diff --git a/extensions/general/mcp-server/__tests__/categorize-category-required.test.ts b/extensions/general/mcp-server/__tests__/categorize-category-required.test.ts
new file mode 100644
index 00000000..b7f301bc
--- /dev/null
+++ b/extensions/general/mcp-server/__tests__/categorize-category-required.test.ts
@@ -0,0 +1,172 @@
+/**
+ * gnubok_categorize_transaction / gnubok_bulk_book_inbox_items: the `category`
+ * argument is required, and hosts don't always enforce inputSchema `required`.
+ *
+ * Issue #1662: a call carrying only account_override reached the enum check
+ * and surfaced as 'Invalid category "undefined"'. These tests pin:
+ *   - missing category -> a clear "category is required" error (before DB work),
+ *   - an unknown category string -> the enum error, unchanged,
+ *   - the happy path stages with the category intact.
+ * Companion suite: categorize-account-override.test.ts.
+ */
+import { describe, it, expect, vi, beforeEach } from 'vitest'
+import { createQueuedMockSupabase } from '@/tests/helpers'
+import { eventBus } from '@/lib/events'
+
+const mockDetectDup = vi.fn()
+vi.mock('@/lib/transactions/booking-duplicate-detection', () => ({
+  detectBookingDuplicate: (...args: unknown[]) => mockDetectDup(...args),
+}))
+
+import { tools } from '../server'
+
+const categorize = tools.find((t) => t.name === 'gnubok_categorize_transaction')!
+const bulkBookInbox = tools.find((t) => t.name === 'gnubok_bulk_book_inbox_items')!
+
+const TX_ID = '00000000-0000-4000-8000-0000000000cc'
+
+/** `transactions` row for categorizeTransactionCore's select('*'). Synthetic. */
+const coreTxRow = () => ({
+  id: TX_ID,
+  date: '2026-07-10',
+  amount: -479,
+  currency: 'SEK',
+  amount_sek: -479,
+  exchange_rate: 1,
+  description: 'SECOND HAND BUTIK',
+  merchant_name: null,
+  cash_account_id: null,
+  document_id: null,
+  journal_entry_id: null,
+  is_business: true,
+})
+
+/** The narrower projection the tool re-fetches for the guard + title. */
+const guardTxRow = () => ({
+  description: 'SECOND HAND BUTIK',
+  merchant_name: null,
+  amount: -479,
+  currency: 'SEK',
+  amount_sek: -479,
+  exchange_rate: 1,
+  date: '2026-07-10',
+  cash_account_id: null,
+})
+
+const settingsRow = { entity_type: 'aktiebolag', fiscal_year_start_month: 1 }
+
+beforeEach(() => {
+  vi.clearAllMocks()
+  eventBus.clear()
+  mockDetectDup.mockResolvedValue(null)
+})
+
+describe('gnubok_categorize_transaction: category presence guard (#1662)', () => {
+  it('declares category as required in the input schema', () => {
+    const schema = categorize.inputSchema as { required?: string[] }
+    expect(schema.required).toContain('category')
+  })
+
+  it('rejects a call with only account_override with a clear "category is required" error', async () => {
+    const { supabase, calls } = createQueuedMockSupabase()
+    let thrown: unknown
+    try {
+      await categorize.execute(
+        { transaction_id: TX_ID, account_override: '4020' },
+        'company-1',
+        'user-1',
+        supabase as never,
+      )
+    } catch (err) {
+      thrown = err
+    }
+    expect(thrown).toBeInstanceOf(Error)
+    const message = (thrown as Error).message
+    expect(message).toMatch(
+      /^category is required; account_override only overrides the category's default account/,
+    )
+    // Never the old 'Invalid category "undefined"' shape, and no DB work.
