diff --git a/app/(dashboard)/transactions/page.tsx b/app/(dashboard)/transactions/page.tsx
index 00a0db4e..a790dce4 100644
--- a/app/(dashboard)/transactions/page.tsx
+++ b/app/(dashboard)/transactions/page.tsx
@@ -519,6 +519,66 @@ export default function TransactionsPage() {
           setProcessingId(null)
           return null
         }
+        if (result?.error?.code === 'ACCOUNTS_NOT_IN_CHART') {
+          // The mapped template/category references one or more accounts
+          // that aren't active in this company's kontoplan. Without an
+          // inline action the user has to navigate to settings, activate
+          // each account, and come back — surface a one-click "Aktivera
+          // och bokför" instead.
+          const accountNumbers: string[] =
+            (Array.isArray(result.error.account_numbers) && result.error.account_numbers) ||
+            (Array.isArray(result.error.details?.account_numbers) && result.error.details.account_numbers) ||
+            []
+          // Synchronous in-flight flag per toast closure: a double-click
+          // would otherwise fire two activate+categorize pairs, where the
+          // second categorize races the first's verifikation insert.
+          let activateInFlight = false
+          toast({
+            title: 'Kontot finns inte i din kontoplan',
+            description: `Bokföringsmallen kräver att följande konton aktiveras: ${accountNumbers.join(', ')}.`,
+            variant: 'destructive',
+            action: accountNumbers.length > 0 ? (
+              <ToastAction altText="Aktivera och bokför" onClick={async () => {
+                if (activateInFlight) return
+                activateInFlight = true
+                try {
+                  const activateRes = await fetch('/api/bookkeeping/accounts/activate', {
+                    method: 'POST',
+                    headers: { 'Content-Type': 'application/json' },
+                    body: JSON.stringify({ account_numbers: accountNumbers }),
+                  })
+                  if (!activateRes.ok) {
+                    const errBody = await activateRes.json().catch(() => null)
+                    toast({
+                      title: 'Kunde inte aktivera konton',
+                      description: getErrorMessage(errBody, { statusCode: activateRes.status }),
+                      variant: 'destructive',
+                    })
+                    return
+                  }
+                  const activateBody = await activateRes.json()
+                  // unknown[] = numbers not in BAS reference at all. Those
+                  // can't be auto-created; tell the user to add them manually.
+                  if (Array.isArray(activateBody.unknown) && activateBody.unknown.length > 0) {
+                    toast({
+                      title: 'Kunde inte hitta alla konton',
+                      description: `Lägg till ${activateBody.unknown.join(', ')} manuellt under Inställningar → Kontoplan.`,
+                      variant: 'destructive',
+                    })
+                    return
+                  }
+                  await runCategorize(args)
+                } finally {
+                  activateInFlight = false
+                }
+              }}>
+                Aktivera och bokför
+              </ToastAction>
+            ) : undefined,
+          })
+          setProcessingId(null)
+          return null
+        }
         toast({
           title: 'Kategorisering misslyckades',
           description: getErrorMessage(result, { context: 'transaction', statusCode: response.status }),
@@ -1250,28 +1310,99 @@ export default function TransactionsPage() {
     let journalEntryId: string | null
     if (!templateId && quickReview?.template?.id && isCounterpartyTemplateId(quickReview.template.id)) {
       const cpTemplateId = extractCounterpartyId(quickReview.template.id)
-      const response = await fetch(`/api/transactions/${id}/categorize`, {
-        method: 'POST',
-        headers: { 'Content-Type': 'application/json' },
-        body: JSON.stringify({
-          is_business: true,
-          counterparty_template_id: cpTemplateId,
-        }),
-      })
-      const result = await response.json()
-      if (!response.ok) {
-        toast({ title: 'Kategorisering misslyckades', description: getErrorMessage(result, { context: 'transaction' }), variant: 'destructive' })
+      const cpCategorize = async (): Promise<{ ok: boolean; journalEntryId: string | null; result: { error?: { code?: string; account_numbers?: string[]; details?: { account_numbers?: string[] } }; journal_entry_id?: string | null }; status: number }> => {
+        const r = await fetch(`/api/transactions/${id}/categorize`, {
+          method: 'POST',
+          headers: { 'Content-Type': 'application/json' },
+          body: JSON.stringify({ is_business: true, counterparty_template_id: cpTemplateId }),
+        })
+        const b = await r.json()
+        return { ok: r.ok, status: r.status, result: b, journalEntryId: b?.journal_entry_id || null }
+      }
+      const { ok: cpOk, status: cpStatus, result, journalEntryId: cpJeId } = await cpCategorize()
+      if (!cpOk) {
+        if (result?.error?.code === 'ACCOUNTS_NOT_IN_CHART') {
+          const accountNumbers: string[] =
+            (Array.isArray(result.error.account_numbers) && result.error.account_numbers) ||
+            (Array.isArray(result.error.details?.account_numbers) && result.error.details?.account_numbers) ||
+            []
+          // Synchronous in-flight flag per toast closure — see same pattern
+          // in runCategorize. Double-click on the counterparty-template
+          // retry would race the second cpCategorize against the first's
+          // verifikation insert.
+          let activateInFlight = false
+          toast({
+            title: 'Kontot finns inte i din kontoplan',
+            description: `Motpartsmallen kräver att följande konton aktiveras: ${accountNumbers.join(', ')}.`,
+            variant: 'destructive',
+            action: accountNumbers.length > 0 ? (
+              <ToastAction altText="Aktivera och bokför" onClick={async () => {
+                if (activateInFlight) return
+                activateInFlight = true
+                try {
+                  const activateRes = await fetch('/api/bookkeeping/accounts/activate', {
+                    method: 'POST',
+                    headers: { 'Content-Type': 'application/json' },
+                    body: JSON.stringify({ account_numbers: accountNumbers }),
+                  })
+                  if (!activateRes.ok) {
+                    const errBody = await activateRes.json().catch(() => null)
+                    toast({ title: 'Kunde inte aktivera konton', description: getErrorMessage(errBody, { statusCode: activateRes.status }), variant: 'destructive' })
+                    return
+                  }
+                  const activateBody = await activateRes.json()
+                  if (Array.isArray(activateBody.unknown) && activateBody.unknown.length > 0) {
+                    toast({ title: 'Kunde inte hitta alla konton', description: `Lägg till ${activateBody.unknown.join(', ')} manuellt under Inställningar → Kontoplan.`, variant: 'destructive' })
+                    return
+                  }
+                  const retry = await cpCategorize()
+                  // Gate on retry.ok alone: a 200 with null journal_entry_id
+                  // is allowed by the declared type (e.g. already-categorized
+                  // flag flip), and showing "Kategorisering misslyckades"
+                  // after the server returned success is misleading. The state
+                  // update conditionally writes the journal_entry_id when it's
+                  // actually present.
+                  if (retry.ok) {
+                    setExitingIds((prev) => new Set(prev).add(id))
+                    setTransactions((prev) =>
+                      prev.map((t) =>
+                        t.id === id
+                          ? { ...t, is_business: true, ...(retry.journalEntryId ? { journal_entry_id: retry.journalEntryId } : {}) }
+                          : t
+                      )
+                    )
+                    toast({ title: 'Bokförd' })
+                  } else {
+                    toast({ title: 'Kategorisering misslyckades', description: getErrorMessage(retry.result, { context: 'transaction', statusCode: retry.status }), variant: 'destructive' })
+                  }
+                } finally {
+                  activateInFlight = false
+                }
+              }}>
+                Aktivera och bokför
+              </ToastAction>
+            ) : undefined,
+          })
+        } else {
+          toast({ title: 'Kategorisering misslyckades', description: getErrorMessage(result, { context: 'transaction', statusCode: cpStatus }), variant: 'destructive' })
+        }
+        // Close the review dialog on hard errors — the toast (with action if
+        // ACCOUNTS_NOT_IN_CHART) carries the message and the recovery path.
+        setQuickReviewOpen(false)
+        setQuickReview(null)
         return null
       }
       setExitingIds((prev) => new Set(prev).add(id))
-      journalEntryId = result.journal_entry_id || null
+      journalEntryId = cpJeId
     } else {
       journalEntryId = await handleCategorize(id, true, category, vatTreatment, accountOverride, templateId)
     }
-    if (journalEntryId) {
-      setQuickReviewOpen(false)
-      setQuickReview(null)
-    }
+    // Always close — whether the server created a verifikation, returned a
+    // structured 4xx (ACCOUNTS_NOT_IN_CHART, INVALID_MAPPING, …), or hit a
+    // partial-success path. The toast from runCategorize already communicates
+    // the outcome; keeping the dialog open serves no purpose.
+    setQuickReviewOpen(false)
+    setQuickReview(null)
     return journalEntryId
   }
 
