diff --git a/app/(auth)/auth/callback/route.ts b/app/(auth)/auth/callback/route.ts
index 224c9ac8..124cd004 100644
--- a/app/(auth)/auth/callback/route.ts
+++ b/app/(auth)/auth/callback/route.ts
@@ -113,12 +113,18 @@ export async function GET(request: NextRequest) {
               source: 'direct',
             })
 
-            // Set active company
-            await serviceClient.from('user_preferences').upsert({
+            // Set active company. Non-fatal on failure — middleware falls
+            // back to the membership created above — but log so silent
+            // persistence failures (#701) are observable.
+            const { error: prefError } = await serviceClient.from('user_preferences').upsert({
               user_id: user.id,
               active_company_id: invite.company_id,
             }, { onConflict: 'user_id' })
 
+            if (prefError) {
+              console.error('[auth/callback] failed to set active company', prefError)
+            }
+
             // Mark invite as accepted
             await serviceClient
               .from('company_invitations')
diff --git a/app/api/team/accept/route.ts b/app/api/team/accept/route.ts
index 5c4a137a..9c020e71 100644
--- a/app/api/team/accept/route.ts
+++ b/app/api/team/accept/route.ts
@@ -112,14 +112,20 @@ export async function POST(request: NextRequest) {
     return NextResponse.json({ error: 'Kunde inte lägga till medlem.' }, { status: 500 })
   }
 
-  // Set active company
-  await serviceClient
+  // Set active company. Non-fatal on failure — the membership insert already
+  // succeeded and middleware falls back to it — but log so silent
+  // persistence failures (#701) are observable.
+  const { error: prefError } = await serviceClient
     .from('user_preferences')
     .upsert({
       user_id: user.id,
       active_company_id: companyInvite.company_id,
     }, { onConflict: 'user_id' })
 
+  if (prefError) {
+    console.error('[team/accept] failed to set active company', prefError)
+  }
+
   // Mark invite as accepted
   await serviceClient
     .from('company_invitations')
diff --git a/components/dashboard/CompanySwitcher.tsx b/components/dashboard/CompanySwitcher.tsx
index 4419f105..f7afb267 100644
--- a/components/dashboard/CompanySwitcher.tsx
+++ b/components/dashboard/CompanySwitcher.tsx
@@ -7,11 +7,13 @@ import { useTranslations } from 'next-intl'
 import { cn } from '@/lib/utils'
 import { useCompany } from '@/contexts/CompanyContext'
 import { switchCompany } from '@/lib/company/actions'
+import { useToast } from '@/components/ui/use-toast'
 import { Check, ChevronsUpDown, Plus, Loader2 } from 'lucide-react'
 
 export default function CompanySwitcher() {
   const { company, companies, isSandbox } = useCompany()
   const t = useTranslations('company_switcher')
+  const { toast } = useToast()
   const [open, setOpen] = useState(false)
   const [isPending, setIsPending] = useState(false)
   const triggerRef = useRef<HTMLButtonElement>(null)
@@ -82,6 +84,10 @@ export default function CompanySwitcher() {
     const result = await switchCompany(companyId)
     if (result.error) {
       setIsPending(false)
+      toast({
+        title: t(result.error === 'not_member' ? 'error_no_access' : 'error_switch_failed'),
+        variant: 'destructive',
+      })
       return
     }
     // Notify every other open tab of the same user so they hard-reload
diff --git a/components/onboarding/BankIdCompanyPicker.tsx b/components/onboarding/BankIdCompanyPicker.tsx
index 6efcc483..7794e249 100644
--- a/components/onboarding/BankIdCompanyPicker.tsx
+++ b/components/onboarding/BankIdCompanyPicker.tsx
@@ -89,7 +89,10 @@ export default function BankIdCompanyPicker({
     setSetup({ kind: 'opening', companyId })
     const result = await switchCompany(companyId)
     if (result.error) {
-      toast({ title: result.error, variant: 'destructive' })
+      toast({
+        title: t(result.error === 'not_member' ? 'error_no_access' : 'error_switch_failed'),
+        variant: 'destructive',
+      })
       setSetup({ kind: 'idle' })
       return
     }
diff --git a/lib/company/__tests__/actions.test.ts b/lib/company/__tests__/actions.test.ts
index 450f9a74..68c51cf9 100644
--- a/lib/company/__tests__/actions.test.ts
+++ b/lib/company/__tests__/actions.test.ts
@@ -8,14 +8,19 @@ vi.mock('@/lib/supabase/server', () => ({
   createClient: vi.fn(),
 }))
 
