diff --git a/app/(dashboard)/transactions/page.tsx b/app/(dashboard)/transactions/page.tsx
index 40dd9a4d..e5ac5dde 100644
--- a/app/(dashboard)/transactions/page.tsx
+++ b/app/(dashboard)/transactions/page.tsx
@@ -30,6 +30,7 @@ import { isCounterpartyTemplateId, extractCounterpartyId } from '@/lib/bookkeepi
 import { isLibraryTemplateId } from '@/lib/bookkeeping/template-library'
 import type { TransactionWithInvoice, ViewMode, CategorizeHandler } from '@/components/transactions/transaction-types'
 import { useCompany } from '@/contexts/CompanyContext'
+import { getErrorMessage } from '@/lib/errors/get-error-message'
 import { formatCurrency, formatDate } from '@/lib/utils'
 import type { TransactionCategory, CreateTransactionInput, Invoice, Customer, VatTreatment, InvoiceInboxItem, EntityType, LinePatternEntry } from '@/types'
 import type { SuggestedCategory, SuggestedTemplate } from '@/lib/transactions/category-suggestions'
@@ -319,7 +320,11 @@ export default function TransactionsPage() {
 
       const result = await response.json()
       if (!response.ok) {
-        toast({ title: 'Kategorisering misslyckades', description: result.error || 'Försök igen.', variant: 'destructive' })
+        toast({
+          title: 'Kategorisering misslyckades',
+          description: getErrorMessage(result, { context: 'transaction', statusCode: response.status }),
+          variant: 'destructive',
+        })
         setProcessingId(null)
         return null
       }
@@ -348,7 +353,11 @@ export default function TransactionsPage() {
                   toast({ title: 'Ångrad', description: 'Kategorisering har ångrats' })
                 } else {
                   const errData = await undoRes.json()
-                  toast({ title: 'Kunde inte ångra', description: errData.error || 'Kategoriseringen kunde inte ångras. Försök igen.', variant: 'destructive' })
+                  toast({
+                    title: 'Kunde inte ångra',
+                    description: getErrorMessage(errData, { context: 'transaction', statusCode: undoRes.status }),
+                    variant: 'destructive',
+                  })
                 }
               } catch {
                 toast({ title: 'Kunde inte ångra', description: 'Kategoriseringen kunde inte ångras. Försök igen.', variant: 'destructive' })
diff --git a/app/api/bookkeeping/fiscal-periods/[id]/currency-revaluation/route.ts b/app/api/bookkeeping/fiscal-periods/[id]/currency-revaluation/route.ts
index 4a373066..4234a1fe 100644
--- a/app/api/bookkeeping/fiscal-periods/[id]/currency-revaluation/route.ts
+++ b/app/api/bookkeeping/fiscal-periods/[id]/currency-revaluation/route.ts
@@ -4,6 +4,7 @@ import {
   previewCurrencyRevaluation,
   executeCurrencyRevaluation,
 } from '@/lib/bookkeeping/currency-revaluation'
+import { bookkeepingErrorResponse } from '@/lib/bookkeeping/errors'
 import { requireCompanyId } from '@/lib/company/context'
 import { requireWritePermission } from '@/lib/auth/require-write'
 
@@ -40,6 +41,8 @@ export async function GET(
     const preview = await previewCurrencyRevaluation(supabase, companyId, period.period_end)
     return NextResponse.json({ data: preview })
   } catch (err) {
+    const typed = bookkeepingErrorResponse(err)
+    if (typed) return typed
     return NextResponse.json(
       { error: err instanceof Error ? err.message : 'Failed to preview currency revaluation' },
       { status: 400 }
@@ -92,6 +95,8 @@ export async function POST(
 
     return NextResponse.json({ data: result })
   } catch (err) {
+    const typed = bookkeepingErrorResponse(err)
+    if (typed) return typed
     return NextResponse.json(
       { error: err instanceof Error ? err.message : 'Failed to execute currency revaluation' },
       { status: 400 }
diff --git a/app/api/bookkeeping/journal-entries/[id]/correct/route.ts b/app/api/bookkeeping/journal-entries/[id]/correct/route.ts
index 1af2cd70..7b065b0f 100644
--- a/app/api/bookkeeping/journal-entries/[id]/correct/route.ts
+++ b/app/api/bookkeeping/journal-entries/[id]/correct/route.ts
@@ -1,7 +1,7 @@
 import { createClient } from '@/lib/supabase/server'
 import { NextResponse } from 'next/server'
 import { correctEntry } from '@/lib/core/bookkeeping/storno-service'
-import { AccountsNotInChartError, accountsNotInChartResponse } from '@/lib/bookkeeping/errors'
+import { bookkeepingErrorResponse } from '@/lib/bookkeeping/errors'
 import { ensureInitialized } from '@/lib/init'
 import { validateBody } from '@/lib/api/validate'
 import { CorrectJournalEntrySchema } from '@/lib/api/schemas'
@@ -35,9 +35,8 @@ export async function POST(
     const result = await correctEntry(supabase, companyId, user.id, id, body.lines)
     return NextResponse.json({ data: result })
   } catch (err) {
-    if (err instanceof AccountsNotInChartError) {
-      return accountsNotInChartResponse(err)
-    }
+    const typed = bookkeepingErrorResponse(err)
+    if (typed) return typed
     return NextResponse.json(
       { error: err instanceof Error ? err.message : 'Failed to correct entry' },
       { status: 400 }
diff --git a/app/api/bookkeeping/journal-entries/[id]/reverse/route.ts b/app/api/bookkeeping/journal-entries/[id]/reverse/route.ts
index 9502ee99..df3aafc3 100644
--- a/app/api/bookkeeping/journal-entries/[id]/reverse/route.ts
+++ b/app/api/bookkeeping/journal-entries/[id]/reverse/route.ts
@@ -1,7 +1,7 @@
 import { createClient } from '@/lib/supabase/server'
 import { NextResponse } from 'next/server'
 import { reverseEntry } from '@/lib/bookkeeping/engine'
-import { AccountsNotInChartError, accountsNotInChartResponse } from '@/lib/bookkeeping/errors'
+import { bookkeepingErrorResponse } from '@/lib/bookkeeping/errors'
 import { ensureInitialized } from '@/lib/init'
 import { requireCompanyId } from '@/lib/company/context'
 import { requireWritePermission } from '@/lib/auth/require-write'
@@ -29,9 +29,8 @@ export async function POST(
     const reversalEntry = await reverseEntry(supabase, companyId, user.id, id)
     return NextResponse.json({ data: reversalEntry })
   } catch (err) {
-    if (err instanceof AccountsNotInChartError) {
-      return accountsNotInChartResponse(err)
-    }
+    const typed = bookkeepingErrorResponse(err)
+    if (typed) return typed
     return NextResponse.json(
       { error: err instanceof Error ? err.message : 'Failed to reverse entry' },
       { status: 400 }
diff --git a/app/api/bookkeeping/journal-entries/route.ts b/app/api/bookkeeping/journal-entries/route.ts
index e7337ffb..c1c38ce0 100644
--- a/app/api/bookkeeping/journal-entries/route.ts
+++ b/app/api/bookkeeping/journal-entries/route.ts
@@ -1,7 +1,7 @@
 import { createClient } from '@/lib/supabase/server'
 import { NextResponse } from 'next/server'
 import { createJournalEntry } from '@/lib/bookkeeping/engine'
-import { AccountsNotInChartError, accountsNotInChartResponse } from '@/lib/bookkeeping/errors'
+import { bookkeepingErrorResponse } from '@/lib/bookkeeping/errors'
 import { ensureInitialized } from '@/lib/init'
 import { validateBody } from '@/lib/api/validate'
 import { CreateJournalEntrySchema } from '@/lib/api/schemas'
@@ -124,9 +124,8 @@ export async function POST(request: Request) {
     const entry = await createJournalEntry(supabase, companyId, user.id, body)
     return NextResponse.json({ data: entry })
   } catch (err) {
-    if (err instanceof AccountsNotInChartError) {
-      return accountsNotInChartResponse(err)
-    }
+    const typed = bookkeepingErrorResponse(err)
+    if (typed) return typed
     return NextResponse.json(
       { error: err instanceof Error ? err.message : 'Failed to create journal entry' },
       { status: 400 }
diff --git a/app/api/import/opening-balance/execute/route.ts b/app/api/import/opening-balance/execute/route.ts
index 3519fc48..5f7b07bb 100644
--- a/app/api/import/opening-balance/execute/route.ts
+++ b/app/api/import/opening-balance/execute/route.ts
@@ -6,7 +6,7 @@ import { OpeningBalanceExecuteSchema } from '@/lib/api/schemas'
 import { requireWritePermission } from '@/lib/auth/require-write'
 import { requireCompanyId } from '@/lib/company/context'
 import { createJournalEntry } from '@/lib/bookkeeping/engine'
-import { AccountsNotInChartError, accountsNotInChartResponse } from '@/lib/bookkeeping/errors'
+import { bookkeepingErrorResponse } from '@/lib/bookkeeping/errors'
 import { getBASReference } from '@/lib/bookkeeping/bas-reference'
 import { fetchAllRows } from '@/lib/supabase/fetch-all'
 import type { CreateJournalEntryLineInput } from '@/types'
@@ -242,9 +242,8 @@ export async function POST(request: Request) {
       },
     })
   } catch (error) {
-    if (error instanceof AccountsNotInChartError) {
-      return accountsNotInChartResponse(error)
-    }
+    const typed = bookkeepingErrorResponse(error)
+    if (typed) return typed
     console.error('Opening balance execute error:', error)
     return NextResponse.json(
       { error: error instanceof Error ? error.message : 'Importen misslyckades' },
diff --git a/app/api/invoices/[id]/mark-paid/route.ts b/app/api/invoices/[id]/mark-paid/route.ts
index 894309a1..2aee05bf 100644
--- a/app/api/invoices/[id]/mark-paid/route.ts
+++ b/app/api/invoices/[id]/mark-paid/route.ts
@@ -5,7 +5,7 @@ import {
   createInvoiceCashEntry,
 } from '@/lib/bookkeeping/invoice-entries'
 import { createJournalEntry, findFiscalPeriod } from '@/lib/bookkeeping/engine'
-import { AccountsNotInChartError, accountsNotInChartResponse } from '@/lib/bookkeeping/errors'
+import { bookkeepingErrorResponse } from '@/lib/bookkeeping/errors'
 import { MarkInvoicePaidSchema } from '@/lib/api/schemas'
 import { ensureInitialized } from '@/lib/init'
 import { requireCompanyId } from '@/lib/company/context'
@@ -161,9 +161,8 @@ export async function POST(
         journalEntryId = journalEntry?.id ?? null
       }
     } catch (err) {
-      if (err instanceof AccountsNotInChartError) {
-        return accountsNotInChartResponse(err)
-      }
+      const typed = bookkeepingErrorResponse(err)
+      if (typed) return typed
       console.error('Failed to create payment journal entry:', err)
       return NextResponse.json(
         { error: 'Kunde inte bokföra betalningen' },
diff --git a/app/api/pending-operations/[id]/commit/route.ts b/app/api/pending-operations/[id]/commit/route.ts
index e335452a..56025e75 100644
--- a/app/api/pending-operations/[id]/commit/route.ts
+++ b/app/api/pending-operations/[id]/commit/route.ts
@@ -16,7 +16,11 @@ import {
   createInvoiceJournalEntry,
 } from '@/lib/bookkeeping/invoice-entries'
 import { reverseEntry } from '@/lib/bookkeeping/engine'
-import { AccountsNotInChartError, accountsNotInChartResponse } from '@/lib/bookkeeping/errors'
+import {
+  AccountsNotInChartError,
+  bookkeepingErrorResponse,
+  isBookkeepingError,
+} from '@/lib/bookkeeping/errors'
 import { getEmailService } from '@/lib/email/service'
 import {
   generateInvoiceEmailHtml,
@@ -220,7 +224,7 @@ async function commitCategorizeTransaction(
       journalEntryId = journalEntry.id
     }
   } catch (err) {
-    if (err instanceof AccountsNotInChartError) throw err
+    if (isBookkeepingError(err)) throw err
     log.error('Failed to create journal entry:', err)
     return { error: err instanceof Error ? err.message : 'Failed to create journal entry', status: 500 }
   }
@@ -787,7 +791,7 @@ async function commitMatchTransactionInvoice(
       journalEntryId = je?.id ?? null
     }
   } catch (err) {
-    if (err instanceof AccountsNotInChartError) throw err
+    if (isBookkeepingError(err)) throw err
     log.error('Failed to create match journal entry:', err)
   }
 