diff --git a/app/api/transactions/[id]/categorize/__tests__/route.test.ts b/app/api/transactions/[id]/categorize/__tests__/route.test.ts
index 5d00e66e..01d29d8f 100644
--- a/app/api/transactions/[id]/categorize/__tests__/route.test.ts
+++ b/app/api/transactions/[id]/categorize/__tests__/route.test.ts
@@ -47,6 +47,17 @@ vi.mock('@/lib/bookkeeping/counterparty-templates', () => ({
   upsertCounterpartyTemplate: vi.fn().mockResolvedValue(undefined),
 }))
 
+const mockFindMissingActiveAccounts = vi.fn()
+vi.mock('@/lib/bookkeeping/account-validation', async () => {
+  const actual = await vi.importActual<typeof import('@/lib/bookkeeping/account-validation')>(
+    '@/lib/bookkeeping/account-validation',
+  )
+  return {
+    ...actual,
+    findMissingActiveAccounts: (...args: unknown[]) => mockFindMissingActiveAccounts(...args),
+  }
+})
+
 import { POST } from '../route'
 
 describe('POST /api/transactions/[id]/categorize', () => {
@@ -69,6 +80,9 @@ describe('POST /api/transactions/[id]/categorize', () => {
     eventBus.clear()
     mockSupabase.auth.getUser.mockResolvedValue({ data: { user: mockUser } })
     mockBuildMappingResultFromCategory.mockReturnValue(defaultMappingResult)
+    // Default: every mapped account exists and is active. Tests covering the
+    // missing-account path override this per-case.
+    mockFindMissingActiveAccounts.mockResolvedValue([])
   })
 
   it('returns 401 when not authenticated', async () => {
@@ -524,4 +538,114 @@ describe('POST /api/transactions/[id]/categorize', () => {
     // Should NOT save mapping rule for private transactions
     expect(mockSaveUserMappingRule).not.toHaveBeenCalled()
   })
+
+  it('returns 400 ACCOUNTS_NOT_IN_CHART when the mapped debit account is not active in the chart', async () => {
+    const tx = makeTransaction({
+      id: 'tx-1',
+      amount: -500,
+      merchant_name: 'GitHub',
+      journal_entry_id: null,
+    })
+
+    // Fetch transaction
+    enqueue({ data: tx, error: null })
+    // Fetch company settings
+    enqueue({ data: { entity_type: 'enskild_firma', fiscal_year_start_month: 1 }, error: null })
+
+    // Mapping built from category — but the debit account is missing/inactive
+    // in this company's kontoplan. findMissingActiveAccounts is mocked at the
+    // module level; flag the debit account here to simulate the same outcome
+    // the engine would otherwise hit at AccountsNotInChartError.
+    mockFindMissingActiveAccounts.mockResolvedValueOnce(['6200'])
+
+    const request = createMockRequest('/api/transactions/tx-1/categorize', {
+      method: 'POST',
+      body: { is_business: true, category: 'expense_software' },
+    })
+    const response = await POST(request, createMockRouteParams({ id: 'tx-1' }))
+    const { status, body } = await parseJsonResponse<{
+      error: { code: string; account_numbers: string[]; message: string }
+    }>(response)
+
+    expect(status).toBe(400)
+    expect(body.error.code).toBe('ACCOUNTS_NOT_IN_CHART')
+    expect(body.error.account_numbers).toEqual(['6200'])
+    expect(body.error.message).toMatch(/Följande konton behöver aktiveras/)
+    // Engine must NOT be called once validation flagged a missing account.
+    expect(mockCreateTransactionJournalEntry).not.toHaveBeenCalled()
+    // No save of mapping rule either — the categorization didn't go through.
+    expect(mockSaveUserMappingRule).not.toHaveBeenCalled()
+  })
+
+  it('returns 400 ACCOUNTS_NOT_IN_CHART listing every missing/inactive account', async () => {
+    const tx = makeTransaction({
+      id: 'tx-1',
+      amount: -1000,
+      merchant_name: 'Acme',
+      journal_entry_id: null,
+    })
+    enqueue({ data: tx, error: null })
+    enqueue({ data: { entity_type: 'enskild_firma', fiscal_year_start_month: 1 }, error: null })
+
+    // Multiple accounts missing — covers the common "imported a template with
+    // accounts that this kontoplan never enabled" case.
+    mockFindMissingActiveAccounts.mockResolvedValueOnce(['5410', '2641'])
+
+    const request = createMockRequest('/api/transactions/tx-1/categorize', {
+      method: 'POST',
+      body: { is_business: true, category: 'expense_office' },
+    })
+    const response = await POST(request, createMockRouteParams({ id: 'tx-1' }))
+    const { status, body } = await parseJsonResponse<{
+      error: { code: string; account_numbers: string[]; message: string }
+    }>(response)
+
+    expect(status).toBe(400)
+    expect(body.error.code).toBe('ACCOUNTS_NOT_IN_CHART')
+    // AccountsNotInChartError sorts + dedupes its input.
+    expect(body.error.account_numbers).toEqual(['2641', '5410'])
+    expect(body.error.message).toContain('2641')
+    expect(body.error.message).toContain('5410')
+    expect(mockCreateTransactionJournalEntry).not.toHaveBeenCalled()
+  })
+
+  it('returns 400 ACCOUNTS_NOT_IN_CHART when the engine throws AccountsNotInChartError (defense in depth)', async () => {
+    const tx = makeTransaction({
+      id: 'tx-1',
+      amount: -500,
+      merchant_name: 'GitHub',
+      journal_entry_id: null,
+    })
+
+    enqueue({ data: tx, error: null })
+    enqueue({ data: { entity_type: 'enskild_firma', fiscal_year_start_month: 1 }, error: null })
+    // ensureFiscalPeriod existing-period check
+    enqueue({ data: [{ id: 'period-1' }], error: null })
+
+    // Pre-validation says everything is fine — simulates a race where an
+    // account got deactivated between our chart_of_accounts read and the
+    // engine's resolveAccountIds read. The engine throws and the route must
+    // surface a structured 400 rather than the partial-success path that
+    // would have marked the row bokförd with no verifikation.
+    const { AccountsNotInChartError } = await import('@/lib/bookkeeping/errors')
+    mockCreateTransactionJournalEntry.mockRejectedValue(
+      new AccountsNotInChartError(['6200']),
+    )
+
+    const request = createMockRequest('/api/transactions/tx-1/categorize', {
+      method: 'POST',
+      body: { is_business: true, category: 'expense_software' },
+    })
+    const response = await POST(request, createMockRouteParams({ id: 'tx-1' }))
+    const { status, body } = await parseJsonResponse<{
+      error: { code: string; account_numbers: string[] }
+    }>(response)
+
+    expect(status).toBe(400)
+    expect(body.error.code).toBe('ACCOUNTS_NOT_IN_CHART')
+    expect(body.error.account_numbers).toEqual(['6200'])
+    // Transaction update must NOT have run — if it had, the test would have
+    // had to enqueue a response for it. The absence of an enqueue here plus
+    // the 400 status is the assertion that the route did not fall through.
+  })
 })
diff --git a/app/api/transactions/[id]/categorize/route.ts b/app/api/transactions/[id]/categorize/route.ts
index 6f8fb20b..956c8a01 100644
--- a/app/api/transactions/[id]/categorize/route.ts
+++ b/app/api/transactions/[id]/categorize/route.ts
@@ -15,7 +15,8 @@ import {
   escapeLikePattern,
   normalizeOcrReference,
 } from '@/lib/invoices/duplicate-payment-guard'
-import { isBookkeepingError } from '@/lib/bookkeeping/errors'
+import { AccountsNotInChartError, accountsNotInChartResponse, isBookkeepingError } from '@/lib/bookkeeping/errors'
+import { collectMappingResultAccounts, findMissingActiveAccounts } from '@/lib/bookkeeping/account-validation'
 import { getErrorMessage } from '@/lib/errors/get-error-message'
 import type { Logger } from '@/lib/logger'
 import type { CategorizationTemplate } from '@/types'
@@ -238,6 +239,7 @@ export const POST = withRouteContext(
         .select('account_number, account_class')
         .eq('company_id', companyId)
         .eq('account_number', body.account_override)
+        .eq('is_active', true)
         .single()
 