-vi.mock('@/lib/company/context', () => ({
+// Keep the real CompanyContextError so instanceof checks in switchCompany
+// see the same class the tests throw.
+vi.mock('@/lib/company/context', async (importOriginal) => ({
+  ...(await importOriginal<typeof import('@/lib/company/context')>()),
   setActiveCompany: vi.fn().mockResolvedValue(undefined),
 }))
 
 import { createClient } from '@/lib/supabase/server'
-import { createCompanyFromOnboarding } from '../actions'
+import { setActiveCompany, CompanyContextError } from '@/lib/company/context'
+import { createCompanyFromOnboarding, switchCompany } from '../actions'
 
 const mockCreateClient = vi.mocked(createClient)
+const mockSetActiveCompany = vi.mocked(setActiveCompany)
 
 type CapturedCall = { table: string; method: string; args: unknown[] }
 
@@ -80,6 +85,62 @@ beforeEach(() => {
   vi.clearAllMocks()
 })
 
+describe('switchCompany', () => {
+  it('returns {} when the switch persists', async () => {
+    const { supabase } = buildSupabase({ user: { id: 'user-1' } })
+    mockCreateClient.mockResolvedValue(supabase as never)
+
+    const result = await switchCompany('company-2')
+
+    expect(result).toEqual({})
+    expect(mockSetActiveCompany).toHaveBeenCalledWith(supabase, 'user-1', 'company-2')
+  })
+
+  it('returns Unauthorized when there is no user', async () => {
+    const { supabase } = buildSupabase({ user: null })
+    mockCreateClient.mockResolvedValue(supabase as never)
+
+    const result = await switchCompany('company-2')
+
+    expect(result).toEqual({ error: 'Unauthorized' })
+    expect(mockSetActiveCompany).not.toHaveBeenCalled()
+  })
+
+  it('maps a membership failure to the not_member code', async () => {
+    const { supabase } = buildSupabase({ user: { id: 'user-1' } })
+    mockCreateClient.mockResolvedValue(supabase as never)
+    mockSetActiveCompany.mockRejectedValueOnce(
+      new CompanyContextError('User is not a member of this company', 'not_member'),
+    )
+
+    const result = await switchCompany('company-2')
+
+    expect(result).toEqual({ error: 'not_member' })
+  })
+
+  it('maps a failed user_preferences write to persist_failed, not a permissions error (#701)', async () => {
+    const { supabase } = buildSupabase({ user: { id: 'user-1' } })
+    mockCreateClient.mockResolvedValue(supabase as never)
+    mockSetActiveCompany.mockRejectedValueOnce(
+      new CompanyContextError('Failed to persist active company: timeout', 'persist_failed'),
+    )
+
+    const result = await switchCompany('company-2')
+
+    expect(result).toEqual({ error: 'persist_failed' })
+  })
+
+  it('maps unexpected errors to persist_failed rather than claiming missing access', async () => {
+    const { supabase } = buildSupabase({ user: { id: 'user-1' } })
+    mockCreateClient.mockResolvedValue(supabase as never)
+    mockSetActiveCompany.mockRejectedValueOnce(new Error('cookies unavailable'))
+
+    const result = await switchCompany('company-2')
+
+    expect(result).toEqual({ error: 'persist_failed' })
+  })
+})
+
 describe('createCompanyFromOnboarding — org_number validation', () => {
   it('rejects malformed org_numbers at the guard boundary', async () => {
     const { supabase } = buildSupabase({
diff --git a/lib/company/__tests__/context.test.ts b/lib/company/__tests__/context.test.ts
new file mode 100644
index 00000000..4fe2b497
--- /dev/null
+++ b/lib/company/__tests__/context.test.ts
@@ -0,0 +1,111 @@
+import { describe, it, expect, vi, beforeEach } from 'vitest'
+
+const { mockCookieSet } = vi.hoisted(() => ({ mockCookieSet: vi.fn() }))
+
+vi.mock('next/headers', () => ({
+  cookies: vi.fn(async () => ({ set: mockCookieSet })),
+}))
+
+import { setActiveCompany, CompanyContextError } from '../context'
+
+type CapturedCall = { table: string; method: string; args: unknown[] }
+
+/**
+ * Chainable Supabase mock (same approach as actions.test.ts): a chain method
+ * terminates with `results[table][method]` when seeded, otherwise keeps
+ * chaining. setActiveCompany ends both its queries on `.single()`, on