@@ -915,9 +919,8 @@ export async function POST(
         return NextResponse.json({ error: 'Unknown operation type' }, { status: 400 })
     }
   } catch (err) {
-    if (err instanceof AccountsNotInChartError) {
-      return accountsNotInChartResponse(err)
-    }
+    const typed = bookkeepingErrorResponse(err)
+    if (typed) return typed
     throw err
   }
 
diff --git a/app/api/salary/runs/[id]/correct/route.ts b/app/api/salary/runs/[id]/correct/route.ts
index 82d50e81..194b0f7d 100644
--- a/app/api/salary/runs/[id]/correct/route.ts
+++ b/app/api/salary/runs/[id]/correct/route.ts
@@ -4,7 +4,7 @@ import { ensureInitialized } from '@/lib/init'
 import { requireCompanyId } from '@/lib/company/context'
 import { requireWritePermission } from '@/lib/auth/require-write'
 import { reverseEntry } from '@/lib/bookkeeping/engine'
-import { AccountsNotInChartError, accountsNotInChartResponse } from '@/lib/bookkeeping/errors'
+import { bookkeepingErrorResponse, EntryAlreadyReversedError } from '@/lib/bookkeeping/errors'
 
 ensureInitialized()
 
@@ -62,14 +62,12 @@ export async function POST(
     try {
       await reverseEntry(supabase, companyId, user.id, entryId)
     } catch (err) {
-      if (err instanceof AccountsNotInChartError) {
-        return accountsNotInChartResponse(err)
-      }
       // Entry may already be reversed — continue
+      if (err instanceof EntryAlreadyReversedError) continue
+      const typed = bookkeepingErrorResponse(err)
+      if (typed) return typed
       const msg = err instanceof Error ? err.message : ''
-      if (!msg.includes('already reversed')) {
-        return NextResponse.json({ error: `Kunde inte makulera verifikation: ${msg}` }, { status: 500 })
-      }
+      return NextResponse.json({ error: `Kunde inte makulera verifikation: ${msg}` }, { status: 500 })
     }
   }
 
diff --git a/app/api/supplier-invoices/[id]/credit/route.ts b/app/api/supplier-invoices/[id]/credit/route.ts
index 44388edd..2fa223a8 100644
--- a/app/api/supplier-invoices/[id]/credit/route.ts
+++ b/app/api/supplier-invoices/[id]/credit/route.ts
@@ -3,7 +3,7 @@ import { NextResponse } from 'next/server'
 import { eventBus } from '@/lib/events'
 import { ensureInitialized } from '@/lib/init'
 import { createSupplierCreditNoteEntry } from '@/lib/bookkeeping/supplier-invoice-entries'
-import { AccountsNotInChartError, accountsNotInChartResponse } from '@/lib/bookkeeping/errors'
+import { bookkeepingErrorResponse } from '@/lib/bookkeeping/errors'
 import { requireCompanyId } from '@/lib/company/context'
 import { requireWritePermission } from '@/lib/auth/require-write'
 import type { SupplierInvoice, SupplierInvoiceItem, AccountingMethod } from '@/types'
@@ -138,9 +138,8 @@ export async function POST(
       // momsdeklaration-integrity concern as the POST-route rollback.
       await supabase.from('supplier_invoices').delete().eq('id', creditNote.id).eq('company_id', companyId)
 
-      if (err instanceof AccountsNotInChartError) {
-        return accountsNotInChartResponse(err)
-      }
+      const typed = bookkeepingErrorResponse(err)
+      if (typed) return typed
       console.error('Failed to create credit note journal entry:', err)
       return NextResponse.json(
         { error: 'Kunde inte bokföra kreditfakturan — försök igen eller ändra datum om perioden är låst.' },
diff --git a/app/api/supplier-invoices/[id]/mark-paid/route.ts b/app/api/supplier-invoices/[id]/mark-paid/route.ts
index bd2e3a53..4ff471c5 100644
--- a/app/api/supplier-invoices/[id]/mark-paid/route.ts
+++ b/app/api/supplier-invoices/[id]/mark-paid/route.ts
@@ -6,7 +6,7 @@ import {
   createSupplierInvoicePaymentEntry,
   createSupplierInvoiceCashEntry,
 } from '@/lib/bookkeeping/supplier-invoice-entries'
-import { AccountsNotInChartError, accountsNotInChartResponse } from '@/lib/bookkeeping/errors'
+import { bookkeepingErrorResponse } from '@/lib/bookkeeping/errors'
 import { validateBody } from '@/lib/api/validate'
 import { MarkSupplierInvoicePaidSchema } from '@/lib/api/schemas'
 import { requireCompanyId } from '@/lib/company/context'
@@ -99,9 +99,8 @@ export async function POST(
       if (journalEntry) journalEntryId = journalEntry.id
     }
   } catch (err) {
-    if (err instanceof AccountsNotInChartError) {
-      return accountsNotInChartResponse(err)
-    }
+    const typed = bookkeepingErrorResponse(err)
+    if (typed) return typed
     console.error('Failed to create payment journal entry:', err)
     return NextResponse.json(
       { error: 'Kunde inte bokföra betalningen' },
diff --git a/app/api/supplier-invoices/[id]/uncredit/__tests__/route.test.ts b/app/api/supplier-invoices/[id]/uncredit/__tests__/route.test.ts
index 70d3f611..5606b739 100644
--- a/app/api/supplier-invoices/[id]/uncredit/__tests__/route.test.ts
+++ b/app/api/supplier-invoices/[id]/uncredit/__tests__/route.test.ts
@@ -30,6 +30,8 @@ vi.mock('@/lib/bookkeeping/engine', () => ({
   reverseEntry: (...args: unknown[]) => mockReverseEntry(...args),
 }))
 
+import { CannotReverseNonPostedError } from '@/lib/bookkeeping/errors'
+
 import { eventBus } from '@/lib/events'
 
 import { POST } from '../route'
@@ -222,7 +224,7 @@ describe('POST /api/supplier-invoices/[id]/uncredit', () => {
       data: { id: 'credit-1', registration_journal_entry_id: 'je-credit' },
       error: null,
     })
-    mockReverseEntry.mockRejectedValue(new Error('Can only reverse posted entries'))
+    mockReverseEntry.mockRejectedValue(new CannotReverseNonPostedError('draft'))
     enqueue({ data: null, error: null })
     enqueue({
       data: { ...original, status: 'approved', remaining_amount: 5000 },
diff --git a/app/api/supplier-invoices/[id]/uncredit/route.ts b/app/api/supplier-invoices/[id]/uncredit/route.ts
index 38832005..2b3c2ee8 100644
--- a/app/api/supplier-invoices/[id]/uncredit/route.ts
+++ b/app/api/supplier-invoices/[id]/uncredit/route.ts
@@ -3,7 +3,11 @@ import { NextResponse } from 'next/server'
 import { eventBus } from '@/lib/events'
 import { ensureInitialized } from '@/lib/init'
 import { reverseEntry } from '@/lib/bookkeeping/engine'
-import { AccountsNotInChartError, accountsNotInChartResponse } from '@/lib/bookkeeping/errors'
+import {
+  bookkeepingErrorResponse,
+  CannotReverseNonPostedError,
+  EntryAlreadyReversedError,
+} from '@/lib/bookkeeping/errors'
 import { getErrorMessage } from '@/lib/errors/get-error-message'
 import { requireCompanyId } from '@/lib/company/context'
 import { requireWritePermission } from '@/lib/auth/require-write'
@@ -69,16 +73,12 @@ export async function POST(
       )
       reversalEntryId = reversal.id
     } catch (err) {
-      const msg = err instanceof Error ? err.message : String(err)
       // Already reversed (manually or by another concurrent uncredit) — fine, continue.
-      if (
-        /Can only reverse posted entries/i.test(msg) ||
-        /already reversed by a concurrent operation/i.test(msg)
-      ) {
+      if (err instanceof CannotReverseNonPostedError || err instanceof EntryAlreadyReversedError) {
         // proceed to row cleanup
-      } else if (err instanceof AccountsNotInChartError) {
-        return accountsNotInChartResponse(err)
       } else {
+        const typed = bookkeepingErrorResponse(err)
+        if (typed) return typed
         // Period lock and similar trigger errors — surface a clear Swedish message
         // so the user knows WHY the action failed (per project's error-UX guidelines).
         return NextResponse.json(
diff --git a/app/api/supplier-invoices/route.ts b/app/api/supplier-invoices/route.ts
index dd9689ea..1418be5c 100644
--- a/app/api/supplier-invoices/route.ts
+++ b/app/api/supplier-invoices/route.ts
@@ -2,7 +2,7 @@ import { createClient } from '@/lib/supabase/server'
 import { NextResponse } from 'next/server'
 import { eventBus } from '@/lib/events'
 import { createSupplierInvoiceRegistrationEntry } from '@/lib/bookkeeping/supplier-invoice-entries'
-import { AccountsNotInChartError, accountsNotInChartResponse } from '@/lib/bookkeeping/errors'
+import { bookkeepingErrorResponse } from '@/lib/bookkeeping/errors'
 import { ensureInitialized } from '@/lib/init'
 import { validateBody } from '@/lib/api/validate'
 import { CreateSupplierInvoiceSchema } from '@/lib/api/schemas'
@@ -272,9 +272,8 @@ export async function POST(request: Request) {
       // which silently understates the momsdeklaration for the period.
       await supabase.from('supplier_invoices').delete().eq('id', invoice.id).eq('company_id', companyId)
 