       if (!accountExists) {
@@ -268,6 +270,23 @@ export const POST = withRouteContext(
       })
     }
 
+    // Pre-validate every account the engine will resolve. Templates,
+    // counterparty templates, and category defaults can all reference accounts
+    // that aren't activated in this company's kontoplan. Without this check,
+    // the engine throws AccountsNotInChartError mid-flight and the legacy
+    // catch below silently marks the transaction as bokförd with no
+    // verifikation. Catching it here means the row stays in "Att bokföra"
+    // and the user gets a clear actionable message.
+    const missingAccounts = await findMissingActiveAccounts(
+      supabase,
+      companyId,
+      collectMappingResultAccounts(mappingResult),
+    )
+    if (missingAccounts.length > 0) {
+      txLog.warn('mapping references inactive/missing accounts', { missingAccounts })
+      return accountsNotInChartResponse(new AccountsNotInChartError(missingAccounts))
+    }
+
     if (body.confirm_no_match && /^244\d$/.test(mappingResult.debit_account)) {
       txLog.warn('supplier-invoice match suggestion bypassed', {
         reason: 'confirm_no_match=true',
@@ -511,6 +530,15 @@ export const POST = withRouteContext(
       }
     } catch (err) {
       txLog.error('failed to create transaction journal entry', err as Error)
+      // AccountsNotInChartError means an account was deactivated between our
+      // pre-validation and the engine call (rare race). Don't fall through to
+      // the partial-success path — that would mark the transaction bokförd
+      // with no verifikation and leave the user staring at an unclosable
+      // dialog. Return a structured 400 so the row stays in "Att bokföra"
+      // and the user can re-activate the account and retry.
+      if (err instanceof AccountsNotInChartError) {
+        return accountsNotInChartResponse(err)
+      }
       // Bookkeeping errors map to Swedish via the registry. Other errors get
       // their raw message — the categorization is preserved either way so the
       // user can still re-book the verifikation manually.
diff --git a/app/api/v1/companies/[companyId]/transactions/[id]/__tests__/route.test.ts b/app/api/v1/companies/[companyId]/transactions/[id]/__tests__/route.test.ts
index c577264b..b0bbf5ae 100644
--- a/app/api/v1/companies/[companyId]/transactions/[id]/__tests__/route.test.ts
+++ b/app/api/v1/companies/[companyId]/transactions/[id]/__tests__/route.test.ts
@@ -28,12 +28,16 @@ vi.mock('@supabase/supabase-js', async () => {
 })
 
 // Engine stubs — happy-path returns reusable across cases.
-const { createTxJE, reverseEntryMock, createInvPmtJE, createInvCashJE, createSupplierInvPmtJE } = vi.hoisted(() => ({
+const { createTxJE, reverseEntryMock, createInvPmtJE, createInvCashJE, createSupplierInvPmtJE, findMissingAccountsMock } = vi.hoisted(() => ({
   createTxJE: vi.fn().mockResolvedValue({ id: 'je-fresh' }),
   reverseEntryMock: vi.fn().mockResolvedValue(undefined),
   createInvPmtJE: vi.fn().mockResolvedValue({ id: 'je-invpmt' }),
   createInvCashJE: vi.fn().mockResolvedValue({ id: 'je-invcash' }),
   createSupplierInvPmtJE: vi.fn().mockResolvedValue({ id: 'je-sipmt' }),
+  // Default: no missing accounts. Per-case overrides simulate the
+  // template-references-inactive-account bug or a race where deactivation
+  // happened between our validation and the engine's resolveAccountIds.
+  findMissingAccountsMock: vi.fn().mockResolvedValue([]),
 }))
 
 vi.mock('@/lib/bookkeeping/transaction-entries', () => ({
@@ -60,6 +64,15 @@ vi.mock('@/lib/bookkeeping/counterparty-templates', () => ({
   upsertCounterpartyTemplate: vi.fn().mockResolvedValue(undefined),
   buildMappingResultFromCounterpartyTemplate: vi.fn(),
 }))
+vi.mock('@/lib/bookkeeping/account-validation', async () => {
+  const actual = await vi.importActual<typeof import('@/lib/bookkeeping/account-validation')>(
+    '@/lib/bookkeeping/account-validation',
+  )
+  return {
+    ...actual,
+    findMissingActiveAccounts: findMissingAccountsMock,
+  }
+})
 // category mapping is real — provides the debit/credit account guarantees.
 