+ * different tables, so seeding `single` per table drives each branch.
+ */
+function buildSupabase(results: Record<string, Record<string, { data?: unknown; error?: unknown }>>) {
+  const calls: CapturedCall[] = []
+
+  function makeChain(table: string) {
+    const chain: Record<string, unknown> = {}
+    const methods = ['select', 'eq', 'is', 'order', 'limit', 'maybeSingle', 'single', 'insert', 'upsert', 'delete', 'update']
+    for (const m of methods) {
+      chain[m] = (...args: unknown[]) => {
+        calls.push({ table, method: m, args })
+        const terminal = results[table]?.[m]
+        if (terminal) {
+          return Promise.resolve({ data: terminal.data ?? null, error: terminal.error ?? null })
+        }
+        return chain
+      }
+    }
+    chain.then = (resolve: (v: unknown) => void) => resolve({ data: null, error: null })
+    return chain
+  }
+
+  const supabase = {
+    from: vi.fn().mockImplementation((table: string) => makeChain(table)),
+  }
+
+  return { supabase, calls }
+}
+
+beforeEach(() => {
+  vi.clearAllMocks()
+})
+
+describe('setActiveCompany', () => {
+  it('throws not_member and never writes when the user lacks membership', async () => {
+    const { supabase, calls } = buildSupabase({
+      company_members: { single: { data: null, error: { message: 'no rows' } } },
+    })
+
+    const err = await setActiveCompany(supabase as never, 'user-1', 'company-2').catch((e) => e)
+
+    expect(err).toBeInstanceOf(CompanyContextError)
+    expect(err.code).toBe('not_member')
+    expect(calls.find((c) => c.table === 'user_preferences')).toBeUndefined()
+    expect(mockCookieSet).not.toHaveBeenCalled()
+  })
+
+  it('throws persist_failed and does NOT set the cookie when the upsert errors (#701)', async () => {
+    const { supabase } = buildSupabase({
+      company_members: { single: { data: { company_id: 'company-2' } } },
+      user_preferences: { single: { data: null, error: { message: 'permission denied' } } },
+    })
+
+    const err = await setActiveCompany(supabase as never, 'user-1', 'company-2').catch((e) => e)
+
+    expect(err).toBeInstanceOf(CompanyContextError)
+    expect(err.code).toBe('persist_failed')
+    expect(err.message).toContain('permission denied')
+    // The exact regression from #701: cookie must not diverge from the DB.
+    expect(mockCookieSet).not.toHaveBeenCalled()
+  })
+
+  it('throws persist_failed when the read-back does not return the new company', async () => {
+    // An RLS-filtered UPDATE affects zero rows without an error; the
+    // read-back is what catches it. Simulate a stale/foreign row coming back.
+    const { supabase } = buildSupabase({
+      company_members: { single: { data: { company_id: 'company-2' } } },
+      user_preferences: { single: { data: { active_company_id: 'company-1' } } },
+    })
+
+    const err = await setActiveCompany(supabase as never, 'user-1', 'company-2').catch((e) => e)
+
+    expect(err).toBeInstanceOf(CompanyContextError)
+    expect(err.code).toBe('persist_failed')
+    expect(mockCookieSet).not.toHaveBeenCalled()
+  })
+
+  it('sets the cookie only after the write is verified', async () => {
+    const { supabase, calls } = buildSupabase({
+      company_members: { single: { data: { company_id: 'company-2' } } },
+      user_preferences: { single: { data: { active_company_id: 'company-2' } } },
+    })
+
+    await expect(setActiveCompany(supabase as never, 'user-1', 'company-2')).resolves.toBeUndefined()
+
+    const upsert = calls.find((c) => c.table === 'user_preferences' && c.method === 'upsert')
+    expect(upsert?.args[0]).toEqual({ user_id: 'user-1', active_company_id: 'company-2' })
+    expect(mockCookieSet).toHaveBeenCalledTimes(1)
+    expect(mockCookieSet).toHaveBeenCalledWith(
+      'gnubok-company-id',
+      'company-2',
+      expect.objectContaining({ httpOnly: true, path: '/' }),
+    )
+  })
+})
diff --git a/lib/company/actions.ts b/lib/company/actions.ts
index 5a0c0415..3573c1e1 100644
--- a/lib/company/actions.ts
+++ b/lib/company/actions.ts
@@ -1,11 +1,17 @@
 'use server'
 
