diff --git a/extensions/general/mcp-server/__tests__/undo-sie-import-stage.test.ts b/extensions/general/mcp-server/__tests__/undo-sie-import-stage.test.ts
index 6ef1246f..fefaf4fb 100644
--- a/extensions/general/mcp-server/__tests__/undo-sie-import-stage.test.ts
+++ b/extensions/general/mcp-server/__tests__/undo-sie-import-stage.test.ts
@@ -139,16 +139,25 @@ describe('gnubok_undo_sie_import: stage-time validation', () => {
     ).rejects.toThrow(/import_id/i)
   })
 
-  it('rejects reason longer than 500 characters', async () => {
+  it('rejects reason longer than 500 characters as VALIDATION_ERROR with a Swedish message', async () => {
     const { supabase } = createQueuedMockSupabase()
-    await expect(
-      undoTool.execute(
+    const { getStructuredError } = await import('@/lib/errors/get-structured-error')
+    let thrown: unknown
+    try {
+      await undoTool.execute(
         { import_id: 'imp-1', reason: 'x'.repeat(501) },
         'company-1',
         'user-1',
         supabase as never,
         { type: 'api_key' },
-      ),
-    ).rejects.toThrow(/500/)
+      )
+    } catch (err) {
+      thrown = err
+    }
+    expect(thrown).toBeInstanceOf(Error)
+    const envelope = getStructuredError(thrown)
+    expect(envelope.code).toBe('VALIDATION_ERROR')
+    expect(envelope.message_sv).toBe('Motiveringen får vara högst 500 tecken.')
+    expect(envelope.message_en).toMatch(/500 characters or fewer/)
   })
 })
diff --git a/extensions/general/mcp-server/__tests__/voucher-tools.test.ts b/extensions/general/mcp-server/__tests__/voucher-tools.test.ts
index 18c0f99b..37ef6ee4 100644
--- a/extensions/general/mcp-server/__tests__/voucher-tools.test.ts
+++ b/extensions/general/mcp-server/__tests__/voucher-tools.test.ts
@@ -749,6 +749,29 @@ describe('gnubok_reverse_journal_entry: staging gates', () => {
     ).rejects.toThrow(/entry_id is required/i)
   })
 
+  it('rejects a reason over 500 characters with a VALIDATION_ERROR code and a Swedish message', async () => {
+    // Customer report: the envelope said "Något gick fel. Försök igen." in
+    // Swedish while the cause was only in the English field.
+    const { supabase } = createQueuedMockSupabase()
+    const { getStructuredError } = await import('@/lib/errors/get-structured-error')
+    let thrown: unknown
+    try {
+      await reverseEntry.execute(
+        { entry_id: '11111111-1111-1111-1111-111111111111', reason: 'x'.repeat(501) },
+        'company-1',
+        'user-1',
+        supabase as never,
+      )
+    } catch (err) {
+      thrown = err
+    }
+    expect(thrown).toBeInstanceOf(Error)
+    const envelope = getStructuredError(thrown)
+    expect(envelope.code).toBe('VALIDATION_ERROR')
+    expect(envelope.message_sv).toBe('Motiveringen får vara högst 500 tecken.')
+    expect(envelope.message_en).toMatch(/500 characters or fewer/)
+  })
+
   it('rejects when the original entry is not posted', async () => {
     const { supabase, enqueue } = createQueuedMockSupabase()
     enqueue({
diff --git a/extensions/general/mcp-server/server.ts b/extensions/general/mcp-server/server.ts
index 2c994049..07fc182e 100644
--- a/extensions/general/mcp-server/server.ts
+++ b/extensions/general/mcp-server/server.ts
@@ -592,6 +592,18 @@ const AUTO_PERIOD_DATE_KEYS = [
 