 import { validateApiKey, createServiceClientNoCookies } from '@/lib/auth/api-keys'
@@ -218,6 +231,90 @@ describe('POST :id/categorize', () => {
     const body = await res.json()
     expect(body.error.code).toBe('TX_CATEGORIZE_TX_NOT_FOUND')
   })
+
+  it('returns 400 ACCOUNTS_NOT_IN_CHART when mapped accounts are not active in the kontoplan', async () => {
+    mockServiceClient.mockReturnValue(
+      makeFlexibleSupabase({
+        company_members: { data: { company_id: COMPANY_ID, role: 'owner' }, error: null },
+        transactions: {
+          data: {
+            id: TX_ID,
+            company_id: COMPANY_ID,
+            date: '2026-05-12',
+            amount: -349.5,
+            currency: 'SEK',
+            merchant_name: 'ICA',
+            journal_entry_id: null,
+          },
+          error: null,
+        },
+        company_settings: { data: { entity_type: 'enskild_firma' }, error: null },
+      }),
+    )
+    // Simulate the user-reported bug: a category/template that maps to an
+    // account they haven't activated in their kontoplan.
+    findMissingAccountsMock.mockResolvedValueOnce(['5410'])
+
+    const res = await categorizePOST(
+      makeRequest(
+        `https://x.test/api/v1/companies/${COMPANY_ID}/transactions/${TX_ID}/categorize`,
+        { is_business: true, category: 'expense_office' },
+      ),
+      txParams(TX_ID),
+    )
+    expect(res.status).toBe(400)
+    const body = await res.json()
+    expect(body.error.code).toBe('ACCOUNTS_NOT_IN_CHART')
+    // The v1 envelope routes typed bookkeeping errors through
+    // extractBookkeepingDetails, which places account_numbers under details.
+    expect(body.error.details.account_numbers).toEqual(['5410'])
+    // Engine and transaction-update must NOT run — the row stays in the
+    // categorization queue so the user can re-activate and retry.
+    expect(createTxJE).not.toHaveBeenCalled()
+  })
+
+  it('returns 400 ACCOUNTS_NOT_IN_CHART when the engine throws mid-flight (defense in depth)', async () => {
+    mockServiceClient.mockReturnValue(
+      makeFlexibleSupabase({
+        company_members: { data: { company_id: COMPANY_ID, role: 'owner' }, error: null },
+        transactions: {
+          data: {
+            id: TX_ID,
+            company_id: COMPANY_ID,
+            date: '2026-05-12',
+            amount: -349.5,
+            currency: 'SEK',
+            merchant_name: 'ICA',
+            journal_entry_id: null,
+          },
+          error: null,
+        },
+        company_settings: { data: { entity_type: 'enskild_firma' }, error: null },
+        fiscal_periods: { data: { id: 'period-1', is_closed: false, locked_at: null }, error: null },
+      }),
+    )
+    // Pre-validation passes — race condition where an account got
+    // deactivated between our chart_of_accounts read and the engine's
+    // resolveAccountIds read. The engine throws and the catch in the route
+    // must short-circuit to a structured 400 rather than falling through
+    // to the partial-success branch that would mark the row bokförd with
+    // no verifikation.
+    findMissingAccountsMock.mockResolvedValueOnce([])
+    const { AccountsNotInChartError } = await import('@/lib/bookkeeping/errors')
+    createTxJE.mockRejectedValueOnce(new AccountsNotInChartError(['5410']))
+
+    const res = await categorizePOST(
+      makeRequest(
+        `https://x.test/api/v1/companies/${COMPANY_ID}/transactions/${TX_ID}/categorize`,
+        { is_business: true, category: 'expense_office' },
+      ),
+      txParams(TX_ID),
+    )
+    expect(res.status).toBe(400)
+    const body = await res.json()
+    expect(body.error.code).toBe('ACCOUNTS_NOT_IN_CHART')
+    expect(body.error.details.account_numbers).toEqual(['5410'])
+  })
 })
 
 describe('POST :id/uncategorize', () => {
diff --git a/app/api/v1/companies/[companyId]/transactions/[id]/categorize/route.ts b/app/api/v1/companies/[companyId]/transactions/[id]/categorize/route.ts
index 52fdd610..a7a9a08f 100644
--- a/app/api/v1/companies/[companyId]/transactions/[id]/categorize/route.ts
+++ b/app/api/v1/companies/[companyId]/transactions/[id]/categorize/route.ts
@@ -34,7 +34,8 @@ import {
 import { createTransactionJournalEntry } from '@/lib/bookkeeping/transaction-entries'
 import { reverseEntry } from '@/lib/bookkeeping/engine'
 import { saveUserMappingRule } from '@/lib/bookkeeping/mapping-engine'
-import { isBookkeepingError } from '@/lib/bookkeeping/errors'
+import { AccountsNotInChartError, isBookkeepingError } from '@/lib/bookkeeping/errors'
+import { collectMappingResultAccounts, findMissingActiveAccounts } from '@/lib/bookkeeping/account-validation'
 import { getErrorMessage } from '@/lib/errors/get-error-message'
 import { eventBus } from '@/lib/events'
 import type {
@@ -250,6 +251,7 @@ export const POST = withApiV1<{ params: Promise<{ companyId: string; id: string
         .select('account_number, account_class')
         .eq('company_id', ctx.companyId!)
         .eq('account_number', body.account_override)
+        .eq('is_active', true)
         .single()
       if (!accountExists) {
         return v1ErrorResponseFromCode('TX_CATEGORIZE_INVALID_ACCOUNT', txLog, {
@@ -285,6 +287,25 @@ export const POST = withApiV1<{ params: Promise<{ companyId: string; id: string
       })
     }
 
