diff --git a/src/app/layout.tsx b/src/app/layout.tsx
index b6c96e7..11c7889 100644
--- a/src/app/layout.tsx
+++ b/src/app/layout.tsx
@@ -3,6 +3,7 @@ import { Source_Code_Pro } from 'next/font/google'
 import './globals.css'
 import { ThemeProvider } from '@/components/theme-provider'
 import { AuthProvider } from '@/context/auth-context'
+import { AppToastProvider } from '@/context/toast-context'
 
 const sourceCodePro = Source_Code_Pro({
   subsets: ['latin'],
@@ -51,7 +52,9 @@ export default function RootLayout({
           enableSystem={true}
           disableTransitionOnChange
         >
-          <AuthProvider>{children}</AuthProvider>
+          <AuthProvider>
+            <AppToastProvider>{children}</AppToastProvider>
+          </AuthProvider>
         </ThemeProvider>
       </body>
     </html>
diff --git a/src/app/u/[handle]/page.test.tsx b/src/app/u/[handle]/page.test.tsx
index a983b6f..49aa239 100644
--- a/src/app/u/[handle]/page.test.tsx
+++ b/src/app/u/[handle]/page.test.tsx
@@ -21,6 +21,14 @@ vi.mock('@/hooks/use-auth', () => ({
   }),
 }))
 
+// Mock useToast hook (used by BlockMuteButton via useRequireAuth)
+vi.mock('@/hooks/use-toast', () => ({
+  useToast: () => ({
+    toast: vi.fn(),
+    dismiss: vi.fn(),
+  }),
+}))
+
 // Mock next/navigation
 vi.mock('next/navigation', () => ({
   useRouter: () => ({
diff --git a/src/app/u/[handle]/page.tsx b/src/app/u/[handle]/page.tsx
index 70abbdf..277f9ea 100644
--- a/src/app/u/[handle]/page.tsx
+++ b/src/app/u/[handle]/page.tsx
@@ -14,6 +14,7 @@ import { Breadcrumbs } from '@/components/breadcrumbs'
 import { ProfileHeader } from '@/components/profile/profile-header'
 import { ProfileSkeleton } from '@/components/profile/profile-skeleton'
 import { getUserProfile } from '@/lib/api/client'
+import { useAuth } from '@/hooks/use-auth'
 import type { UserProfile } from '@/lib/api/types'
 
 interface UserProfilePageProps {
@@ -32,6 +33,7 @@ export default function UserProfilePage({ params }: UserProfilePageProps) {
   const [error, setError] = useState<string | null>(null)
   const [isBlocked, setIsBlocked] = useState(false)
   const [isMuted, setIsMuted] = useState(false)
+  const { user } = useAuth()
 
   // Resolve Next.js async params
   useEffect(() => {
@@ -134,6 +136,7 @@ export default function UserProfilePage({ params }: UserProfilePageProps) {
           isMuted={isMuted}
           onBlockToggle={setIsBlocked}
           onMuteToggle={setIsMuted}
+          viewerDid={user?.did ?? null}
         />
 
         {/* Recent activity */}
diff --git a/src/components/block-mute-button.test.tsx b/src/components/block-mute-button.test.tsx
index f5c71a9..0744bbe 100644
--- a/src/components/block-mute-button.test.tsx
+++ b/src/components/block-mute-button.test.tsx
@@ -8,6 +8,7 @@ import userEvent from '@testing-library/user-event'
 import { BlockMuteButton } from './block-mute-button'
 
 const mockGetAccessToken = vi.fn<() => string | null>(() => 'mock-access-token')
+const mockToast = vi.fn()
 
 vi.mock('@/hooks/use-auth', () => ({
   useAuth: () => ({
@@ -27,6 +28,13 @@ vi.mock('@/hooks/use-auth', () => ({
   }),
 }))
 
+vi.mock('@/hooks/use-toast', () => ({
+  useToast: () => ({
+    toast: mockToast,
+    dismiss: vi.fn(),
+  }),
+}))
+
 // Mock localStorage
 const mockStorage: Record<string, string> = {}
 
diff --git a/src/components/block-mute-button.tsx b/src/components/block-mute-button.tsx
index e3c0bc7..b3f59f1 100644
--- a/src/components/block-mute-button.tsx
+++ b/src/components/block-mute-button.tsx
@@ -1,6 +1,7 @@
 /**
  * Block/mute toggle button for user actions.
  * Used in user profiles and post context menus.
+ * Shows a login prompt toast for unauthenticated users.
  * @see specs/prd-web.md Section M8
  */
 