-      if (err instanceof AccountsNotInChartError) {
-        return accountsNotInChartResponse(err)
-      }
+      const typed = bookkeepingErrorResponse(err)
+      if (typed) return typed
       console.error('Failed to create registration journal entry:', err)
       return NextResponse.json(
         { error: 'Kunde inte bokföra leverantörsfakturan — försök igen eller ändra datum om perioden är låst.' },
diff --git a/app/api/transactions/[id]/book/route.ts b/app/api/transactions/[id]/book/route.ts
index a76da58f..77bb09b2 100644
--- a/app/api/transactions/[id]/book/route.ts
+++ b/app/api/transactions/[id]/book/route.ts
@@ -3,7 +3,7 @@ import { NextResponse } from 'next/server'
 import { eventBus } from '@/lib/events'
 import { ensureInitialized } from '@/lib/init'
 import { createJournalEntry } from '@/lib/bookkeeping/engine'
-import { AccountsNotInChartError, accountsNotInChartResponse } from '@/lib/bookkeeping/errors'
+import { bookkeepingErrorResponse } from '@/lib/bookkeeping/errors'
 import { validateBody } from '@/lib/api/validate'
 import { BookTransactionSchema } from '@/lib/api/schemas'
 import { requireCompanyId } from '@/lib/company/context'
@@ -66,9 +66,8 @@ export async function POST(
       lines,
     })
   } catch (err) {
-    if (err instanceof AccountsNotInChartError) {
-      return accountsNotInChartResponse(err)
-    }
+    const typed = bookkeepingErrorResponse(err)
+    if (typed) return typed
     return NextResponse.json(
       { error: err instanceof Error ? err.message : 'Failed to create journal entry' },
       { status: 400 }
diff --git a/app/api/transactions/[id]/categorize/route.ts b/app/api/transactions/[id]/categorize/route.ts
index 7ada5465..d87d7740 100644
--- a/app/api/transactions/[id]/categorize/route.ts
+++ b/app/api/transactions/[id]/categorize/route.ts
@@ -5,6 +5,8 @@ import { ensureInitialized } from '@/lib/init'
 import { buildMappingResultFromCategory } from '@/lib/bookkeeping/category-mapping'
 import { getTemplateById, buildMappingResultFromTemplate, validateTemplateForEntity } from '@/lib/bookkeeping/booking-templates'
 import { createTransactionJournalEntry } from '@/lib/bookkeeping/transaction-entries'
+import { bookkeepingErrorResponse } from '@/lib/bookkeeping/errors'
+import { getErrorMessage } from '@/lib/errors/get-error-message'
 import { saveUserMappingRule } from '@/lib/bookkeeping/mapping-engine'
 import { upsertCounterpartyTemplate, buildMappingResultFromCounterpartyTemplate } from '@/lib/bookkeeping/counterparty-templates'
 import { requireCompanyId } from '@/lib/company/context'
@@ -297,7 +299,15 @@ export async function POST(
     }
   } catch (err) {
     console.error('Failed to create journal entry:', err)
-    journalEntryError = err instanceof Error ? err.message : 'Unknown error'
+    // Typed bookkeeping errors: surface a Swedish translation in the response
+    // so the client toast can display it directly.
+    const typedResp = bookkeepingErrorResponse(err)
+    if (typedResp) {
+      const body = (await typedResp.json()) as unknown
+      journalEntryError = getErrorMessage(body, { context: 'transaction' })
+    } else {
+      journalEntryError = err instanceof Error ? err.message : 'Unknown error'
+    }
     // Continue - we still want to save the categorization
   }
 
diff --git a/app/api/transactions/[id]/match-invoice/route.ts b/app/api/transactions/[id]/match-invoice/route.ts
index 64815732..9a95f681 100644
--- a/app/api/transactions/[id]/match-invoice/route.ts
+++ b/app/api/transactions/[id]/match-invoice/route.ts
@@ -5,7 +5,12 @@ import {
   createInvoiceCashEntry,
 } from '@/lib/bookkeeping/invoice-entries'
 import { reverseEntry } from '@/lib/bookkeeping/engine'
-import { AccountsNotInChartError, accountsNotInChartResponse } from '@/lib/bookkeeping/errors'
+import {
+  AccountsNotInChartError,
+  accountsNotInChartResponse,
+  bookkeepingErrorResponse,
+} from '@/lib/bookkeeping/errors'
+import { getErrorMessage } from '@/lib/errors/get-error-message'
 import { validateBody } from '@/lib/api/validate'
 import { MatchInvoiceSchema } from '@/lib/api/schemas'
 import { logMatchEvent } from '@/lib/invoices/match-log'
@@ -122,9 +127,8 @@ export async function POST(
         newState: { journal_entry_id: null },
       })
     } catch (err) {
-      if (err instanceof AccountsNotInChartError) {
-        return accountsNotInChartResponse(err)
-      }
+      const typed = bookkeepingErrorResponse(err)
+      if (typed) return typed
       console.error('Failed to storno conflicting journal entry:', err)
       return NextResponse.json(
         { error: 'Failed to reverse conflicting journal entry' },
@@ -204,11 +208,21 @@ export async function POST(
       journalEntryId = journalEntry?.id ?? null
     }
   } catch (err) {
+    // AccountsNotInChart returns the structured 400 so the UI can open the
+    // account-activation dialog. Other errors are logged and attached to
+    // `journal_entry_error` — the match itself is a valuable business event,
+    // and the user can re-book the payment verifikation separately.
     if (err instanceof AccountsNotInChartError) {
       return accountsNotInChartResponse(err)
     }
     console.error('Failed to create payment journal entry:', err)
-    journalEntryError = err instanceof Error ? err.message : 'Unknown error'
+    const typedResp = bookkeepingErrorResponse(err)
+    if (typedResp) {
+      const body = (await typedResp.json()) as unknown
+      journalEntryError = getErrorMessage(body, { context: 'invoice' })
+    } else {
+      journalEntryError = err instanceof Error ? err.message : 'Unknown error'
+    }
     // Continue - we still want to update the invoice and transaction
   }
 
diff --git a/app/api/transactions/[id]/match-supplier-invoice/route.ts b/app/api/transactions/[id]/match-supplier-invoice/route.ts
index b51aae27..7bf99e55 100644
--- a/app/api/transactions/[id]/match-supplier-invoice/route.ts
+++ b/app/api/transactions/[id]/match-supplier-invoice/route.ts
@@ -4,7 +4,7 @@ import {
   createSupplierInvoicePaymentEntry,
   createSupplierInvoiceCashEntry,
 } from '@/lib/bookkeeping/supplier-invoice-entries'
-import { AccountsNotInChartError, accountsNotInChartResponse } from '@/lib/bookkeeping/errors'
+import { bookkeepingErrorResponse } from '@/lib/bookkeeping/errors'
 import { validateBody } from '@/lib/api/validate'
 import { MatchSupplierInvoiceSchema } from '@/lib/api/schemas'
 import { logMatchEvent } from '@/lib/invoices/match-log'
@@ -128,9 +128,8 @@ export async function POST(
       if (journalEntry) journalEntryId = journalEntry.id
     }
   } catch (err) {
-    if (err instanceof AccountsNotInChartError) {
-      return accountsNotInChartResponse(err)
-    }
+    const typed = bookkeepingErrorResponse(err)
+    if (typed) return typed
     console.error('Failed to create payment journal entry:', err)
   }
 
diff --git a/app/api/transactions/[id]/uncategorize/route.ts b/app/api/transactions/[id]/uncategorize/route.ts
index 14f19063..caac6125 100644
--- a/app/api/transactions/[id]/uncategorize/route.ts
+++ b/app/api/transactions/[id]/uncategorize/route.ts
@@ -1,7 +1,7 @@
 import { createClient } from '@/lib/supabase/server'
 import { NextResponse } from 'next/server'
 import { reverseEntry } from '@/lib/bookkeeping/engine'
-import { AccountsNotInChartError, accountsNotInChartResponse } from '@/lib/bookkeeping/errors'
+import { bookkeepingErrorResponse } from '@/lib/bookkeeping/errors'
 import { ensureInitialized } from '@/lib/init'
 import { requireCompanyId } from '@/lib/company/context'
 import { requireWritePermission } from '@/lib/auth/require-write'
@@ -61,9 +61,8 @@ export async function POST(
   try {
     await reverseEntry(supabase, companyId, user.id, transaction.journal_entry_id)
   } catch (err) {
-    if (err instanceof AccountsNotInChartError) {
-      return accountsNotInChartResponse(err)
-    }
+    const typed = bookkeepingErrorResponse(err)
+    if (typed) return typed
     const message = err instanceof Error ? err.message : 'Reversal failed'
     return NextResponse.json({ error: message }, { status: 500 })
   }
diff --git a/components/bookkeeping/CorrectionEntryDialog.tsx b/components/bookkeeping/CorrectionEntryDialog.tsx
index ff4356c5..62a25334 100644
--- a/components/bookkeeping/CorrectionEntryDialog.tsx
+++ b/components/bookkeeping/CorrectionEntryDialog.tsx
@@ -15,6 +15,7 @@ import { Badge } from '@/components/ui/badge'
 import { AccountNumber } from '@/components/ui/account-number'
 import AccountCombobox from '@/components/bookkeeping/AccountCombobox'
 import { useToast } from '@/components/ui/use-toast'
+import { getErrorMessage } from '@/lib/errors/get-error-message'
 import { Plus, Trash2 } from 'lucide-react'
 import type { JournalEntry, JournalEntryLine, BASAccount } from '@/types'
 
@@ -109,7 +110,10 @@ export default function CorrectionEntryDialog({ entry, open, onOpenChange, onCor
       const result = await res.json()
 
       if (!res.ok) {
-        throw new Error(result.error || 'Failed to create correction')
+        const error = new Error('Failed to create correction') as Error & { body?: unknown; status?: number }
+        error.body = result
+        error.status = res.status
+        throw error
       }
 
       const correctedId = result.data?.corrected?.id
@@ -126,9 +130,10 @@ export default function CorrectionEntryDialog({ entry, open, onOpenChange, onCor
       onOpenChange(false)
       onCorrected()
     } catch (err) {
+      const anyErr = err as { body?: unknown; status?: number }
       toast({
         title: 'Fel',
-        description: err instanceof Error ? err.message : 'Kunde inte skapa ändringsverifikation',
+        description: getErrorMessage(anyErr.body ?? err, { context: 'journal_entry', statusCode: anyErr.status }),
         variant: 'destructive',
       })
     } finally {
diff --git a/components/invoices/PaymentBookingDialog.tsx b/components/invoices/PaymentBookingDialog.tsx
index def7f0c7..aeb009e5 100644
--- a/components/invoices/PaymentBookingDialog.tsx
+++ b/components/invoices/PaymentBookingDialog.tsx
@@ -16,6 +16,7 @@ import { Badge } from '@/components/ui/badge'
 import { useToast } from '@/components/ui/use-toast'
 import AccountCombobox from '@/components/bookkeeping/AccountCombobox'
 import { proposePaymentLines } from '@/lib/bookkeeping/propose-payment-lines'
+import { getErrorMessage } from '@/lib/errors/get-error-message'
 import { formatCurrency } from '@/lib/utils'
 import { createClient } from '@/lib/supabase/client'
 import { useCompany } from '@/contexts/CompanyContext'
@@ -187,15 +188,19 @@ export default function PaymentBookingDialog({
 
       if (!response.ok) {
         const data = await response.json()
-        throw new Error(data.error || 'Kunde inte markera som betald')
+        const error = new Error('Kunde inte markera som betald') as Error & { body?: unknown; status?: number }
+        error.body = data
+        error.status = response.status
+        throw error
       }
 
       onOpenChange(false)
       onSuccess()
     } catch (error) {
+      const anyErr = error as { body?: unknown; status?: number }
       toast({
         title: 'Bokföring misslyckades',
-        description: error instanceof Error ? error.message : 'Försök igen.',
+        description: getErrorMessage(anyErr.body ?? error, { context: 'invoice', statusCode: anyErr.status }),
         variant: 'destructive',
       })
     }
diff --git a/lib/bookkeeping/__tests__/engine.test.ts b/lib/bookkeeping/__tests__/engine.test.ts
index bf1882b2..6e99c230 100644
--- a/lib/bookkeeping/__tests__/engine.test.ts
+++ b/lib/bookkeeping/__tests__/engine.test.ts
@@ -1,5 +1,6 @@
 import { describe, it, expect, vi, beforeEach } from 'vitest'
 import { validateBalance, getSwedishLocalDate, createDraftEntry, reverseEntry } from '../engine'
+import { BookkeepingDatabaseError } from '../errors'
 import type { CreateJournalEntryLineInput, JournalEntryStatus } from '@/types'
 