+    expect(message).not.toMatch(/undefined/)
+    expect(calls).toHaveLength(0)
+  })
+
+  it('rejects a non-string or blank category the same way', async () => {
+    const { supabase } = createQueuedMockSupabase()
+    for (const category of [42, '', '   ', null]) {
+      await expect(
+        categorize.execute(
+          { transaction_id: TX_ID, category },
+          'company-1',
+          'user-1',
+          supabase as never,
+        ),
+      ).rejects.toThrow(/category is required/)
+    }
+  })
+
+  it('still returns the enum error for an unknown category string', async () => {
+    const { supabase } = createQueuedMockSupabase()
+    await expect(
+      categorize.execute(
+        { transaction_id: TX_ID, category: 'expense_unicorns' },
+        'company-1',
+        'user-1',
+        supabase as never,
+      ),
+    ).rejects.toThrow(/Invalid category "expense_unicorns"\. Valid categories:/)
+  })
+
+  it('happy path: a valid category stages with the category intact', async () => {
+    const { supabase, enqueue, findCall } = createQueuedMockSupabase()
+    enqueue({ data: coreTxRow() }) // core: transactions
+    enqueue({ data: settingsRow }) // core: company_settings
+    enqueue({ data: guardTxRow() }) // tool: transactions re-fetch
+    enqueue({ data: null }) // resolvePeriodStatusForDate: company_settings
+    enqueue({ data: null }) // resolvePeriodStatusForDate: fiscal_periods
+    enqueue({ data: { id: 'op-cat-1' } }) // pending_operations insert
+
+    const result = (await categorize.execute(
+      { transaction_id: TX_ID, category: 'expense_other' },
+      'company-1',
+      'user-1',
+      supabase as never,
+      { type: 'api_key' },
+    )) as { staged: boolean; operation_id?: string; preview: Record<string, unknown> }
+
+    expect(result.staged).toBe(true)
+    expect(result.operation_id).toBe('op-cat-1')
+    expect(result.preview.category).toBe('expense_other')
+
+    const insertArgs = findCall('pending_operations', 'insert')
+    expect(insertArgs).toBeDefined()
+    const payload = (insertArgs as unknown[])[0] as { params?: { category?: string } }
+    expect(payload.params?.category).toBe('expense_other')
+  })
+})
+
+describe('gnubok_bulk_book_inbox_items: category guard at staging', () => {
+  it('rejects a missing category before any DB work', async () => {
+    const { supabase, calls } = createQueuedMockSupabase()
+    await expect(
+      bulkBookInbox.execute(
+        { item_ids: ['i1'] },
+        'company-1',
+        'user-1',
+        supabase as never,
+      ),
+    ).rejects.toThrow(/^category is required/)
+    expect(calls).toHaveLength(0)
+  })
+
+  it('rejects an unknown category string with the enum error', async () => {
+    const { supabase } = createQueuedMockSupabase()
+    await expect(
+      bulkBookInbox.execute(
+        { item_ids: ['i1'], category: 'expense_unicorns' },
+        'company-1',
+        'user-1',
+        supabase as never,
+      ),
+    ).rejects.toThrow(/Invalid category "expense_unicorns"/)
+  })
+})
diff --git a/extensions/general/mcp-server/server.ts b/extensions/general/mcp-server/server.ts
index a64b9095..7e4b1360 100644
--- a/extensions/general/mcp-server/server.ts
+++ b/extensions/general/mcp-server/server.ts
@@ -4287,7 +4287,7 @@ export const tools: McpTool[] = [
         category: { type: 'string', description: 'Transaction category', enum: [...VALID_CATEGORIES] },
         vat_treatment: { type: 'string', description: 'VAT treatment override. Defaults to standard_25 for business expenses. Set reverse_charge ONLY when the underlag confirms the seller did NOT charge VAT (omvänd skattskyldighet). An invoice with foreign VAT already debited is NOT reverse charge.', enum: [...VALID_VAT_TREATMENTS] },
         vat_amount: { type: 'number', exclusiveMinimum: 0, description: 'The underlag\'s exact moms (> 0) when it differs from rate × belopp: e.g. dricks carries no VAT. Requires a rate-based vat_treatment. Swedish moms only: foreign VAT is never deductible. For a 0-moms document use vat_treatment="exempt".' },
-        account_override: { type: 'string', pattern: '^\\d{4}$', description: 'Books the business side (debit when money goes out, credit when money comes in) on this kontoplan account instead of the category default: the ONLY way to reach company-custom accounts (e.g. VMB). Must exist and be active (gnubok_list_accounts; create via gnubok_create_account). VMB purchases/sales carry no deductible moms: use vat_treatment "exempt". Without an explicit vat_treatment (or vat_amount) the override books GROSS with no auto-VAT line: a moms leg is never guessed onto a custom account. Class-2 overrides outside 2610-2649 always drop auto-VAT. Not valid with category "private". State the actual affärshändelse in notes (BFL 5 kap).' },
+        account_override: { type: 'string', pattern: '^\\d{4}$', description: 'Books the business side (debit when money goes out, credit when money comes in) on this kontoplan account instead of the category default: the ONLY way to reach company-custom accounts (e.g. VMB). category is still required: it decides direction and VAT; the override only replaces its default account. Must exist and be active (gnubok_list_accounts; create via gnubok_create_account). VMB purchases/sales carry no deductible moms: use vat_treatment "exempt". Without an explicit vat_treatment (or vat_amount) the override books GROSS with no auto-VAT line: a moms leg is never guessed onto a custom account. Class-2 overrides outside 2610-2649 always drop auto-VAT. Not valid with category "private". State the actual affärshändelse in notes (BFL 5 kap).' },
         notes: { type: 'string', description: 'Audit-trail context appended to the verifikation description. For category=representation use this to record deltagare + syfte ("Anna Andersson (Acme AB), kundmöte om Y"). For project work, include the project ref. Keep under 200 chars; pure metadata, not a re-description of the transaction.' },
         dimensions: {
           type: 'object',
@@ -4322,10 +4322,22 @@ export const tools: McpTool[] = [
         throw new Error('account_override must be exactly 4 digits, e.g. "4020".')