@@ -11,6 +12,7 @@ import { Prohibit, SpeakerSimpleSlash, WarningCircle } from '@phosphor-icons/rea
 import { cn } from '@/lib/utils'
 import { blockUser, unblockUser, muteUser, unmuteUser } from '@/lib/api/client'
 import { useAuth } from '@/hooks/use-auth'
+import { useRequireAuth } from '@/hooks/use-require-auth'
 
 interface BlockMuteButtonProps {
   targetDid: string
@@ -28,39 +30,42 @@ export function BlockMuteButton({
   className,
 }: BlockMuteButtonProps) {
   const { getAccessToken } = useAuth()
+  const { requireAuth } = useRequireAuth()
   const [loading, setLoading] = useState(false)
   const [error, setError] = useState(false)
 
-  const handleClick = async () => {
-    setLoading(true)
-    setError(false)
+  const handleClick = () => {
+    requireAuth(async () => {
+      setLoading(true)
+      setError(false)
 
-    const token = getAccessToken()
-    if (!token) {
-      setLoading(false)
-      return
-    }
+      const token = getAccessToken()
+      if (!token) {
+        setLoading(false)
+        return
+      }
 
-    try {
-      if (action === 'block') {
-        if (isActive) {
-          await unblockUser(targetDid, token)
-        } else {
-          await blockUser(targetDid, token)
-        }
-      } else {
-        if (isActive) {
-          await unmuteUser(targetDid, token)
+      try {
+        if (action === 'block') {
+          if (isActive) {
+            await unblockUser(targetDid, token)
+          } else {
+            await blockUser(targetDid, token)
+          }
         } else {
-          await muteUser(targetDid, token)
+          if (isActive) {
+            await unmuteUser(targetDid, token)
+          } else {
+            await muteUser(targetDid, token)
+          }
         }
+        onToggle(!isActive)
+      } catch {
+        setError(true)
+      } finally {
+        setLoading(false)
       }
-      onToggle(!isActive)
-    } catch {
-      setError(true)
-    } finally {
-      setLoading(false)
-    }
+    })
   }
 
   const Icon = action === 'block' ? Prohibit : SpeakerSimpleSlash
diff --git a/src/components/profile/profile-header.tsx b/src/components/profile/profile-header.tsx
index 3ddaac5..c69e932 100644
--- a/src/components/profile/profile-header.tsx
+++ b/src/components/profile/profile-header.tsx
@@ -1,5 +1,6 @@
 /**
  * ProfileHeader - Displays user profile card with banner, avatar, bio, stats, and actions.
+ * Hides block/mute buttons when viewing own profile.
  * @see specs/prd-web.md Section M8
  */
 
@@ -21,6 +22,8 @@ interface ProfileHeaderProps {
   isMuted: boolean
   onBlockToggle: (blocked: boolean) => void
   onMuteToggle: (muted: boolean) => void
+  /** DID of the currently authenticated viewer (null if logged out) */
+  viewerDid: string | null
 }
 
 export function ProfileHeader({
@@ -33,7 +36,10 @@ export function ProfileHeader({
   isMuted,
   onBlockToggle,
   onMuteToggle,
+  viewerDid,
 }: ProfileHeaderProps) {
+  const isOwnProfile = viewerDid !== null && viewerDid === profile.did
+
   return (
     <div className="overflow-hidden rounded-lg border border-border bg-card">
       {/* Banner */}
@@ -79,21 +85,23 @@ export function ProfileHeader({
               </span>
             </div>
 