 // Mock Supabase client for createDraftEntry/reverseEntry tests
@@ -167,7 +168,7 @@ describe('createDraftEntry — cancelled status on line-insert failure', () => {
           { account_number: '3001', debit_amount: 0, credit_amount: 1000 },
         ],
       })
-    ).rejects.toThrow('Failed to create journal entry lines')
+    ).rejects.toThrow(BookkeepingDatabaseError)
 
     // Should call update with cancelled status, NOT delete
     expect(updateMock).toHaveBeenCalledWith({ status: 'cancelled' })
diff --git a/lib/bookkeeping/__tests__/errors.test.ts b/lib/bookkeeping/__tests__/errors.test.ts
new file mode 100644
index 00000000..43c31572
--- /dev/null
+++ b/lib/bookkeeping/__tests__/errors.test.ts
@@ -0,0 +1,258 @@
+import { describe, it, expect } from 'vitest'
+import {
+  AccountsNotInChartError,
+  BookkeepingDatabaseError,
+  CannotCorrectNonPostedError,
+  CannotReverseNonPostedError,
+  CurrencyRevaluationAlreadyExistsError,
+  EntryAlreadyReversedError,
+  EntryDateOutsideFiscalPeriodError,
+  FiscalPeriodNotFoundError,
+  InvalidMappingResultError,
+  JournalEntryNotBalancedError,
+  JournalEntryNotFoundError,
+  accountsNotInChartResponse,
+  bookkeepingErrorResponse,
+  isAccountsNotInChartError,
+  isBookkeepingError,
+} from '../errors'
+
+describe('Typed bookkeeping errors', () => {
+  it('AccountsNotInChartError carries sorted, deduped account numbers', () => {
+    const err = new AccountsNotInChartError(['2641', '1930', '1930', '1510'])
+    expect(err.code).toBe('ACCOUNTS_NOT_IN_CHART')
+    expect(err.name).toBe('AccountsNotInChartError')
+    expect(err.accountNumbers).toEqual(['1510', '1930', '2641'])
+    expect(err).toBeInstanceOf(Error)
+  })
+
+  it('JournalEntryNotBalancedError preserves amounts and kind', () => {
+    const err = new JournalEntryNotBalancedError(100, 80, 'correction')
+    expect(err.code).toBe('JOURNAL_ENTRY_NOT_BALANCED')
+    expect(err.totalDebit).toBe(100)
+    expect(err.totalCredit).toBe(80)
+    expect(err.kind).toBe('correction')
+    expect(err.message).toContain('100')
+    expect(err.message).toContain('80')
+  })
+
+  it('JournalEntryNotBalancedError defaults kind to "draft"', () => {
+    const err = new JournalEntryNotBalancedError(100, 80)
+    expect(err.kind).toBe('draft')
+  })
+
+  it('FiscalPeriodNotFoundError has fixed message', () => {
+    const err = new FiscalPeriodNotFoundError()
+    expect(err.code).toBe('FISCAL_PERIOD_NOT_FOUND')
+    expect(err.message).toBe('Fiscal period not found')
+  })
+
+  it('EntryDateOutsideFiscalPeriodError carries period context', () => {
+    const err = new EntryDateOutsideFiscalPeriodError('2024-06-15', 'FY 2025', '2025-01-01', '2025-12-31')
+    expect(err.code).toBe('ENTRY_DATE_OUTSIDE_FISCAL_PERIOD')
+    expect(err.entryDate).toBe('2024-06-15')
+    expect(err.periodName).toBe('FY 2025')
+    expect(err.periodStart).toBe('2025-01-01')
+    expect(err.periodEnd).toBe('2025-12-31')
+  })
+
+  it('JournalEntryNotFoundError has fixed message', () => {
+    const err = new JournalEntryNotFoundError()
+    expect(err.code).toBe('JOURNAL_ENTRY_NOT_FOUND')
+    expect(err.message).toBe('Journal entry not found')
+  })
+
+  it('CannotReverseNonPostedError carries current status', () => {
+    const err = new CannotReverseNonPostedError('draft')
+    expect(err.code).toBe('CANNOT_REVERSE_NON_POSTED')
+    expect(err.currentStatus).toBe('draft')
+  })
+
+  it('CannotCorrectNonPostedError carries current status', () => {
+    const err = new CannotCorrectNonPostedError('reversed')
+    expect(err.code).toBe('CANNOT_CORRECT_NON_POSTED')
+    expect(err.currentStatus).toBe('reversed')
+  })
+
+  it('EntryAlreadyReversedError has fixed message', () => {
+    const err = new EntryAlreadyReversedError()
+    expect(err.code).toBe('ENTRY_ALREADY_REVERSED')
+  })
+
+  it('CurrencyRevaluationAlreadyExistsError has fixed message', () => {
+    const err = new CurrencyRevaluationAlreadyExistsError()
+    expect(err.code).toBe('CURRENCY_REVALUATION_ALREADY_EXISTS')
+  })
+
+  it('InvalidMappingResultError carries mapping details', () => {
+    const err = new InvalidMappingResultError(null, '3001')
+    expect(err.code).toBe('INVALID_MAPPING_RESULT')
+    expect(err.debitAccount).toBeNull()
+    expect(err.creditAccount).toBe('3001')
+  })
+
+  it('BookkeepingDatabaseError carries operation tag and cause', () => {
+    const err = new BookkeepingDatabaseError('commit_entry', 'constraint violation')
+    expect(err.code).toBe('BOOKKEEPING_DATABASE_ERROR')
+    expect(err.operation).toBe('commit_entry')
+    expect(err.cause).toBe('constraint violation')
+    expect(err.message).toContain('commit_entry')
+    expect(err.message).toContain('constraint violation')
+  })
+
+  it('BookkeepingDatabaseError handles undefined cause', () => {
+    const err = new BookkeepingDatabaseError('commit_entry', undefined)
+    expect(err.message).toContain('commit_entry')
+    expect(err.message).not.toContain('undefined')
+  })
+})
+
+describe('isAccountsNotInChartError', () => {
+  it('returns true for AccountsNotInChartError', () => {
+    expect(isAccountsNotInChartError(new AccountsNotInChartError(['1930']))).toBe(true)
+  })
+
+  it('returns false for other errors', () => {
+    expect(isAccountsNotInChartError(new Error('plain'))).toBe(false)
+    expect(isAccountsNotInChartError(new FiscalPeriodNotFoundError())).toBe(false)
+    expect(isAccountsNotInChartError(null)).toBe(false)
+    expect(isAccountsNotInChartError(undefined)).toBe(false)
+  })
+})
+
+describe('isBookkeepingError', () => {
+  it('returns true for all typed bookkeeping errors', () => {
+    expect(isBookkeepingError(new AccountsNotInChartError(['1930']))).toBe(true)
+    expect(isBookkeepingError(new JournalEntryNotBalancedError(100, 80))).toBe(true)
+    expect(isBookkeepingError(new FiscalPeriodNotFoundError())).toBe(true)
+    expect(isBookkeepingError(new EntryDateOutsideFiscalPeriodError('2024-06-15', 'FY', '2025-01-01', '2025-12-31'))).toBe(true)
+    expect(isBookkeepingError(new JournalEntryNotFoundError())).toBe(true)
+    expect(isBookkeepingError(new CannotReverseNonPostedError('draft'))).toBe(true)
+    expect(isBookkeepingError(new CannotCorrectNonPostedError('draft'))).toBe(true)
+    expect(isBookkeepingError(new EntryAlreadyReversedError())).toBe(true)
+    expect(isBookkeepingError(new CurrencyRevaluationAlreadyExistsError())).toBe(true)
+    expect(isBookkeepingError(new InvalidMappingResultError('1930', '3001'))).toBe(true)
+    expect(isBookkeepingError(new BookkeepingDatabaseError('commit_entry', 'x'))).toBe(true)
+  })
+
+  it('returns false for plain Error', () => {
+    expect(isBookkeepingError(new Error('plain'))).toBe(false)
+    expect(isBookkeepingError(null)).toBe(false)
+    expect(isBookkeepingError('string')).toBe(false)
+  })
+})
+
+describe('accountsNotInChartResponse', () => {
+  it('returns 400 with Swedish message and account_numbers', async () => {
+    const err = new AccountsNotInChartError(['1930', '2641'])
+    const response = accountsNotInChartResponse(err)
+    expect(response.status).toBe(400)
+    const body = await response.json()
+    expect(body.error.code).toBe('ACCOUNTS_NOT_IN_CHART')
+    expect(body.error.message).toContain('1930')
+    expect(body.error.message).toContain('2641')
+    expect(body.error.account_numbers).toEqual(['1930', '2641'])
+  })
+})
+
+describe('bookkeepingErrorResponse', () => {
+  it('returns null for plain Error', () => {
+    expect(bookkeepingErrorResponse(new Error('plain'))).toBeNull()
+    expect(bookkeepingErrorResponse(null)).toBeNull()
+    expect(bookkeepingErrorResponse('string')).toBeNull()
+  })
+
+  it('returns 400 for AccountsNotInChartError (via accountsNotInChartResponse)', async () => {
+    const response = bookkeepingErrorResponse(new AccountsNotInChartError(['1930']))!
+    expect(response.status).toBe(400)
+    const body = await response.json()
+    expect(body.error.code).toBe('ACCOUNTS_NOT_IN_CHART')
+    expect(body.error.account_numbers).toEqual(['1930'])
+  })
+
+  it('returns 400 for JournalEntryNotBalancedError with totalDebit/totalCredit', async () => {
+    const response = bookkeepingErrorResponse(new JournalEntryNotBalancedError(100, 80, 'draft'))!
+    expect(response.status).toBe(400)
+    const body = await response.json()
+    expect(body.error.code).toBe('JOURNAL_ENTRY_NOT_BALANCED')
+    expect(body.error.details).toEqual({ totalDebit: 100, totalCredit: 80, kind: 'draft' })
+  })
+
+  it('returns 404 for FiscalPeriodNotFoundError', async () => {
+    const response = bookkeepingErrorResponse(new FiscalPeriodNotFoundError())!
+    expect(response.status).toBe(404)
+    const body = await response.json()
+    expect(body.error.code).toBe('FISCAL_PERIOD_NOT_FOUND')
+  })
+
+  it('returns 400 for EntryDateOutsideFiscalPeriodError with period details', async () => {
+    const response = bookkeepingErrorResponse(
+      new EntryDateOutsideFiscalPeriodError('2024-06-15', 'FY 2025', '2025-01-01', '2025-12-31')
+    )!
+    expect(response.status).toBe(400)
+    const body = await response.json()
+    expect(body.error.code).toBe('ENTRY_DATE_OUTSIDE_FISCAL_PERIOD')
+    expect(body.error.details).toEqual({
+      entryDate: '2024-06-15',
+      periodName: 'FY 2025',
+      periodStart: '2025-01-01',
+      periodEnd: '2025-12-31',
+    })
+  })
+
+  it('returns 404 for JournalEntryNotFoundError', async () => {
+    const response = bookkeepingErrorResponse(new JournalEntryNotFoundError())!
+    expect(response.status).toBe(404)
+    const body = await response.json()
+    expect(body.error.code).toBe('JOURNAL_ENTRY_NOT_FOUND')
+  })
+
+  it('returns 400 for CannotReverseNonPostedError with currentStatus', async () => {
+    const response = bookkeepingErrorResponse(new CannotReverseNonPostedError('draft'))!
+    expect(response.status).toBe(400)
+    const body = await response.json()
+    expect(body.error.code).toBe('CANNOT_REVERSE_NON_POSTED')
+    expect(body.error.details).toEqual({ currentStatus: 'draft' })
+  })
+
+  it('returns 400 for CannotCorrectNonPostedError with currentStatus', async () => {
+    const response = bookkeepingErrorResponse(new CannotCorrectNonPostedError('reversed'))!
+    expect(response.status).toBe(400)
+    const body = await response.json()
+    expect(body.error.code).toBe('CANNOT_CORRECT_NON_POSTED')
+    expect(body.error.details).toEqual({ currentStatus: 'reversed' })
+  })
+
+  it('returns 409 for EntryAlreadyReversedError (concurrent conflict)', async () => {
+    const response = bookkeepingErrorResponse(new EntryAlreadyReversedError())!
+    expect(response.status).toBe(409)
+    const body = await response.json()
+    expect(body.error.code).toBe('ENTRY_ALREADY_REVERSED')
+  })
+
+  it('returns 409 for CurrencyRevaluationAlreadyExistsError (duplicate)', async () => {
+    const response = bookkeepingErrorResponse(new CurrencyRevaluationAlreadyExistsError())!
+    expect(response.status).toBe(409)
+    const body = await response.json()
+    expect(body.error.code).toBe('CURRENCY_REVALUATION_ALREADY_EXISTS')
+  })
+
+  it('returns 400 for InvalidMappingResultError with mapping details', async () => {
+    const response = bookkeepingErrorResponse(new InvalidMappingResultError(null, '3001'))!
+    expect(response.status).toBe(400)
+    const body = await response.json()
+    expect(body.error.code).toBe('INVALID_MAPPING_RESULT')
+    expect(body.error.details).toEqual({ debitAccount: null, creditAccount: '3001' })
+  })
+
+  it('returns 500 for BookkeepingDatabaseError with operation tag', async () => {
+    const response = bookkeepingErrorResponse(
+      new BookkeepingDatabaseError('commit_entry', 'constraint violation')
+    )!
+    expect(response.status).toBe(500)
+    const body = await response.json()
+    expect(body.error.code).toBe('BOOKKEEPING_DATABASE_ERROR')
+    expect(body.error.details).toEqual({ operation: 'commit_entry' })
+    expect(body.error.message).toContain('constraint violation')
+  })
+})
diff --git a/lib/bookkeeping/__tests__/voucher-atomicity.test.ts b/lib/bookkeeping/__tests__/voucher-atomicity.test.ts
index 1283477b..561904f6 100644
--- a/lib/bookkeeping/__tests__/voucher-atomicity.test.ts
+++ b/lib/bookkeeping/__tests__/voucher-atomicity.test.ts
@@ -7,6 +7,7 @@ vi.mock('@/lib/events', () => ({
 }))
 
