 12; month++) {
-        const monthOffset = settings.vat_taxable_base_over_40m ? 1 : 2
-        const deadlineMonth = (month + monthOffset) % 12
-        const deadlineYear = year + Math.floor((month + monthOffset) / 12)
-        // Above SEK 40M the 26th applies year-round; 26 December is annandag
-        // jul, and the banking-day adjustment yields Skatteverket's 27th.
-        const day = settings.vat_taxable_base_over_40m
-          ? 26
-          : (deadlineMonth === 0 || deadlineMonth === 7 ? 17 : 12)
-        instances.push({
-          day,
-          month: deadlineMonth,
-          year: deadlineYear,
-          period: `${year}-${String(month + 1).padStart(2, '0')}`,
-          periodLabel: getMonthLabel(month, year),
-        })
-      }
-      return instances
-    },
+    generateDates: (year, settings) => Array.from(
+      { length: 12 },
+      (_, month) => getVatDeadlineForPeriod('monthly', year, month + 1, settings)!,
+    ),
   },
 
   // Momsdeklaration (quarterly)
@@ -213,14 +286,10 @@ export const TAX_DEADLINE_CONFIGS: TaxDeadlineConfig[] = [
     condition: (s) => s.vat_registered && s.moms_period === 'quarterly',
     priority: 'important',
     linkedReportType: 'vat',
-    generateDates: (year) => {
-      return [
-        { day: 12, month: 4, year, period: `${year}-Q1`, periodLabel: `Q1 ${year}` },
-        { day: 17, month: 7, year, period: `${year}-Q2`, periodLabel: `Q2 ${year}` },
-        { day: 12, month: 10, year, period: `${year}-Q3`, periodLabel: `Q3 ${year}` },
-        { day: 12, month: 1, year: year + 1, period: `${year}-Q4`, periodLabel: `Q4 ${year}` },
-      ]
-    },
+    generateDates: (year, settings) => Array.from(
+      { length: 4 },
+      (_, quarter) => getVatDeadlineForPeriod('quarterly', year, quarter + 1, settings)!,
+    ),
   },
 
   // Momsdeklaration (yearly)

lib/vat/__tests__/period-defaults.test.ts

@@ -47,6 +47,18 @@ describe('mostRecentEndedVatPeriod', () => {
       })
     })
 
+    it('keeps the M-2 period through an adjusted banking-day deadline', () => {
+      // 17 January 2026 is Saturday, so the deadline moves to Monday 19th.
+      expect(mostRecentEndedVatPeriod('monthly', new Date(2026, 0, 19))).toEqual({
+        year: 2025,
+        period: 11,
+      })
+      expect(mostRecentEndedVatPeriod('monthly', new Date(2026, 0, 20))).toEqual({
+        year: 2025,
+        period: 12,
+      })
+    })
+
     it('over-40M filers (month M due the 26th of M+1) always get M-1', () => {
       expect(
         mostRecentEndedVatPeriod('monthly', new Date(2026, 7, 6), { over40m: true }),

lib/vat/period-defaults.ts

@@ -14,12 +14,22 @@
  * most recently ended month the due one year-round.
  */
 
+import { getVatDeadlineForPeriod } from '@/lib/tax/deadline-config'
+import { adjustDeadlineToNextBankingDay } from '@/lib/tax/swedish-holidays'
+
 export interface VatPeriodDefault {
   year: number
   /** 1-12 for monthly, 1-4 for quarterly. */
   period: number
 }
 
+/** Return the monthly VAT period a fixed number of months before a month. */
+function previousMonth(year: number, month: number, monthsBack: number): VatPeriodDefault {
+  const date = new Date(year, month - 1 - monthsBack, 1)
+  return { year: date.getFullYear(), period: date.getMonth() + 1 }
+}
+
+/** Select the latest ended VAT period that is currently relevant for filing. */
 export function mostRecentEndedVatPeriod(
   periodType: 'monthly' | 'quarterly',
   today: Date = new Date(),
@@ -29,19 +39,23 @@ export function mostRecentEndedVatPeriod(
   const month = today.getMonth() + 1
 
   if (periodType === 'monthly') {
-    let m = month - 1
-    let y = year
-    if (!opts.over40m) {
-      // Deadline day of the CURRENT month for the M-2 declaration: 17 in
-      // January and August, else 12 (deadline-config huvudregel).
-      const deadlineDay = month === 1 || month === 8 ? 17 : 12
-      if (today.getDate() <= deadlineDay) m -= 1
-    }
-    while (m < 1) {
-      m += 12
-      y -= 1
-    }
-    return { year: y, period: m }
+    const mostRecentEnded = previousMonth(year, month, 1)
+    if (opts.over40m) return mostRecentEnded
+
+    const periodDueThisMonth = previousMonth(year, month, 2)
+    const deadline = getVatDeadlineForPeriod(
+      'monthly',
+      periodDueThisMonth.year,
+      periodDueThisMonth.period,
+      { vat_taxable_base_over_40m: false },
+    )
+    const adjustedDeadline = deadline
+      ? adjustDeadlineToNextBankingDay(new Date(deadline.year, deadline.month, deadline.day))
+      : null
+    const currentDate = new Date(year, month - 1, today.getDate())
+    return adjustedDeadline && currentDate <= adjustedDeadline
+      ? periodDueThisMonth
+      : mostRecentEnded
   }
 
   const quarter = Math.ceil(month / 3)