+    // Pre-validate every account in the mapping against the company's
+    // chart_of_accounts. Template / counterparty-template / category paths
+    // all bypass the older account_override check; without this catch they
+    // would reach the engine and throw AccountsNotInChartError mid-flight,
+    // leaving the legacy partial-success branch to silently mark the row as
+    // bokförd with no verifikation. We validate in both live AND dry-run
+    // paths so previews surface the same actionable error.
+    const missingAccounts = await findMissingActiveAccounts(
+      ctx.supabase,
+      ctx.companyId!,
+      collectMappingResultAccounts(mappingResult),
+    )
+    if (missingAccounts.length > 0) {
+      txLog.warn('mapping references inactive/missing accounts', { missingAccounts })
+      return v1ErrorResponse(new AccountsNotInChartError(missingAccounts), txLog, {
+        requestId: ctx.requestId,
+      })
+    }
+
     // Dry-run stops here — caller sees the resolved mapping without burning
     // a voucher number or mutating any state.
     if (ctx.dryRun) {
@@ -341,6 +362,14 @@ export const POST = withApiV1<{ params: Promise<{ companyId: string; id: string
       if (journalEntry) journalEntryId = journalEntry.id
     } catch (err) {
       txLog.error('transactions.categorize: journal entry creation failed', err as Error)
+      // AccountsNotInChartError means an account was deactivated between our
+      // pre-validation and the engine call (race). Don't fall through to the
+      // partial-success path that would mark the row bokförd with no
+      // verifikation — return a structured 400 so the row stays in the
+      // categorization queue and the caller can retry after re-activating.
+      if (err instanceof AccountsNotInChartError) {
+        return v1ErrorResponse(err, txLog, { requestId: ctx.requestId })
+      }
       if (isBookkeepingError(err)) {
         journalEntryError = getErrorMessage(err, { context: 'transaction' })
       } else {
diff --git a/app/api/v1/companies/[companyId]/transactions/batch-categorize/__tests__/route.test.ts b/app/api/v1/companies/[companyId]/transactions/batch-categorize/__tests__/route.test.ts
new file mode 100644
index 00000000..575e35b9
--- /dev/null
+++ b/app/api/v1/companies/[companyId]/transactions/batch-categorize/__tests__/route.test.ts
@@ -0,0 +1,214 @@
+/**
+ * Integration tests for POST /api/v1/companies/{companyId}/transactions/batch-categorize.
+ *
+ * Covers the missing-account guard: when a categorization references an
+ * account that isn't active in the company's kontoplan, the per-item result
+ * must surface as ACCOUNTS_NOT_IN_CHART without ever marking the row bokförd.
+ * Other items in the same batch continue independently (partial-success
+ * semantics).
+ */
+import { beforeAll, beforeEach, describe, expect, it, vi } from 'vitest'
+
+beforeAll(() => {
+  if (process.env.NODE_ENV !== 'test') throw new Error('NODE_ENV=test required')
+  process.env.NEXT_PUBLIC_SUPABASE_URL ||= 'http://localhost:54321'
+  process.env.NEXT_PUBLIC_SUPABASE_ANON_KEY ||= 'test-anon-key'
+})
+
+vi.mock('@/lib/auth/api-keys', async () => {
+  const actual = await vi.importActual<typeof import('@/lib/auth/api-keys')>('@/lib/auth/api-keys')
+  return { ...actual, validateApiKey: vi.fn(), createServiceClientNoCookies: vi.fn() }
+})
+vi.mock('@supabase/supabase-js', async () => {
+  const actual = await vi.importActual<typeof import('@supabase/supabase-js')>('@supabase/supabase-js')
+  return { ...actual, createClient: vi.fn().mockReturnValue({}) }
+})
+
+const { createTxJE, findMissingAccountsMock } = vi.hoisted(() => ({
+  createTxJE: vi.fn().mockResolvedValue({ id: 'je-fresh' }),
+  // Default: every mapped account exists and is active. Per-test overrides
+  // simulate the bug surface.
+  findMissingAccountsMock: vi.fn().mockResolvedValue([]),
+}))
+
+vi.mock('@/lib/bookkeeping/transaction-entries', () => ({
+  createTransactionJournalEntry: createTxJE,
+}))
+vi.mock('@/lib/bookkeeping/engine', () => ({
+  reverseEntry: vi.fn().mockResolvedValue(undefined),
+}))
+vi.mock('@/lib/bookkeeping/account-validation', async () => {
+  const actual = await vi.importActual<typeof import('@/lib/bookkeeping/account-validation')>(
+    '@/lib/bookkeeping/account-validation',
+  )
+  return {
+    ...actual,
+    findMissingActiveAccounts: findMissingAccountsMock,
+  }
+})
+// category mapping is real — gives the route real BAS accounts to validate.
+
+import { validateApiKey, createServiceClientNoCookies } from '@/lib/auth/api-keys'
+import { POST } from '../route'
+
+const mockValidate = validateApiKey as ReturnType<typeof vi.fn>
+const mockServiceClient = createServiceClientNoCookies as ReturnType<typeof vi.fn>
+
+type MockResult = { data?: unknown; error?: unknown }
+function makeFlexibleSupabase(byTable: Record<string, MockResult | MockResult[]>) {
+  const queues = new Map<string, MockResult[]>()
+  for (const [t, val] of Object.entries(byTable)) {
+    queues.set(t, Array.isArray(val) ? [...val] : [val])
+  }
+  const buildChain = (table: string): unknown => {
+    const handler: ProxyHandler<object> = {
+      get(_target, prop) {
+        if (prop === 'then') {
+          return (resolve: (v: unknown) => void) => {
+            const q = queues.get(table)
+            const next = q && q.length > 1 ? q.shift()! : (q?.[0] ?? { data: null, error: null })
+            resolve(next)
+          }
+        }
+        return (..._args: unknown[]) => buildChain(table)
+      },
+    }
+    return new Proxy({}, handler)
+  }
+  return { from: vi.fn((table: string) => buildChain(table)) }
+}
+
+const COMPANY_ID = 'aaaaaaaa-aaaa-4aaa-8aaa-aaaaaaaaaaaa'
+const TX_A = 'bbbbbbbb-bbbb-4bbb-8bbb-bbbbbbbbbbbb'
+const TX_B = 'cccccccc-cccc-4ccc-8ccc-cccccccccccc'
+
+function makeRequest(url: string, body: unknown): Request {
+  return new Request(url, {
+    method: 'POST',
+    headers: {
+      Authorization: 'Bearer test-fixture-not-a-real-key',
+      'Content-Type': 'application/json',
+      'Idempotency-Key': 'idem1234-aaaa-4abc-8def-1234567890ab',
+    },
+    body: JSON.stringify(body),
+  })
+}
+function batchParams() {
+  return { params: Promise.resolve({ companyId: COMPANY_ID }) }
+}
+
+beforeEach(() => {
+  vi.clearAllMocks()
+  findMissingAccountsMock.mockResolvedValue([])
+  mockValidate.mockResolvedValue({
+    userId: 'user-1',
+    companyId: COMPANY_ID,
+    apiKeyId: 'ak_1',
+    scopes: ['transactions:write'],
+    mode: 'live',
+  })
+})
+
+describe('POST batch-categorize', () => {
+  it('returns per-item ACCOUNTS_NOT_IN_CHART for items whose mapping references inactive accounts; clean items still succeed', async () => {
+    mockServiceClient.mockReturnValue(
+      makeFlexibleSupabase({
+        company_members: { data: { company_id: COMPANY_ID, role: 'owner' }, error: null },
+        // Each `transactions` lookup returns the same shape; the flexible
+        // proxy serves both items from this single result. amount is < 0 so
+        // both map to an expense flow.
+        transactions: {
+          data: {
+            company_id: COMPANY_ID,
+            date: '2026-05-12',
+            amount: -349.5,
+            currency: 'SEK',
+            merchant_name: 'ICA',
+            journal_entry_id: null,
+          },
+          error: null,
+        },
+        company_settings: { data: { entity_type: 'enskild_firma' }, error: null },
+        fiscal_periods: { data: { id: 'period-1', is_closed: false, locked_at: null }, error: null },
+      }),
+    )
+
+    // First item: mapping references an inactive account. Second item: clean.
+    findMissingAccountsMock
+      .mockResolvedValueOnce(['5410'])
+      .mockResolvedValueOnce([])
+
+    const res = await POST(
+      makeRequest(
+        `https://x.test/api/v1/companies/${COMPANY_ID}/transactions/batch-categorize`,
+        {
+          items: [
+            { transaction_id: TX_A, categorization: { is_business: true, category: 'expense_office' } },
+            { transaction_id: TX_B, categorization: { is_business: true, category: 'expense_office' } },
+          ],
+        },
+      ),
+      batchParams(),
+    )
+
+    expect(res.status).toBe(200)
+    const body = await res.json()
+    expect(body.data.results).toHaveLength(2)
+    expect(body.data.results[0].ok).toBe(false)
+    expect(body.data.results[0].request_index).toBe(0)
+    expect(body.data.results[0].error.code).toBe('ACCOUNTS_NOT_IN_CHART')
+    expect(body.data.results[0].error.details.account_numbers).toEqual(['5410'])
+    expect(body.data.results[1].ok).toBe(true)
+    expect(body.data.results[1].request_index).toBe(1)
+    expect(body.data.summary).toEqual({ total: 2, succeeded: 1, failed: 1 })
+
+    // Engine must only be called for the clean item.
+    expect(createTxJE).toHaveBeenCalledTimes(1)
+  })
+
+  it('returns ACCOUNTS_NOT_IN_CHART when the engine throws AccountsNotInChartError mid-flight (defense in depth)', async () => {
+    mockServiceClient.mockReturnValue(
+      makeFlexibleSupabase({
+        company_members: { data: { company_id: COMPANY_ID, role: 'owner' }, error: null },
+        transactions: {
+          data: {
+            company_id: COMPANY_ID,
+            date: '2026-05-12',
+            amount: -349.5,
+            currency: 'SEK',
+            merchant_name: 'ICA',
+            journal_entry_id: null,
+          },
+          error: null,
+        },
+        company_settings: { data: { entity_type: 'enskild_firma' }, error: null },
+        fiscal_periods: { data: { id: 'period-1', is_closed: false, locked_at: null }, error: null },
+      }),
+    )
+    // Pre-validation passes — race where an account got deactivated between
+    // our chart_of_accounts read and the engine's resolveAccountIds read.
+    findMissingAccountsMock.mockResolvedValueOnce([])
+    const { AccountsNotInChartError } = await import('@/lib/bookkeeping/errors')
+    createTxJE.mockRejectedValueOnce(new AccountsNotInChartError(['5410']))
+
+    const res = await POST(
+      makeRequest(
+        `https://x.test/api/v1/companies/${COMPANY_ID}/transactions/batch-categorize`,
+        {
+          items: [
+            { transaction_id: TX_A, categorization: { is_business: true, category: 'expense_office' } },
+          ],
+        },
+      ),
+      batchParams(),
+    )
+
+    expect(res.status).toBe(200)
+    const body = await res.json()
+    expect(body.data.results).toHaveLength(1)
+    expect(body.data.results[0].ok).toBe(false)
+    expect(body.data.results[0].error.code).toBe('ACCOUNTS_NOT_IN_CHART')
+    expect(body.data.results[0].error.details.account_numbers).toEqual(['5410'])
+    expect(body.data.summary).toEqual({ total: 1, succeeded: 0, failed: 1 })
+  })
+})
diff --git a/app/api/v1/companies/[companyId]/transactions/batch-categorize/route.ts b/app/api/v1/companies/[companyId]/transactions/batch-categorize/route.ts
index cd32d041..e71dd677 100644
--- a/app/api/v1/companies/[companyId]/transactions/batch-categorize/route.ts
+++ b/app/api/v1/companies/[companyId]/transactions/batch-categorize/route.ts
@@ -26,7 +26,8 @@ import {
 } from '@/lib/bookkeeping/booking-templates'
 import { createTransactionJournalEntry } from '@/lib/bookkeeping/transaction-entries'
 import { reverseEntry } from '@/lib/bookkeeping/engine'
-import { isBookkeepingError } from '@/lib/bookkeeping/errors'
+import { AccountsNotInChartError, isBookkeepingError } from '@/lib/bookkeeping/errors'
+import { collectMappingResultAccounts, findMissingActiveAccounts } from '@/lib/bookkeeping/account-validation'
 import { getErrorMessage } from '@/lib/errors/get-error-message'
 import { eventBus } from '@/lib/events'
 import type { Logger } from '@/lib/logger'
@@ -197,6 +198,31 @@ async function categorizeOne(
     }
   }
 