 import { createClient } from '@/lib/supabase/server'
-import { setActiveCompany } from '@/lib/company/context'
+import { setActiveCompany, CompanyContextError } from '@/lib/company/context'
 import { revalidatePath } from 'next/cache'
 import { normalizeOrgNumber } from '@/lib/company-lookup/normalize-org-number'
 import type { CompanyLookupResult } from '@/lib/company-lookup/types'
 
+/**
+ * Switch the active company. Returns an error *code* (translated by the
+ * caller, same pattern as `org_number_invalid` below): 'not_member' when the
+ * user lacks membership, 'persist_failed' when the user_preferences write
+ * failed or could not be verified (#701).
+ */
 export async function switchCompany(companyId: string): Promise<{ error?: string }> {
   const supabase = await createClient()
   const { data: { user } } = await supabase.auth.getUser()
@@ -21,8 +27,14 @@ export async function switchCompany(companyId: string): Promise<{ error?: string
     // every React/router/fetch cache wholesale. revalidatePath would be a
     // no-op and would just race with the hard reload.
     return {}
-  } catch {
-    return { error: 'Du har inte tillgång till detta företag.' }
+  } catch (err) {
+    console.error('[switchCompany] failed', err)
+    if (err instanceof CompanyContextError && err.code === 'not_member') {
+      return { error: 'not_member' }
+    }
+    // persist_failed and anything unexpected: a retryable failure, not a
+    // permissions problem — don't tell the user they lack access.
+    return { error: 'persist_failed' }
   }
 }
 
diff --git a/lib/company/context.ts b/lib/company/context.ts
index 0f1aa13d..4e2b6c37 100644
--- a/lib/company/context.ts
+++ b/lib/company/context.ts
@@ -3,6 +3,21 @@ import { cookies } from 'next/headers'
 
 const COMPANY_COOKIE = 'gnubok-company-id'
 
+/**
+ * Thrown by setActiveCompany so callers can tell a permissions problem
+ * ('not_member') apart from a failed/unverified database write
+ * ('persist_failed') and surface the right message to the user.
+ */
+export class CompanyContextError extends Error {
+  constructor(
+    message: string,
+    readonly code: 'not_member' | 'persist_failed'
+  ) {
+    super(message)
+    this.name = 'CompanyContextError'
+  }
+}
+
 /**
  * Get the active company ID for the authenticated user.
  *
@@ -105,18 +120,39 @@ export async function setActiveCompany(
     .single()
 
   if (!membership) {
-    throw new Error('User is not a member of this company')
+    throw new CompanyContextError('User is not a member of this company', 'not_member')
   }
 
-  // Update user_preferences — this is the authoritative value RLS reads
-  await supabase
+  // Update user_preferences — this is the authoritative value RLS reads.
+  // The write MUST be verified: an UPDATE filtered out by RLS affects zero
+  // rows without raising an error, which previously made failed switches
+  // look successful while middleware kept resolving the old company (#701).
+  // `.select().single()` reads the row back, so both an explicit error and
+  // a silent zero-row write surface as a thrown CompanyContextError.
+  const { data: persisted, error: upsertError } = await supabase
     .from('user_preferences')
     .upsert(
       { user_id: userId, active_company_id: companyId },
       { onConflict: 'user_id' }
     )
+    .select('active_company_id')
+    .single()
 