 import { commitEntry, getNextVoucherNumber, createJournalEntry } from '../engine'
+import { BookkeepingDatabaseError } from '../errors'
 
 describe('voucher number atomicity', () => {
   beforeEach(() => {
@@ -39,7 +40,7 @@ describe('voucher number atomicity', () => {
 
     await expect(
       getNextVoucherNumber(supabase as never, 'co-1', 'fp-1', 'A')
-    ).rejects.toThrow('Failed to get next voucher number: connection lost')
+    ).rejects.toThrow(BookkeepingDatabaseError)
   })
 
   /**
@@ -59,7 +60,7 @@ describe('voucher number atomicity', () => {
 
     await expect(
       commitEntry(supabase as never, 'co-1', 'user-1', 'entry-1')
-    ).rejects.toThrow('Failed to commit journal entry: Journal entry is not balanced')
+    ).rejects.toThrow(BookkeepingDatabaseError)
 
     // The atomic RPC was called — it failed, rolling back both the
     // sequence increment and the status update. No burned number.
@@ -231,7 +232,7 @@ describe('createJournalEntry orphan draft cleanup', () => {
           { account_number: '1510', debit_amount: 0, credit_amount: 1000 },
         ],
       })
-    ).rejects.toThrow('Failed to commit journal entry')
+    ).rejects.toThrow(BookkeepingDatabaseError)
 
     // The orphan draft must have been cancelled with CAS guard (status='draft')
     expect(cancelUpdate).toHaveBeenCalledWith({ status: 'cancelled' })
diff --git a/lib/bookkeeping/currency-revaluation.ts b/lib/bookkeeping/currency-revaluation.ts
index cfc140dd..dc3bb51b 100644
--- a/lib/bookkeeping/currency-revaluation.ts
+++ b/lib/bookkeeping/currency-revaluation.ts
@@ -1,6 +1,10 @@
 import type { SupabaseClient } from '@supabase/supabase-js'
 import { fetchMultipleRates } from '@/lib/currency/riksbanken'
 import { createJournalEntry } from '@/lib/bookkeeping/engine'
+import {
+  BookkeepingDatabaseError,
+  CurrencyRevaluationAlreadyExistsError,
+} from '@/lib/bookkeeping/errors'
 import type {
   Currency,
   Invoice,
@@ -29,7 +33,7 @@ export async function getOpenForeignCurrencyReceivables(
     .not('exchange_rate', 'is', null)
 
   if (error) {
-    throw new Error(`Failed to fetch foreign currency receivables: ${error.message}`)
+    throw new BookkeepingDatabaseError('fetch_currency_receivables', error.message)
   }
 
   return (data || []) as Invoice[]
@@ -53,7 +57,7 @@ export async function getOpenForeignCurrencyPayables(
     .not('exchange_rate', 'is', null)
 
   if (error) {
-    throw new Error(`Failed to fetch foreign currency payables: ${error.message}`)
+    throw new BookkeepingDatabaseError('fetch_currency_payables', error.message)
   }
 
   return (data || []) as SupplierInvoice[]
@@ -290,11 +294,11 @@ export async function executeCurrencyRevaluation(
     .eq('status', 'posted')
 
   if (checkError) {
-    throw new Error(`Failed to check existing revaluation: ${checkError.message}`)
+    throw new BookkeepingDatabaseError('check_existing_revaluation', checkError.message)
   }
 
   if ((count ?? 0) > 0) {
-    throw new Error('Currency revaluation already exists for this period')
+    throw new CurrencyRevaluationAlreadyExistsError()
   }
 
   const preview = await previewCurrencyRevaluation(supabase, companyId, closingDate)
diff --git a/lib/bookkeeping/engine.ts b/lib/bookkeeping/engine.ts
index b45d7e65..4151df85 100644
--- a/lib/bookkeeping/engine.ts
+++ b/lib/bookkeeping/engine.ts
@@ -1,6 +1,15 @@
 import type { SupabaseClient } from '@supabase/supabase-js'
 import { eventBus } from '@/lib/events'
-import { AccountsNotInChartError } from '@/lib/bookkeeping/errors'
+import {
+  AccountsNotInChartError,
+  BookkeepingDatabaseError,
+  CannotReverseNonPostedError,
+  EntryAlreadyReversedError,
+  EntryDateOutsideFiscalPeriodError,
+  FiscalPeriodNotFoundError,
+  JournalEntryNotBalancedError,
+  JournalEntryNotFoundError,
+} from '@/lib/bookkeeping/errors'
 import type {
   CreateJournalEntryInput,
   CreateJournalEntryLineInput,
@@ -48,7 +57,7 @@ export async function getNextVoucherNumber(
   })
 
   if (error) {
-    throw new Error(`Failed to get next voucher number: ${error.message}`)
+    throw new BookkeepingDatabaseError('get_next_voucher_number', error.message)
   }
 
   return data as number
@@ -87,7 +96,7 @@ async function resolveAccountIds(
   const { data: accounts, error } = await query
 
   if (error) {
-    throw new Error(`Failed to resolve account IDs: ${error.message}`)
+    throw new BookkeepingDatabaseError('resolve_account_ids', error.message)
   }
 
   const map = new Map<string, string>()
@@ -165,9 +174,7 @@ export async function createDraftEntry(
   // Validate balance
   const balance = validateBalance(input.lines)
   if (!balance.valid) {
-    throw new Error(
-      `Journal entry is not balanced: debits (${balance.totalDebit}) != credits (${balance.totalCredit})`
-    )
+    throw new JournalEntryNotBalancedError(balance.totalDebit, balance.totalCredit, 'draft')
   }
 
   // Validate that entry_date falls within the selected fiscal period
@@ -179,12 +186,15 @@ export async function createDraftEntry(
     .single()
 
   if (periodError || !period) {
-    throw new Error('Fiscal period not found')
+    throw new FiscalPeriodNotFoundError()
   }
 
   if (input.entry_date < period.period_start || input.entry_date > period.period_end) {
-    throw new Error(
-      `Entry date ${input.entry_date} is outside fiscal period "${period.name}" (${period.period_start} - ${period.period_end})`
+    throw new EntryDateOutsideFiscalPeriodError(
+      input.entry_date,
+      period.name,
+      period.period_start,
+      period.period_end
     )
   }
 
@@ -218,7 +228,7 @@ export async function createDraftEntry(
     .single()
 
   if (entryError || !entry) {
-    throw new Error(`Failed to create draft journal entry: ${entryError?.message}`)
+    throw new BookkeepingDatabaseError('create_draft_entry', entryError?.message)
   }
 