+  // Pre-validate every account in the mapping against the company's
+  // chart_of_accounts. Templates and category defaults can reference accounts
+  // that aren't activated in this company's kontoplan; without this check the
+  // engine throws AccountsNotInChartError mid-flight and the legacy
+  // partial-success branch silently marks the row bokförd with no
+  // verifikation. Validate in both dry-run and live paths so previews
+  // surface the same actionable error.
+  const missingAccounts = await findMissingActiveAccounts(
+    supabase,
+    companyId,
+    collectMappingResultAccounts(mappingResult),
+  )
+  if (missingAccounts.length > 0) {
+    return {
+      ok: false,
+      request_index: index,
+      transaction_id: transactionId,
+      error: {
+        code: 'ACCOUNTS_NOT_IN_CHART',
+        message: `Följande konton behöver aktiveras: ${missingAccounts.join(', ')}`,
+        details: { account_numbers: missingAccounts },
+      },
+    }
+  }
+
   if (dryRun) {
     return {
       ok: true,
@@ -279,6 +305,23 @@ async function categorizeOne(
       request_index: index,
       transactionId,
     })
+    // AccountsNotInChartError means an account was deactivated between our
+    // pre-validation and the engine call (rare race). Return the per-item
+    // failure WITHOUT the transaction update below so the row stays in
+    // "Att bokföra" — partial-success on a missing-account error would
+    // mark it bokförd with no verifikation.
+    if (err instanceof AccountsNotInChartError) {
+      return {
+        ok: false,
+        request_index: index,
+        transaction_id: transactionId,
+        error: {
+          code: 'ACCOUNTS_NOT_IN_CHART',
+          message: `Följande konton behöver aktiveras: ${err.accountNumbers.join(', ')}`,
+          details: { account_numbers: err.accountNumbers },
+        },
+      }
+    }
     if (isBookkeepingError(err)) {
       journalEntryError = getErrorMessage(err, { context: 'transaction' })
     } else {
diff --git a/app/api/v1/companies/[companyId]/transactions/ingest/__tests__/route.test.ts b/app/api/v1/companies/[companyId]/transactions/ingest/__tests__/route.test.ts
index 65027bec..2b3363a2 100644
--- a/app/api/v1/companies/[companyId]/transactions/ingest/__tests__/route.test.ts
+++ b/app/api/v1/companies/[companyId]/transactions/ingest/__tests__/route.test.ts
@@ -19,7 +19,7 @@ vi.mock('@supabase/supabase-js', async () => {
   return { ...actual, createClient: vi.fn().mockReturnValue({}) }
 })
 