 const ISO_DATE_RE = /^\d{4}-\d{2}-\d{2}$/
 
+/**
+ * `reason` is capped at 500 characters (inputSchema maxLength). Hosts do not
+ * always enforce inputSchema, so the tools re-check at runtime. The message
+ * text is what the error layer keys on: getStructuredError infers
+ * VALIDATION_ERROR from it and getErrorMessage maps it to a specific Swedish
+ * line (a structured `code` on the error would win over the message and
+ * collapse the Swedish text to the generic validation fallback).
+ */
+function reasonTooLongError(): Error {
+  return new Error('reason must be 500 characters or fewer')
+}
+
 /**
  * Coerce a single account-number argument to a trimmed string (validated
  * against ACCOUNT_NUMBER_RE from lib/invariants/account-number). Accepts a
@@ -15107,7 +15119,7 @@ export const tools: McpTool[] = [
 
       if (!importId) throw new Error('import_id is required')
       if (reason !== undefined && reason.length > 500) {
-        throw new Error('reason must be 500 characters or fewer')
+        throw reasonTooLongError()
       }
 
       // Pre-flight mirrors undoSIEImport: confirm row exists, belongs to
@@ -15734,7 +15746,7 @@ export const tools: McpTool[] = [
         throw new Error('reversal_date must be ISO yyyy-MM-dd')
       }
       if (reason !== undefined && reason.length > 500) {
-        throw new Error('reason must be 500 characters or fewer')
+        throw reasonTooLongError()
       }
 
       const entryId = await resolveJournalEntryRef(supabase, companyId, entryRef)
diff --git a/lib/errors/__tests__/get-structured-error.test.ts b/lib/errors/__tests__/get-structured-error.test.ts
index 0e667466..5be0d1da 100644
--- a/lib/errors/__tests__/get-structured-error.test.ts
+++ b/lib/errors/__tests__/get-structured-error.test.ts
@@ -35,6 +35,12 @@ describe('getStructuredError', () => {
     expect(result.remediation?.tool).toBe('gnubok_lock_period')
   })
 
+  it('maps an over-long reason to VALIDATION_ERROR with a specific Swedish message', () => {
+    const result = getStructuredError(new Error('reason must be 500 characters or fewer'))
+    expect(result.code).toBe('VALIDATION_ERROR')
+    expect(result.message_sv).toBe('Motiveringen får vara högst 500 tecken.')
+  })
+
   it('infers PERIOD_HAS_UNBOOKED_TRANSACTIONS from Swedish lock-error message', () => {
     const result = getStructuredError(
       new Error('Kan inte låsa period: 3 affärstransaktion(er) saknar bokföring.')
diff --git a/lib/errors/get-error-message.ts b/lib/errors/get-error-message.ts
index e2cee47f..8230a4d6 100644
--- a/lib/errors/get-error-message.ts
+++ b/lib/errors/get-error-message.ts
@@ -98,6 +98,10 @@ const GENERIC_FALLBACK: Bilingual = { sv: 'Något gick fel. Försök igen.', en:
 
 // Known error patterns → user-friendly Swedish messages
 const ERROR_PATTERN_MAP: [RegExp, string | null][] = [
+  [
+    /reason must be 500 characters or fewer/i,
+    'Motiveringen får vara högst 500 tecken.',
+  ],
   [
     /locked\/closed fiscal period/i,
     'Perioden är låst. Verifikationen kan inte skapas i en stängd eller låst period.',
diff --git a/lib/errors/get-structured-error.ts b/lib/errors/get-structured-error.ts
index 479166d7..abdacfa7 100644
--- a/lib/errors/get-structured-error.ts
+++ b/lib/errors/get-structured-error.ts
@@ -158,6 +158,7 @@ function inferCode(message: string): string | null {
   if (/Bokföringen är låst/i.test(message)) return 'PERIOD_LOCKED'
   if (/Transaction not found/i.test(message)) return 'NOT_FOUND'
   if (/Invoice not found/i.test(message)) return 'NOT_FOUND'
+  if (/must be \d+ characters or fewer/i.test(message)) return 'VALIDATION_ERROR'
   return null
 }