-            {/* Block/Mute actions */}
-            <div className="mt-3 flex gap-2">
-              <BlockMuteButton
-                targetDid={profile.did}
-                action="block"
-                isActive={isBlocked}
-                onToggle={onBlockToggle}
-              />
-              <BlockMuteButton
-                targetDid={profile.did}
-                action="mute"
-                isActive={isMuted}
-                onToggle={onMuteToggle}
-              />
-            </div>
+            {/* Block/Mute actions (hidden on own profile) */}
+            {!isOwnProfile && (
+              <div className="mt-3 flex gap-2">
+                <BlockMuteButton
+                  targetDid={profile.did}
+                  action="block"
+                  isActive={isBlocked}
+                  onToggle={onBlockToggle}
+                />
+                <BlockMuteButton
+                  targetDid={profile.did}
+                  action="mute"
+                  isActive={isMuted}
+                  onToggle={onMuteToggle}
+                />
+              </div>
+            )}
           </div>
         </div>
       </div>
diff --git a/src/components/ui/toast.tsx b/src/components/ui/toast.tsx
new file mode 100644
index 0000000..29af6c9
--- /dev/null
+++ b/src/components/ui/toast.tsx
@@ -0,0 +1,111 @@
+'use client'
+
+import * as React from 'react'
+import * as ToastPrimitives from '@radix-ui/react-toast'
+import { X } from '@phosphor-icons/react'
+
+import { cn } from '@/lib/utils'
+
+const ToastProvider = ToastPrimitives.Provider
+
+const ToastViewport = React.forwardRef<
+  React.ComponentRef<typeof ToastPrimitives.Viewport>,
+  React.ComponentPropsWithoutRef<typeof ToastPrimitives.Viewport>
+>(({ className, ...props }, ref) => (
+  <ToastPrimitives.Viewport
+    ref={ref}
+    className={cn(
+      'fixed bottom-0 right-0 z-[100] flex max-h-screen w-full flex-col-reverse gap-2 p-4 sm:max-w-[420px]',
+      className
+    )}
+    {...props}
+  />
+))
+ToastViewport.displayName = ToastPrimitives.Viewport.displayName
+
+const Toast = React.forwardRef<
+  React.ComponentRef<typeof ToastPrimitives.Root>,
+  React.ComponentPropsWithoutRef<typeof ToastPrimitives.Root> & {
+    variant?: 'default' | 'destructive'
+  }
+>(({ className, variant = 'default', ...props }, ref) => (
+  <ToastPrimitives.Root
+    ref={ref}
+    className={cn(
+      'group pointer-events-auto relative flex w-full items-center gap-3 overflow-hidden rounded-lg border p-4 shadow-lg transition-all',
+      'data-[swipe=cancel]:translate-x-0 data-[swipe=end]:translate-x-[var(--radix-toast-swipe-end-x)] data-[swipe=move]:translate-x-[var(--radix-toast-swipe-move-x)] data-[swipe=move]:transition-none',
+      'data-[state=open]:animate-in data-[state=closed]:animate-out data-[swipe=end]:animate-out data-[state=closed]:fade-out-80 data-[state=closed]:slide-out-to-right-full data-[state=open]:slide-in-from-bottom-full',
+      variant === 'default' && 'border-border bg-card text-foreground',
+      variant === 'destructive' &&
+        'border-destructive/50 bg-destructive text-destructive-foreground',
+      className
+    )}
+    {...props}
+  />
+))
+Toast.displayName = ToastPrimitives.Root.displayName
+
+const ToastClose = React.forwardRef<
+  React.ComponentRef<typeof ToastPrimitives.Close>,
+  React.ComponentPropsWithoutRef<typeof ToastPrimitives.Close>
+>(({ className, ...props }, ref) => (
+  <ToastPrimitives.Close
+    ref={ref}
+    className={cn(
+      'absolute right-2 top-2 rounded-md p-1 text-foreground/50 opacity-0 transition-opacity hover:text-foreground focus:opacity-100 focus:outline-none focus:ring-2 group-hover:opacity-100',
+      className
+    )}
+    toast-close=""
+    {...props}
+  >
+    <X size={16} weight="bold" />
+  </ToastPrimitives.Close>
+))
+ToastClose.displayName = ToastPrimitives.Close.displayName
+
+const ToastTitle = React.forwardRef<
+  React.ComponentRef<typeof ToastPrimitives.Title>,
+  React.ComponentPropsWithoutRef<typeof ToastPrimitives.Title>
+>(({ className, ...props }, ref) => (
+  <ToastPrimitives.Title ref={ref} className={cn('text-sm font-semibold', className)} {...props} />
+))
+ToastTitle.displayName = ToastPrimitives.Title.displayName
+
+const ToastDescription = React.forwardRef<
+  React.ComponentRef<typeof ToastPrimitives.Description>,
+  React.ComponentPropsWithoutRef<typeof ToastPrimitives.Description>
+>(({ className, ...props }, ref) => (
+  <ToastPrimitives.Description
+    ref={ref}
+    className={cn('text-sm opacity-90', className)}
+    {...props}
+  />
+))
+ToastDescription.displayName = ToastPrimitives.Description.displayName
+
+const ToastAction = React.forwardRef<
+  React.ComponentRef<typeof ToastPrimitives.Action>,