-const { ingestMock, createTxJE } = vi.hoisted(() => ({
+const { ingestMock, createTxJE, findMissingAccountsMock } = vi.hoisted(() => ({
   ingestMock: vi.fn().mockResolvedValue({
     imported: 2,
     duplicates: 1,
@@ -30,6 +30,12 @@ const { ingestMock, createTxJE } = vi.hoisted(() => ({
     transaction_ids: ['11111111-1111-4111-8111-111111111111', '22222222-2222-4222-8222-222222222222'],
   }),
   createTxJE: vi.fn().mockResolvedValue({ id: 'je-bc' }),
+  // batch-categorize pre-validates every mapped account against chart_of_accounts
+  // (commit 6afb13aa). The flexible-Supabase proxy returns { data: null } for
+  // unmocked tables, which would make the real implementation report ALL
+  // accounts as missing and short-circuit every item with ACCOUNTS_NOT_IN_CHART.
+  // Stub it to "no missing accounts" so the happy path is exercised.
+  findMissingAccountsMock: vi.fn().mockResolvedValue([]),
 }))
 
 vi.mock('@/lib/transactions/ingest', () => ({
@@ -38,6 +44,15 @@ vi.mock('@/lib/transactions/ingest', () => ({
 vi.mock('@/lib/bookkeeping/transaction-entries', () => ({
   createTransactionJournalEntry: createTxJE,
 }))
+vi.mock('@/lib/bookkeeping/account-validation', async () => {
+  const actual = await vi.importActual<typeof import('@/lib/bookkeeping/account-validation')>(
+    '@/lib/bookkeeping/account-validation',
+  )
+  return {
+    ...actual,
+    findMissingActiveAccounts: findMissingAccountsMock,
+  }
+})
 
 import { validateApiKey, createServiceClientNoCookies } from '@/lib/auth/api-keys'
 import { POST as ingestPOST } from '../route'
diff --git a/components/transactions/QuickReviewDialog.tsx b/components/transactions/QuickReviewDialog.tsx
index 5b80671d..417707ac 100644
--- a/components/transactions/QuickReviewDialog.tsx
+++ b/components/transactions/QuickReviewDialog.tsx
@@ -229,6 +229,11 @@ export default function QuickReviewDialog({
       setShowUploadZone(false)
     } catch {
       setError('Ett fel uppstod vid bokföring.')
+    } finally {
+      // Always reset isProcessing — without this, an onConfirm that resolves
+      // with null (e.g. server returned a structured 4xx error like
+      // ACCOUNTS_NOT_IN_CHART) leaves the dialog frozen because the
+      // <Dialog onOpenChange> below disables backdrop/ESC while processing.
       setIsProcessing(false)
     }
   }
diff --git a/extensions/general/mcp-server/server.ts b/extensions/general/mcp-server/server.ts
index 45124371..8ad078dc 100644
--- a/extensions/general/mcp-server/server.ts
+++ b/extensions/general/mcp-server/server.ts
@@ -6819,14 +6819,15 @@ export async function handleMcpRequest(request: Request): Promise<Response> {
             '• When the user asks "how do I do X" or you\'re unsure of the correct sequence (month-end close, VAT review, year-end, invoicing, payroll), call gnubok_list_skills first — domain workflows are documented as loadable skills with tool references.',
             '',
             'Common workflows:',
-            '• Categorize transactions: gnubok_list_uncategorized_transactions → gnubok_suggest_categories → gnubok_categorize_transaction (stages for user approval). Use gnubok_match_transaction_to_invoice to apply income to a specific invoice.',
+            '• Categorize transactions: gnubok_list_uncategorized_transactions → gnubok_suggest_categories → gnubok_categorize_transaction (stages) → gnubok_approve_pending_operation (after user confirms in chat). Use gnubok_match_transaction_to_invoice to apply income to a specific invoice.',
             '• Invoicing: gnubok_list_customers (or gnubok_create_customer) → gnubok_create_invoice → gnubok_send_invoice or gnubok_mark_invoice_as_sent → gnubok_mark_invoice_as_paid. Refund via gnubok_credit_invoice.',
             '• VAT: gnubok_get_vat_report(period_type, year, period). Ruta49 = VAT to pay (positive) or refund (negative).',
             '• Reporting: gnubok_get_trial_balance / _income_statement / _balance_sheet / _kpi_report / _ar_ledger / _supplier_ledger — all default to the most recent fiscal period.',
             '• Year-end: gnubok_lock_period → gnubok_run_year_end → gnubok_set_opening_balances → gnubok_close_period. Each stages for human approval; closing is irreversible per BFL.',
             '• Payroll: gnubok_create_salary_run → gnubok_calculate_salary_run → review/approve in web UI → gnubok_generate_agi.',
+            '• Reviewing & approving staged operations: gnubok_list_pending_operations (filter status / risk_level / operation_type) shows what is staged. After the user reviews and explicitly authorizes in chat, call gnubok_approve_pending_operation to commit (high-risk ops require confirmed=true). Use gnubok_reject_pending_operation to discard.',
             '',
-            'Write operations stage a pending_operation (risk_level: low/medium/high) — the user approves in the gnubok web app before any DB write. Pass dry_run=true to preview without staging. Pass idempotency_key to make a write safely retryable.',
+            'Write operations stage a pending_operation (risk_level: low/medium/high). Nothing posts until the user approves. Two equally valid approval paths: (a) the user explicitly authorizes in chat ("approve", "yes go ahead", "commit it") → call gnubok_approve_pending_operation, or (b) the user reviews and approves later in the gnubok web app at /pending. The chat-approval path is the intended flow for conversational use — do not refuse it on segregation-of-duties grounds when the user has explicitly authorized commitment. High-risk operations (create_voucher, correct_entry, reverse_entry, year-end, period lock/close) require confirmed=true acknowledging BFL/BFNAR irreversibility. Pass dry_run=true to preview without staging. Pass idempotency_key to make a write safely retryable.',
             'All amounts are SEK unless currency is specified. All dates ISO YYYY-MM-DD. Account numbers are strings (e.g. "1930").',
           ].join('\n'),
         })
diff --git a/lib/bookkeeping/__tests__/errors.test.ts b/lib/bookkeeping/__tests__/errors.test.ts
index 43c31572..25c709d3 100644
--- a/lib/bookkeeping/__tests__/errors.test.ts
+++ b/lib/bookkeeping/__tests__/errors.test.ts
@@ -26,6 +26,53 @@ describe('Typed bookkeeping errors', () => {
     expect(err).toBeInstanceOf(Error)
   })
 