       }
 
+      // Presence guard (hosts don't always enforce inputSchema `required`).
+      // Without it a call carrying only account_override reached the enum
+      // check and surfaced as 'Invalid category "undefined"'. The enum check
+      // in categorizeTransactionCore still handles unknown category strings.
+      const category = typeof args.category === 'string' ? args.category.trim() : ''
+      if (!category) {
+        throw new Error(
+          'category is required; account_override only overrides the category\'s default account. ' +
+          `Valid categories: ${VALID_CATEGORIES.join(', ')}`,
+        )
+      }
+
       // Compute the preview (accounts, amounts, VAT lines)
       const result = await categorizeTransactionCore(
         args.transaction_id as string,
-        args.category as TransactionCategory,
+        category as TransactionCategory,
         args.vat_treatment as VatTreatment | undefined,
         vatAmount,
         accountOverride,
@@ -4408,7 +4420,7 @@ export const tools: McpTool[] = [
         `Kategorisera: ${txDesc}`,
         {
           transaction_id: args.transaction_id,
-          category: args.category,
+          category,
           vat_treatment: args.vat_treatment || null,
           vat_amount: vatAmount ?? null,
           account_override: accountOverride ?? null,
@@ -8902,6 +8914,16 @@ export const tools: McpTool[] = [
     async execute(args, companyId, userId, supabase, actor) {
       const itemIds = args.item_ids as string[]
       if (!Array.isArray(itemIds) || itemIds.length === 0) throw new Error('item_ids is required (non-empty)')
+      // Presence + enum guard at the boundary (hosts don't always enforce
+      // inputSchema `required`/`enum`): without it a missing or unknown
+      // category was staged as-is and only rejected at approval time.
+      const category = typeof args.category === 'string' ? args.category.trim() : ''
+      if (!category) {
+        throw new Error(`category is required. Valid categories: ${VALID_CATEGORIES.join(', ')}`)
+      }
+      if (!VALID_CATEGORIES.includes(category as typeof VALID_CATEGORIES[number])) {
+        throw new Error(`Invalid category "${category}". Valid categories: ${VALID_CATEGORIES.join(', ')}`)
+      }
       const vatAmount = typeof args.vat_amount === 'number' && Number.isFinite(args.vat_amount)
         ? args.vat_amount
         : undefined
@@ -8971,7 +8993,7 @@ export const tools: McpTool[] = [
           // Stage only the bookable items: the executor re-checks each and
           // skips any that changed state between staging and approval.
           item_ids: bookable.map((it) => it.id as string),
-          category: args.category,
+          category,
           vat_treatment: args.vat_treatment ?? null,
           vat_amount: vatAmount ?? null,
           notes: notes ?? null,
@@ -8988,7 +9010,7 @@ export const tools: McpTool[] = [
           already_booked: alreadyBooked,
           not_found: notFound,
           total_sek: Math.round(totalSek * 100) / 100,
-          category: args.category,
+          category,
           vat_treatment: args.vat_treatment ?? null,
           ...(resolvedDimensions && Object.keys(resolvedDimensions).length > 0
             ? { dimensions: resolvedDimensions }