   // Insert journal entry lines with dimensions
@@ -230,7 +240,7 @@ export async function createDraftEntry(
 
   if (linesError) {
     await supabase.from('journal_entries').update({ status: 'cancelled' }).eq('id', entry.id)
-    throw new Error(`Failed to create journal entry lines: ${linesError.message}`)
+    throw new BookkeepingDatabaseError('create_entry_lines', linesError.message)
   }
 
   // Fetch complete entry with lines
@@ -275,7 +285,7 @@ export async function commitEntry(
   })
 
   if (commitError) {
-    throw new Error(`Failed to commit journal entry: ${commitError.message}`)
+    throw new BookkeepingDatabaseError('commit_entry', commitError.message)
   }
 
   // Fetch complete posted entry with lines
@@ -362,11 +372,11 @@ export async function reverseEntry(
     .single()
 
   if (error || !original) {
-    throw new Error('Journal entry not found')
+    throw new JournalEntryNotFoundError()
   }
 
   if (original.status !== 'posted') {
-    throw new Error('Can only reverse posted entries')
+    throw new CannotReverseNonPostedError(original.status)
   }
 
   const lines = (original.lines as JournalEntryLine[]) || []
@@ -430,7 +440,7 @@ export async function reverseEntry(
     .single()
 
   if (reversalError || !reversalEntry) {
-    throw new Error(`Failed to create reversal entry: ${reversalError?.message}`)
+    throw new BookkeepingDatabaseError('create_reversal_entry', reversalError?.message)
   }
 
   // Insert reversal lines with dimensions
@@ -443,7 +453,7 @@ export async function reverseEntry(
   if (linesError) {
     await supabase.from('journal_entries').update({ status: 'cancelled' }).eq('id', reversalEntry.id)
     await supabase.from('journal_entry_lines').delete().eq('journal_entry_id', reversalEntry.id)
-    throw new Error(`Failed to create reversal lines: ${linesError.message}`)
+    throw new BookkeepingDatabaseError('create_reversal_lines', linesError.message)
   }
 
   // Post the reversal entry
@@ -455,7 +465,7 @@ export async function reverseEntry(
   if (postError) {
     await supabase.from('journal_entries').update({ status: 'cancelled' }).eq('id', reversalEntry.id)
     await supabase.from('journal_entry_lines').delete().eq('journal_entry_id', reversalEntry.id)
-    throw new Error(`Failed to post reversal entry: ${postError.message}`)
+    throw new BookkeepingDatabaseError('post_reversal_entry', postError.message)
   }
 
   // Mark original as reversed with reversed_by_id link (CAS guard: only if still 'posted')
@@ -474,7 +484,7 @@ export async function reverseEntry(
     // reversal as cancelled so it's excluded from reports but remains traceable.
     await supabase.from('journal_entries').update({ status: 'cancelled' }).eq('id', reversalEntry.id)
     await supabase.from('journal_entry_lines').delete().eq('journal_entry_id', reversalEntry.id)
-    throw new Error('Entry was already reversed by a concurrent operation')
+    throw new EntryAlreadyReversedError()
   }
 
   // If this was a payment entry, sync the linked invoice/supplier-invoice status
diff --git a/lib/bookkeeping/errors.ts b/lib/bookkeeping/errors.ts
index e9329667..c7347352 100644
--- a/lib/bookkeeping/errors.ts
+++ b/lib/bookkeeping/errors.ts
@@ -1,6 +1,24 @@
 import { NextResponse } from 'next/server'
 
+// ============================================================================
+// Error codes
+// ============================================================================
+
 export const ACCOUNTS_NOT_IN_CHART = 'ACCOUNTS_NOT_IN_CHART' as const
+export const JOURNAL_ENTRY_NOT_BALANCED = 'JOURNAL_ENTRY_NOT_BALANCED' as const
+export const FISCAL_PERIOD_NOT_FOUND = 'FISCAL_PERIOD_NOT_FOUND' as const
+export const ENTRY_DATE_OUTSIDE_FISCAL_PERIOD = 'ENTRY_DATE_OUTSIDE_FISCAL_PERIOD' as const
+export const JOURNAL_ENTRY_NOT_FOUND = 'JOURNAL_ENTRY_NOT_FOUND' as const
+export const CANNOT_REVERSE_NON_POSTED = 'CANNOT_REVERSE_NON_POSTED' as const
+export const CANNOT_CORRECT_NON_POSTED = 'CANNOT_CORRECT_NON_POSTED' as const
+export const ENTRY_ALREADY_REVERSED = 'ENTRY_ALREADY_REVERSED' as const
+export const CURRENCY_REVALUATION_ALREADY_EXISTS = 'CURRENCY_REVALUATION_ALREADY_EXISTS' as const
+export const INVALID_MAPPING_RESULT = 'INVALID_MAPPING_RESULT' as const
+export const BOOKKEEPING_DATABASE_ERROR = 'BOOKKEEPING_DATABASE_ERROR' as const
+
+// ============================================================================
+// AccountsNotInChartError — kept for back-compat (many existing call sites)
+// ============================================================================
 
 export class AccountsNotInChartError extends Error {
   readonly code = ACCOUNTS_NOT_IN_CHART
@@ -18,10 +36,166 @@ export function isAccountsNotInChartError(err: unknown): err is AccountsNotInCha
   return err instanceof AccountsNotInChartError
 }
 
+// ============================================================================
+// Semantic errors — carry structured data so getErrorMessage can format rich
+// Swedish translations with amounts / period names / status.
+// ============================================================================
+
+export class JournalEntryNotBalancedError extends Error {
+  readonly code = JOURNAL_ENTRY_NOT_BALANCED
+  constructor(
+    public readonly totalDebit: number,
+    public readonly totalCredit: number,
+    public readonly kind: 'draft' | 'correction' = 'draft'
+  ) {
+    super(`Journal entry is not balanced: debits (${totalDebit}) != credits (${totalCredit})`)
+    this.name = 'JournalEntryNotBalancedError'
+  }
+}
+
+export class FiscalPeriodNotFoundError extends Error {
+  readonly code = FISCAL_PERIOD_NOT_FOUND
+  constructor() {
+    super('Fiscal period not found')
+    this.name = 'FiscalPeriodNotFoundError'
+  }
+}
+
+export class EntryDateOutsideFiscalPeriodError extends Error {
+  readonly code = ENTRY_DATE_OUTSIDE_FISCAL_PERIOD
+  constructor(
+    public readonly entryDate: string,
+    public readonly periodName: string,
+    public readonly periodStart: string,
+    public readonly periodEnd: string
+  ) {
+    super(
+      `Entry date ${entryDate} is outside fiscal period "${periodName}" (${periodStart} - ${periodEnd})`
+    )
+    this.name = 'EntryDateOutsideFiscalPeriodError'
+  }
+}
+
+export class JournalEntryNotFoundError extends Error {
+  readonly code = JOURNAL_ENTRY_NOT_FOUND
+  constructor() {
+    super('Journal entry not found')
+    this.name = 'JournalEntryNotFoundError'
+  }
+}
+
+export class CannotReverseNonPostedError extends Error {
+  readonly code = CANNOT_REVERSE_NON_POSTED
+  constructor(public readonly currentStatus: string) {
+    super('Can only reverse posted entries')
+    this.name = 'CannotReverseNonPostedError'
+  }
+}
+
+export class CannotCorrectNonPostedError extends Error {
+  readonly code = CANNOT_CORRECT_NON_POSTED
+  constructor(public readonly currentStatus: string) {
+    super('Can only correct posted entries')
+    this.name = 'CannotCorrectNonPostedError'
+  }
+}
+
+export class EntryAlreadyReversedError extends Error {
+  readonly code = ENTRY_ALREADY_REVERSED
+  constructor() {
+    super('Entry was already reversed by a concurrent operation')
+    this.name = 'EntryAlreadyReversedError'
+  }
+}
+
+export class CurrencyRevaluationAlreadyExistsError extends Error {
+  readonly code = CURRENCY_REVALUATION_ALREADY_EXISTS
+  constructor() {
+    super('Currency revaluation already exists for this period')
+    this.name = 'CurrencyRevaluationAlreadyExistsError'
+  }
+}
+
+export class InvalidMappingResultError extends Error {
+  readonly code = INVALID_MAPPING_RESULT
+  constructor(
+    public readonly debitAccount: string | null | undefined,
+    public readonly creditAccount: string | null | undefined
+  ) {
+    super(
+      `Invalid mapping result: debit_account="${debitAccount}", credit_account="${creditAccount}". Both must be non-empty.`
+    )
+    this.name = 'InvalidMappingResultError'
+  }
+}
+
+// ============================================================================
+// BookkeepingDatabaseError — single wrapper for all "Failed to <op>: <cause>"
+// engine throws. The `operation` tag is preserved for logs; the cause string
+// stays in `message` so period-lock / trigger messages can still be matched
+// by regex patterns in get-error-message.ts.
+// ============================================================================
+
+export type BookkeepingOperation =
+  | 'get_next_voucher_number'
+  | 'resolve_account_ids'
+  | 'create_draft_entry'
+  | 'create_entry_lines'
+  | 'commit_entry'
+  | 'create_reversal_entry'
+  | 'create_reversal_lines'
+  | 'post_reversal_entry'
+  | 'create_corrected_entry'
+  | 'create_corrected_lines'
+  | 'post_corrected_entry'
+  | 'fetch_currency_receivables'
+  | 'fetch_currency_payables'
+  | 'check_existing_revaluation'
+
+export class BookkeepingDatabaseError extends Error {
+  readonly code = BOOKKEEPING_DATABASE_ERROR
+  constructor(
+    public readonly operation: BookkeepingOperation,
+    public readonly cause: string | undefined
+  ) {
+    super(cause ? `Database operation "${operation}" failed: ${cause}` : `Database operation "${operation}" failed`)
+    this.name = 'BookkeepingDatabaseError'
+  }
+}
+
+// ============================================================================
+// Type guard
+// ============================================================================
+
+/**
+ * True if `err` is any typed bookkeeping error. Use this in inner catch blocks
+ * that want to re-throw domain errors so the outer handler can translate them
+ * via bookkeepingErrorResponse().
+ */
+export function isBookkeepingError(err: unknown): boolean {
+  return (
+    err instanceof AccountsNotInChartError ||
+    err instanceof JournalEntryNotBalancedError ||
+    err instanceof FiscalPeriodNotFoundError ||
+    err instanceof EntryDateOutsideFiscalPeriodError ||
+    err instanceof JournalEntryNotFoundError ||
+    err instanceof CannotReverseNonPostedError ||
+    err instanceof CannotCorrectNonPostedError ||
+    err instanceof EntryAlreadyReversedError ||
+    err instanceof CurrencyRevaluationAlreadyExistsError ||
+    err instanceof InvalidMappingResultError ||
+    err instanceof BookkeepingDatabaseError
+  )
+}
+
+// ============================================================================
+// Response helpers
+// ============================================================================
+
 /**
  * Build a structured 400 response for AccountsNotInChartError.
- * API routes catch the typed error and return this so the frontend can
- * open ActivateAccountsDialog and retry the request after activation.
+ * Kept for back-compat with existing callers; new code should prefer
+ * bookkeepingErrorResponse() which covers all typed bookkeeping errors.
  */
 export function accountsNotInChartResponse(err: AccountsNotInChartError) {
   return NextResponse.json(
@@ -35,3 +209,138 @@ export function accountsNotInChartResponse(err: AccountsNotInChartError) {
     { status: 400 }
   )
 }
+
+/**
+ * Build a structured JSON response for any typed bookkeeping error.
+ * Returns null if `err` is not a recognized bookkeeping error so callers can
+ * fall through to their existing generic handling.
+ *
+ * Response shape: { error: { code, message, details? } }
+ * HTTP status: 404 for *_NOT_FOUND, 409 for concurrent/duplicate conflicts,
+ * 500 for BOOKKEEPING_DATABASE_ERROR, 400 otherwise.
+ */
+export function bookkeepingErrorResponse(err: unknown): NextResponse | null {
+  if (err instanceof AccountsNotInChartError) {
+    return accountsNotInChartResponse(err)
+  }
+
+  if (err instanceof JournalEntryNotBalancedError) {
+    return NextResponse.json(
+      {
+        error: {
+          code: err.code,
+          message: err.message,
+          details: {
+            totalDebit: err.totalDebit,
+            totalCredit: err.totalCredit,
+            kind: err.kind,
+          },
+        },
+      },
+      { status: 400 }
+    )
+  }
+
+  if (err instanceof FiscalPeriodNotFoundError) {
+    return NextResponse.json(
+      { error: { code: err.code, message: err.message } },
+      { status: 404 }
+    )
+  }
+
+  if (err instanceof EntryDateOutsideFiscalPeriodError) {
+    return NextResponse.json(
+      {
+        error: {
+          code: err.code,
+          message: err.message,
+          details: {
+            entryDate: err.entryDate,
+            periodName: err.periodName,
+            periodStart: err.periodStart,
+            periodEnd: err.periodEnd,
+          },
+        },
+      },
+      { status: 400 }
+    )
+  }
+
+  if (err instanceof JournalEntryNotFoundError) {
+    return NextResponse.json(
+      { error: { code: err.code, message: err.message } },
+      { status: 404 }
+    )
+  }
+
+  if (err instanceof CannotReverseNonPostedError) {
+    return NextResponse.json(
+      {
+        error: {
+          code: err.code,
+          message: err.message,
+          details: { currentStatus: err.currentStatus },
+        },
+      },
+      { status: 400 }
+    )
+  }
+
+  if (err instanceof CannotCorrectNonPostedError) {
+    return NextResponse.json(
+      {
+        error: {
+          code: err.code,
+          message: err.message,
+          details: { currentStatus: err.currentStatus },
+        },
+      },
+      { status: 400 }
+    )
+  }
+
+  if (err instanceof EntryAlreadyReversedError) {
+    return NextResponse.json(
+      { error: { code: err.code, message: err.message } },
+      { status: 409 }
+    )
+  }
+
+  if (err instanceof CurrencyRevaluationAlreadyExistsError) {
+    return NextResponse.json(
+      { error: { code: err.code, message: err.message } },
+      { status: 409 }
+    )
+  }
+
+  if (err instanceof InvalidMappingResultError) {
+    return NextResponse.json(
+      {
+        error: {
+          code: err.code,
+          message: err.message,
+          details: {
+            debitAccount: err.debitAccount,
+            creditAccount: err.creditAccount,
+          },
+        },
+      },
+      { status: 400 }
+    )
+  }
+
+  if (err instanceof BookkeepingDatabaseError) {
+    return NextResponse.json(
+      {
+        error: {
+          code: err.code,
+          message: err.message,
+          details: { operation: err.operation },
+        },
+      },
+      { status: 500 }
+    )
+  }
+
+  return null
+}
diff --git a/lib/bookkeeping/transaction-entries.ts b/lib/bookkeeping/transaction-entries.ts
index b9a4e87d..6f80a728 100644
--- a/lib/bookkeeping/transaction-entries.ts
+++ b/lib/bookkeeping/transaction-entries.ts
@@ -1,6 +1,7 @@
 import { createJournalEntry, findFiscalPeriod } from './engine'
 import { resolveSekAmount, buildCurrencyMetadata } from './currency-utils'
 import { extractNetAmount, extractVatAmount } from './vat-entries'
+import { InvalidMappingResultError } from '@/lib/bookkeeping/errors'
 import { createLogger } from '@/lib/logger'
 import type { SupabaseClient } from '@supabase/supabase-js'
 import type {
@@ -47,9 +48,7 @@ export async function createTransactionJournalEntry(
   mappingResult: MappingResult
 ): Promise<JournalEntry | null> {
   if (!mappingResult.debit_account || !mappingResult.credit_account) {
-    throw new Error(
-      `Invalid mapping result: debit_account="${mappingResult.debit_account}", credit_account="${mappingResult.credit_account}". Both must be non-empty.`
-    )
+    throw new InvalidMappingResultError(mappingResult.debit_account, mappingResult.credit_account)
   }
 