+  it('AccountsNotInChartError ordering is deterministic across input permutations', () => {
+    // The user-facing toast lists which accounts to activate in Kontoplan.
+    // If the same set of missing accounts came back in different orders on
+    // each call, the user might mistake an identical error for a different
+    // one. Lock in: same set → same array, regardless of input order.
+    const sets: string[][] = [
+      ['5410', '2641'],
+      ['2641', '5410'],
+      ['5410', '5410', '2641', '2641'],
+      ['2641', '2641', '5410'],
+    ]
+    const outputs = sets.map((s) => new AccountsNotInChartError(s).accountNumbers)
+    for (const out of outputs) {
+      expect(out).toEqual(['2641', '5410'])
+    }
+    // And the rendered message stays stable too.
+    expect(new AccountsNotInChartError(['5410', '2641']).message).toBe(
+      new AccountsNotInChartError(['2641', '5410']).message,
+    )
+  })
+
+  it('AccountsNotInChartError sorts numerically, not by UTF-16 code units', () => {
+    // Default string sort would order ['245', '1930'] as ['1930', '245']
+    // because '1' < '2'. That's wrong for accounts — a user looking at the
+    // toast and walking down their kontoplan expects numeric order.
+    const err = new AccountsNotInChartError(['1930', '245', '5410'])
+    expect(err.accountNumbers).toEqual(['245', '1930', '5410'])
+  })
+
+  it('AccountsNotInChartError tie-breaks numerically-equal strings deterministically', () => {
+    // "0245" and "245" compare as Number-equal but are distinct strings; the
+    // comparator must not collapse them by returning 0 unstably. We don't
+    // emit zero-padded BAS codes anywhere internally, but the public surface
+    // shouldn't be sensitive to that defensive case.
+    const err = new AccountsNotInChartError(['245', '0245'])
+    expect(err.accountNumbers).toEqual(['0245', '245'])
+  })
+
+  it('AccountsNotInChartError preserves order across multiple calls with same data', () => {
+    // Same inputs ⇒ identical outputs across separate calls (no hidden
+    // randomness, no Set iteration drift, no dependence on call order).
+    const calls = Array.from({ length: 5 }, () => new AccountsNotInChartError(['5410', '1930', '2641']).accountNumbers)
+    for (let i = 1; i < calls.length; i++) {
+      expect(calls[i]).toEqual(calls[0])
+    }
+  })
+
   it('JournalEntryNotBalancedError preserves amounts and kind', () => {
     const err = new JournalEntryNotBalancedError(100, 80, 'correction')
     expect(err.code).toBe('JOURNAL_ENTRY_NOT_BALANCED')
diff --git a/lib/bookkeeping/account-validation.ts b/lib/bookkeeping/account-validation.ts
new file mode 100644
index 00000000..23bc1c8c
--- /dev/null
+++ b/lib/bookkeeping/account-validation.ts
@@ -0,0 +1,56 @@
+import type { SupabaseClient } from '@supabase/supabase-js'
+import type { MappingResult } from '@/types'
+
+/**
+ * Return the subset of `accountNumbers` that are NOT present-and-active in the
+ * given company's chart_of_accounts. Mirrors the engine's resolveAccountIds
+ * (lib/bookkeeping/engine.ts) so a pre-validation in API routes catches the
+ * same condition (AccountsNotInChartError) before any DB writes happen.
+ *
+ * Empty/duplicate inputs are normalised; preserves first-seen order in output.
+ * On Supabase error: bubbles up. A chart-of-accounts read failure is real
+ * infrastructure trouble and should surface as 500 rather than be silently
+ * masked as "account missing".
+ */
+export async function findMissingActiveAccounts(
+  supabase: SupabaseClient,
+  companyId: string,
+  accountNumbers: readonly string[],
+): Promise<string[]> {
+  const seen = new Set<string>()
+  const unique: string[] = []
+  for (const num of accountNumbers) {
+    if (!num) continue
+    if (seen.has(num)) continue
+    seen.add(num)
+    unique.push(num)
+  }
+  if (unique.length === 0) return []
+
+  const { data, error } = await supabase
+    .from('chart_of_accounts')
+    .select('account_number')
+    .eq('company_id', companyId)
+    .eq('is_active', true)
+    .in('account_number', unique)
+
+  if (error) throw error
+
+  const present = new Set<string>((data ?? []).map((r) => r.account_number as string))
+  return unique.filter((n) => !present.has(n))
+}
+
+/**
+ * Extract every chart account number a MappingResult will post to: the headline
+ * debit/credit plus every account_number in vat_lines. Returns the raw list
+ * (duplicates intact); pass through findMissingActiveAccounts to dedupe.
+ */
+export function collectMappingResultAccounts(mr: MappingResult): string[] {
+  const out: string[] = []
+  if (mr.debit_account) out.push(mr.debit_account)
+  if (mr.credit_account) out.push(mr.credit_account)
+  for (const line of mr.vat_lines ?? []) {
+    if (line.account_number) out.push(line.account_number)
+  }
+  return out
+}
diff --git a/lib/bookkeeping/errors.ts b/lib/bookkeeping/errors.ts
index c7347352..3cf2df0d 100644
--- a/lib/bookkeeping/errors.ts
+++ b/lib/bookkeeping/errors.ts
@@ -25,13 +25,35 @@ export class AccountsNotInChartError extends Error {
   readonly accountNumbers: string[]
 
   constructor(accountNumbers: string[]) {
-    const sorted = [...new Set(accountNumbers)].sort()
+    // Numeric-first sort so mixed-length BAS codes (rare but possible) order
+    // by value rather than by UTF-16 code units — otherwise ['245', '1930']
+    // would sort to ['1930', '245'] under the default string comparator,
+    // confusing a user about which accounts to activate in Kontoplan.
+    // Non-numeric tokens fall back to a stable string compare so the order
+    // is fully deterministic for any input.
+    const sorted = [...new Set(accountNumbers)].sort(compareAccountNumbers)
     super(`Accounts not enabled in chart of accounts: ${sorted.join(', ')}`)
     this.name = 'AccountsNotInChartError'
     this.accountNumbers = sorted
   }
 }
 
+function compareAccountNumbers(a: string, b: string): number {
+  const na = Number(a)
+  const nb = Number(b)
+  const aIsNum = Number.isFinite(na)
+  const bIsNum = Number.isFinite(nb)
+  if (aIsNum && bIsNum) {
+    if (na !== nb) return na - nb
+    // Same numeric value but different string (e.g. "0245" vs "245") —
+    // break the tie deterministically by string.
+    return a < b ? -1 : a > b ? 1 : 0
+  }
+  if (aIsNum) return -1
+  if (bIsNum) return 1
+  return a < b ? -1 : a > b ? 1 : 0
+}
+
 export function isAccountsNotInChartError(err: unknown): err is AccountsNotInChartError {
   return err instanceof AccountsNotInChartError
 }
@@ -203,7 +225,12 @@ export function accountsNotInChartResponse(err: AccountsNotInChartError) {
       error: {
         code: err.code,
         message: `Följande konton behöver aktiveras: ${err.accountNumbers.join(', ')}`,
+        // Dual-emit: top-level for legacy frontend callers, nested under
+        // `details` to match the v1 envelope shape so a single client (MCP
+        // or external) can read `error.details.account_numbers` regardless
+        // of which categorize endpoint it hit.
         account_numbers: err.accountNumbers,
+        details: { account_numbers: err.accountNumbers },
       },
     },
     { status: 400 }