-  // Refresh the cookie as a compat hint
+  if (upsertError) {
+    throw new CompanyContextError(
+      `Failed to persist active company: ${upsertError.message}`,
+      'persist_failed'
+    )
+  }
+  if (persisted?.active_company_id !== companyId) {
+    throw new CompanyContextError(
+      'Active company write did not persist',
+      'persist_failed'
+    )
+  }
+
+  // Refresh the cookie as a compat hint — only after the DB write is
+  // confirmed, so the cookie can never diverge from user_preferences.
   const cookieStore = await cookies()
   cookieStore.set(COMPANY_COOKIE, companyId, {
     path: '/',
diff --git a/lib/supabase/middleware.ts b/lib/supabase/middleware.ts
index 10a5597e..bccdce7c 100644
--- a/lib/supabase/middleware.ts
+++ b/lib/supabase/middleware.ts
@@ -281,12 +281,19 @@ async function resolveCompanyForMiddleware(
 
   // Write the fallback back to user_preferences so future RLS lookups
   // see the same active company without needing this fallback scan.
-  await supabase
+  // Non-fatal on failure: resolution for this request already succeeded,
+  // the write-back is an optimization — but log it so silent persistence
+  // failures (#701) are observable.
+  const { error: writeBackError } = await supabase
     .from('user_preferences')
     .upsert(
       { user_id: userId, active_company_id: firstCompany.company_id },
       { onConflict: 'user_id' }
     )
 
+  if (writeBackError) {
+    console.error('[middleware] active company write-back failed', writeBackError)
+  }
+
   return { companyId: firstCompany.company_id, locale }
 }
diff --git a/messages/en.json b/messages/en.json
index 57c000eb..9451406e 100644
--- a/messages/en.json
+++ b/messages/en.json
@@ -918,7 +918,9 @@
     "toast_create_failed_title": "Could not create company",
     "toast_create_failed_description": "Try again or add it manually.",
     "toast_welcome_title": "Welcome!",
-    "toast_company_ready": "Your company is ready."
+    "toast_company_ready": "Your company is ready.",
+    "error_no_access": "You don't have access to this company.",
+    "error_switch_failed": "The company switch could not be saved. Please try again."
   },
   "companies_new": {
     "step1_title": "New company",
@@ -3421,7 +3423,9 @@
   "company_switcher": {
     "company_label": "Company",
     "default_company_name": "My business",
-    "add_company": "Add company"
+    "add_company": "Add company",
+    "error_no_access": "You don't have access to this company.",
+    "error_switch_failed": "The company switch could not be saved. Please try again."
   },
   "transactions": {
     "page_title": "Transactions",
diff --git a/messages/sv.json b/messages/sv.json
index 7b3a06bb..ee0a985e 100644
--- a/messages/sv.json
+++ b/messages/sv.json
@@ -918,7 +918,9 @@
     "toast_create_failed_title": "Kunde inte skapa företag",
     "toast_create_failed_description": "Försök igen eller lägg till manuellt.",
     "toast_welcome_title": "Välkommen!",
-    "toast_company_ready": "Ditt företag är nu redo."
+    "toast_company_ready": "Ditt företag är nu redo.",
+    "error_no_access": "Du har inte tillgång till detta företag.",
+    "error_switch_failed": "Företagsbytet kunde inte sparas. Försök igen."
   },
   "companies_new": {
     "step1_title": "Nytt företag",
@@ -3421,7 +3423,9 @@
   "company_switcher": {
     "company_label": "Företag",
     "default_company_name": "Min verksamhet",
-    "add_company": "Lägg till företag"
+    "add_company": "Lägg till företag",
+    "error_no_access": "Du har inte tillgång till detta företag.",
+    "error_switch_failed": "Företagsbytet kunde inte sparas. Försök igen."
   },
   "transactions": {
     "page_title": "Transaktioner",