   const fiscalPeriodId = await findFiscalPeriod(supabase, companyId, transaction.date)
diff --git a/lib/core/bookkeeping/__tests__/storno-service.test.ts b/lib/core/bookkeeping/__tests__/storno-service.test.ts
index f6f6ad9d..e1e5add0 100644
--- a/lib/core/bookkeeping/__tests__/storno-service.test.ts
+++ b/lib/core/bookkeeping/__tests__/storno-service.test.ts
@@ -1,6 +1,7 @@
 import { describe, it, expect, vi, beforeEach } from 'vitest'
 import { eventBus } from '@/lib/events/bus'
 import { makeJournalEntry, makeJournalEntryLine } from '@/tests/helpers'
+import { BookkeepingDatabaseError } from '@/lib/bookkeeping/errors'
 
 // ============================================================
 // Mock — separate client (no .then) from query builder (thenable)
@@ -172,7 +173,7 @@ describe('correctEntry', () => {
     const supabase = makeClient()
     await expect(
       correctEntry(supabase as never, 'company-1', 'user-1', 'orig-1', correctedLines)
-    ).rejects.toThrow('Failed to create corrected entry')
+    ).rejects.toThrow(BookkeepingDatabaseError)
   })
 
   it('cancels reversal entry when reversal lines fail', async () => {
@@ -189,7 +190,7 @@ describe('correctEntry', () => {
     const supabase = makeClient()
     await expect(
       correctEntry(supabase as never, 'company-1', 'user-1', 'orig-1', correctedLines)
-    ).rejects.toThrow('Failed to create reversal lines')
+    ).rejects.toThrow(BookkeepingDatabaseError)
   })
 
   it('emits journal_entry.corrected event', async () => {
diff --git a/lib/core/bookkeeping/storno-service.ts b/lib/core/bookkeeping/storno-service.ts
index 264e1b00..5b5ca2ce 100644
--- a/lib/core/bookkeeping/storno-service.ts
+++ b/lib/core/bookkeeping/storno-service.ts
@@ -6,7 +6,14 @@ import type {
   JournalEntryLine,
 } from '@/types'
 import { validateBalance, getNextVoucherNumber, getSwedishLocalDate } from '@/lib/bookkeeping/engine'
-import { AccountsNotInChartError } from '@/lib/bookkeeping/errors'
+import {
+  AccountsNotInChartError,
+  BookkeepingDatabaseError,
+  CannotCorrectNonPostedError,
+  EntryAlreadyReversedError,
+  JournalEntryNotBalancedError,
+  JournalEntryNotFoundError,
+} from '@/lib/bookkeeping/errors'
 
 /**
  * Storno Service - 3-step correction flow per Bokföringslagen
@@ -55,9 +62,7 @@ export async function correctEntry(
   // Validate the corrected lines are balanced
   const balance = validateBalance(correctedLines)
   if (!balance.valid) {
-    throw new Error(
-      `Corrected entry is not balanced: debits (${balance.totalDebit}) != credits (${balance.totalCredit})`
-    )
+    throw new JournalEntryNotBalancedError(balance.totalDebit, balance.totalCredit, 'correction')
   }
 
   // Fetch original entry with lines
@@ -69,11 +74,11 @@ export async function correctEntry(
     .single()
 
   if (fetchError || !original) {
-    throw new Error('Original journal entry not found')
+    throw new JournalEntryNotFoundError()
   }
 
   if (original.status !== 'posted') {
-    throw new Error('Can only correct posted entries')
+    throw new CannotCorrectNonPostedError(original.status)
   }
 
   const originalLines = (original.lines as JournalEntryLine[]) || []
@@ -104,7 +109,7 @@ export async function correctEntry(
     .single()
 
   if (reversalError || !reversalEntry) {
-    throw new Error(`Failed to create reversal entry: ${reversalError?.message}`)
+    throw new BookkeepingDatabaseError('create_reversal_entry', reversalError?.message)
   }
 
   // Insert reversed lines (swap debit and credit)
@@ -130,7 +135,7 @@ export async function correctEntry(
 
   if (reversalLinesError) {
     await cancelEntry(supabase, reversalEntry.id)
-    throw new Error(`Failed to create reversal lines: ${reversalLinesError.message}`)
+    throw new BookkeepingDatabaseError('create_reversal_lines', reversalLinesError.message)
   }
 
   // Post the reversal entry
@@ -141,7 +146,7 @@ export async function correctEntry(
 
   if (postReversalError) {
     await cancelEntry(supabase, reversalEntry.id)
-    throw new Error(`Failed to post reversal entry: ${postReversalError.message}`)
+    throw new BookkeepingDatabaseError('post_reversal_entry', postReversalError.message)
   }
 
   // NOTE: Original entry is NOT marked as 'reversed' here. We defer that
@@ -200,7 +205,7 @@ export async function correctEntry(
       .single()
 
     if (correctedError || !newEntry) {
-      throw new Error(`Failed to create corrected entry: ${correctedError?.message}`)
+      throw new BookkeepingDatabaseError('create_corrected_entry', correctedError?.message)
     }
 
     correctedEntry = newEntry
@@ -230,7 +235,7 @@ export async function correctEntry(
 
     if (correctedLinesError) {
       await cancelEntry(supabase, correctedEntry.id)
-      throw new Error(`Failed to create corrected lines: ${correctedLinesError.message}`)
+      throw new BookkeepingDatabaseError('create_corrected_lines', correctedLinesError.message)
     }
 
     // Post the corrected entry
@@ -241,7 +246,7 @@ export async function correctEntry(
 
     if (postCorrectedError) {
       await cancelEntry(supabase, correctedEntry.id)
-      throw new Error(`Failed to post corrected entry: ${postCorrectedError.message}`)
+      throw new BookkeepingDatabaseError('post_corrected_entry', postCorrectedError.message)
     }
   } catch (err) {
     // Cancel the reversal entry (posted → cancelled). Original was never
@@ -265,7 +270,7 @@ export async function correctEntry(
     // Concurrent reversal beat us — cancel both our entries
     await cancelEntry(supabase, reversalEntry.id)
     await cancelEntry(supabase, correctedEntry!.id)
-    throw new Error('Entry was already reversed by a concurrent operation')
+    throw new EntryAlreadyReversedError()
   }
 
   // ===== Step 3: Fetch complete entries =====
diff --git a/lib/errors/__tests__/get-error-message.test.ts b/lib/errors/__tests__/get-error-message.test.ts
new file mode 100644
index 00000000..22015f4b
--- /dev/null
+++ b/lib/errors/__tests__/get-error-message.test.ts
@@ -0,0 +1,128 @@
+import { describe, it, expect } from 'vitest'
+import { getErrorMessage } from '../get-error-message'
+
+describe('getErrorMessage — typed bookkeeping error codes', () => {
+  it('ACCOUNTS_NOT_IN_CHART → lists accounts to activate', () => {
+    const msg = getErrorMessage({
+      error: { code: 'ACCOUNTS_NOT_IN_CHART', message: '...', account_numbers: ['1930', '2641'] },
+    })
+    expect(msg).toBe('Följande konton behöver aktiveras: 1930, 2641')
+  })
+
+  it('JOURNAL_ENTRY_NOT_BALANCED with details → rich amount message', () => {
+    const msg = getErrorMessage({
+      error: {
+        code: 'JOURNAL_ENTRY_NOT_BALANCED',
+        message: 'Journal entry is not balanced: debits (100) != credits (80)',
+        details: { totalDebit: 100, totalCredit: 80, kind: 'draft' },
+      },
+    })
+    expect(msg).toContain('balanserar inte')
+    expect(msg).toContain('debet')
+    expect(msg).toContain('kredit')
+    expect(msg).toMatch(/100/)
+    expect(msg).toMatch(/80/)
+  })
+
+  it('JOURNAL_ENTRY_NOT_BALANCED without details → fallback Swedish message', () => {
+    const msg = getErrorMessage({
+      error: { code: 'JOURNAL_ENTRY_NOT_BALANCED', message: '...' },
+    })
+    expect(msg).toBe('Verifikationen balanserar inte. Kontrollera att debet och kredit är lika stora.')
+  })
+
+  it('FISCAL_PERIOD_NOT_FOUND → Swedish message', () => {
+    const msg = getErrorMessage({ error: { code: 'FISCAL_PERIOD_NOT_FOUND', message: '...' } })
+    expect(msg).toBe('Räkenskapsperioden kunde inte hittas.')
+  })
+
+  it('ENTRY_DATE_OUTSIDE_FISCAL_PERIOD → Swedish message', () => {
+    const msg = getErrorMessage({ error: { code: 'ENTRY_DATE_OUTSIDE_FISCAL_PERIOD', message: '...' } })
+    expect(msg).toBe('Datumet ligger utanför det valda räkenskapsåret.')
+  })
+
+  it('JOURNAL_ENTRY_NOT_FOUND → Swedish message', () => {
+    const msg = getErrorMessage({ error: { code: 'JOURNAL_ENTRY_NOT_FOUND', message: '...' } })
+    expect(msg).toBe('Verifikationen kunde inte hittas.')
+  })
+
+  it('CANNOT_REVERSE_NON_POSTED → Swedish message', () => {
+    const msg = getErrorMessage({ error: { code: 'CANNOT_REVERSE_NON_POSTED', message: '...' } })
+    expect(msg).toBe('Endast bokförda verifikationer kan stornas.')
+  })
+
+  it('CANNOT_CORRECT_NON_POSTED → Swedish message', () => {
+    const msg = getErrorMessage({ error: { code: 'CANNOT_CORRECT_NON_POSTED', message: '...' } })
+    expect(msg).toBe('Endast bokförda verifikationer kan rättas.')
+  })
+
+  it('ENTRY_ALREADY_REVERSED → Swedish concurrent-conflict message', () => {
+    const msg = getErrorMessage({ error: { code: 'ENTRY_ALREADY_REVERSED', message: '...' } })
+    expect(msg).toContain('redan stornats')
+    expect(msg).toContain('Ladda om sidan')
+  })
+
+  it('CURRENCY_REVALUATION_ALREADY_EXISTS → Swedish message', () => {
+    const msg = getErrorMessage({ error: { code: 'CURRENCY_REVALUATION_ALREADY_EXISTS', message: '...' } })
+    expect(msg).toBe('En valutaomvärdering finns redan för denna period.')
+  })
+
+  it('INVALID_MAPPING_RESULT → Swedish message', () => {
+    const msg = getErrorMessage({ error: { code: 'INVALID_MAPPING_RESULT', message: '...' } })
+    expect(msg).toBe('Kontering saknas för transaktionen. Kontrollera bokföringsreglerna.')
+  })
+
+  it('BOOKKEEPING_DATABASE_ERROR → generic "kunde inte sparas" when no pattern matches', () => {
+    const msg = getErrorMessage({
+      error: {
+        code: 'BOOKKEEPING_DATABASE_ERROR',
+        message: 'Database operation "commit_entry" failed: some random constraint',
+      },
+    })
+    expect(msg).toBe('Verifikationen kunde inte sparas. Försök igen.')
+  })
+
+  it('BOOKKEEPING_DATABASE_ERROR falls through to regex pattern for period lock', () => {
+    // Period-lock trigger errors come through as DB errors — message should still
+    // match the locked-period pattern and produce the specific Swedish message.
+    const msg = getErrorMessage({
+      error: {
+        code: 'BOOKKEEPING_DATABASE_ERROR',
+        message: 'Cannot create entry in locked/closed fiscal period',
+      },
+    })
+    expect(msg).toBe('Perioden är låst. Verifikationen kan inte skapas i en stängd eller låst period.')
+  })
+})
+
+describe('getErrorMessage — existing patterns still work', () => {
+  it('regex match for "Entry date ... outside fiscal period" on plain string', () => {
+    const msg = getErrorMessage('Entry date 2024-06-15 is outside fiscal period "FY 2025"')
+    expect(msg).toBe('Datumet ligger utanför det valda räkenskapsåret.')
+  })
+
+  it('regex match for "locked/closed fiscal period" on plain string', () => {
+    const msg = getErrorMessage('Cannot create entry in locked/closed fiscal period')
+    expect(msg).toBe('Perioden är låst. Verifikationen kan inte skapas i en stängd eller låst period.')
+  })
+
+  it('Swedish message passes through unchanged', () => {
+    const msg = getErrorMessage('Bokföringen är låst t.o.m. 2024-12-31.')
+    expect(msg).toBe('Bokföringen är låst t.o.m. 2024-12-31.')
+  })
+
+  it('falls through to context fallback when no pattern matches', () => {
+    const msg = getErrorMessage('Random English error', { context: 'transaction' })
+    expect(msg).toBe('Kunde inte hantera transaktionen. Försök igen.')
+  })
+
+  it('falls through to HTTP status map', () => {
+    const msg = getErrorMessage(null, { statusCode: 404 })
+    expect(msg).toBe('Resursen kunde inte hittas.')
+  })
+
+  it('falls through to generic message', () => {
+    const msg = getErrorMessage(null)
+    expect(msg).toBe('Något gick fel. Försök igen.')
+  })
+})
diff --git a/lib/errors/get-error-message.ts b/lib/errors/get-error-message.ts
index 3d335160..a79c0663 100644
--- a/lib/errors/get-error-message.ts
+++ b/lib/errors/get-error-message.ts
@@ -9,6 +9,8 @@
  * 5. Generic fallback
  */
 
+import { formatCurrency } from '@/lib/utils'
+
 type ErrorContext =
   | 'invoice'
   | 'supplier_invoice'
@@ -194,9 +196,11 @@ export function getErrorMessage(
 ): string {
   const { context, statusCode } = options
 
-  // 1. If it's a string, check if it's already Swedish
+  // 1. If it's a string, check if it's already Swedish or matches a known pattern
   if (typeof error === 'string' && error.trim()) {
     if (isSwedishUserMessage(error)) return error
+    const knownError = tryMatchKnownError(error)
+    if (knownError) return knownError
   }
 
   // 2. If it's an object, try various parsing strategies
@@ -205,11 +209,69 @@ export function getErrorMessage(
 
     // Structured application error: { error: { code, message, ... } }
     if (typeof obj.error === 'object' && obj.error !== null) {
-      const structured = obj.error as { code?: unknown; message?: unknown; account_numbers?: unknown }
+      const structured = obj.error as {
+        code?: unknown
+        message?: unknown
+        account_numbers?: unknown
+        details?: unknown
+      }
+
       if (structured.code === 'ACCOUNTS_NOT_IN_CHART' && Array.isArray(structured.account_numbers)) {
         const numbers = structured.account_numbers as string[]
         return `Följande konton behöver aktiveras: ${numbers.join(', ')}`
       }
+
+      if (structured.code === 'JOURNAL_ENTRY_NOT_BALANCED') {
+        const details = structured.details as { totalDebit?: number; totalCredit?: number } | undefined
+        if (details && typeof details.totalDebit === 'number' && typeof details.totalCredit === 'number') {
+          return `Verifikationen balanserar inte (${formatCurrency(details.totalDebit)} debet vs ${formatCurrency(details.totalCredit)} kredit).`
+        }
+        return 'Verifikationen balanserar inte. Kontrollera att debet och kredit är lika stora.'
+      }
+
+      if (structured.code === 'FISCAL_PERIOD_NOT_FOUND') {
+        return 'Räkenskapsperioden kunde inte hittas.'
+      }
+
+      if (structured.code === 'ENTRY_DATE_OUTSIDE_FISCAL_PERIOD') {
+        return 'Datumet ligger utanför det valda räkenskapsåret.'
+      }
+
+      if (structured.code === 'JOURNAL_ENTRY_NOT_FOUND') {
+        return 'Verifikationen kunde inte hittas.'
+      }
+
+      if (structured.code === 'CANNOT_REVERSE_NON_POSTED') {
+        return 'Endast bokförda verifikationer kan stornas.'
+      }
+
+      if (structured.code === 'CANNOT_CORRECT_NON_POSTED') {
+        return 'Endast bokförda verifikationer kan rättas.'
+      }
+
+      if (structured.code === 'ENTRY_ALREADY_REVERSED') {
+        return 'Verifikationen har redan stornats av en annan användare. Ladda om sidan och försök igen.'
+      }
+
+      if (structured.code === 'CURRENCY_REVALUATION_ALREADY_EXISTS') {
+        return 'En valutaomvärdering finns redan för denna period.'
+      }
+
+      if (structured.code === 'INVALID_MAPPING_RESULT') {
+        return 'Kontering saknas för transaktionen. Kontrollera bokföringsreglerna.'
+      }
+
+      if (structured.code === 'BOOKKEEPING_DATABASE_ERROR') {
+        // A DB-layer error may carry a user-relevant cause (e.g. period lock
+        // trigger). Try the known-pattern map before falling back to the
+        // generic "kunde inte sparas" message.
+        if (typeof structured.message === 'string') {
+          const matched = tryMatchKnownError(structured.message)
+          if (matched) return matched
+        }
+        return 'Verifikationen kunde inte sparas. Försök igen.'
+      }
+
       if (typeof structured.message === 'string' && structured.message.trim()) {
         return structured.message
       }