+  React.ComponentPropsWithoutRef<typeof ToastPrimitives.Action>
+>(({ className, ...props }, ref) => (
+  <ToastPrimitives.Action
+    ref={ref}
+    className={cn(
+      'inline-flex shrink-0 items-center justify-center rounded-md border bg-transparent px-3 py-1 text-sm font-medium ring-offset-background transition-colors hover:bg-secondary focus:outline-none focus:ring-2 focus:ring-ring focus:ring-offset-2',
+      className
+    )}
+    {...props}
+  />
+))
+ToastAction.displayName = ToastPrimitives.Action.displayName
+
+export {
+  ToastProvider,
+  ToastViewport,
+  Toast,
+  ToastClose,
+  ToastTitle,
+  ToastDescription,
+  ToastAction,
+}
+
+export type ToastActionElement = React.ReactElement<typeof ToastAction>
diff --git a/src/context/toast-context.tsx b/src/context/toast-context.tsx
new file mode 100644
index 0000000..1992781
--- /dev/null
+++ b/src/context/toast-context.tsx
@@ -0,0 +1,78 @@
+'use client'
+
+import { createContext, useCallback, useMemo, useState } from 'react'
+import type { ReactNode } from 'react'
+import {
+  ToastProvider as RadixToastProvider,
+  ToastViewport,
+  Toast,
+  ToastTitle,
+  ToastDescription,
+  ToastClose,
+  ToastAction,
+} from '@/components/ui/toast'
+
+export interface ToastMessage {
+  id: string
+  title: string
+  description?: string
+  variant?: 'default' | 'destructive'
+  action?: {
+    label: string
+    onClick: () => void
+    altText: string
+  }
+}
+
+export interface ToastContextValue {
+  toast: (message: Omit<ToastMessage, 'id'>) => void
+  dismiss: (id: string) => void
+}
+
+export const ToastContext = createContext<ToastContextValue | null>(null)
+
+let toastCount = 0
+
+export function AppToastProvider({ children }: { children: ReactNode }) {
+  const [toasts, setToasts] = useState<ToastMessage[]>([])
+
+  const toast = useCallback((message: Omit<ToastMessage, 'id'>) => {
+    const id = String(++toastCount)
+    setToasts((prev) => [...prev, { ...message, id }])
+  }, [])
+
+  const dismiss = useCallback((id: string) => {
+    setToasts((prev) => prev.filter((t) => t.id !== id))
+  }, [])
+
+  const value = useMemo<ToastContextValue>(() => ({ toast, dismiss }), [toast, dismiss])
+
+  return (
+    <ToastContext.Provider value={value}>
+      <RadixToastProvider swipeDirection="right">
+        {children}
+        {toasts.map((t) => (
+          <Toast
+            key={t.id}
+            variant={t.variant}
+            onOpenChange={(open) => {
+              if (!open) dismiss(t.id)
+            }}
+          >
+            <div className="flex-1">
+              <ToastTitle>{t.title}</ToastTitle>
+              {t.description && <ToastDescription>{t.description}</ToastDescription>}
+            </div>
+            {t.action && (
+              <ToastAction altText={t.action.altText} onClick={t.action.onClick}>
+                {t.action.label}
+              </ToastAction>
+            )}
+            <ToastClose />
+          </Toast>
+        ))}
+        <ToastViewport />
+      </RadixToastProvider>
+    </ToastContext.Provider>
+  )
+}
diff --git a/src/hooks/use-require-auth.test.ts b/src/hooks/use-require-auth.test.ts
new file mode 100644
index 0000000..4a784c2
--- /dev/null
+++ b/src/hooks/use-require-auth.test.ts
@@ -0,0 +1,104 @@
+/**
+ * Tests for useRequireAuth hook.
+ */
+
+import { describe, it, expect, vi, beforeEach } from 'vitest'
+import { renderHook, act } from '@testing-library/react'
+import { useRequireAuth } from './use-require-auth'
+import { useAuth } from '@/hooks/use-auth'
+
+const mockToast = vi.fn()
+
+vi.mock('@/hooks/use-auth', () => ({
+  useAuth: vi.fn(),
+}))
+
+vi.mock('@/hooks/use-toast', () => ({
+  useToast: () => ({
+    toast: mockToast,
+    dismiss: vi.fn(),
+  }),
+}))
+
+const mockedUseAuth = vi.mocked(useAuth)
+
+beforeEach(() => {
+  vi.clearAllMocks()
+  // Default: logged out
+  mockedUseAuth.mockReturnValue({
+    user: null,
+    isAuthenticated: false,
+    isLoading: false,
+    crossPostScopesGranted: false,
+    getAccessToken: vi.fn(() => null),
+    login: vi.fn(),
+    logout: vi.fn(),
+    setSessionFromCallback: vi.fn(),
+    requestCrossPostAuth: vi.fn(),
+    authFetch: vi.fn(),
+  })
+})
+
+describe('useRequireAuth', () => {
+  it('shows login toast when not authenticated', () => {
+    const { result } = renderHook(() => useRequireAuth())
+    const action = vi.fn()
+
+    act(() => {
+      result.current.requireAuth(action)
+    })
+
+    expect(action).not.toHaveBeenCalled()
+    expect(mockToast).toHaveBeenCalledWith(
+      expect.objectContaining({
+        title: 'Login required',
+        description: 'You need to log in before you can perform this action.',
+      })
+    )
+  })
+
+  it('includes login action in toast', () => {
+    const { result } = renderHook(() => useRequireAuth())
+
+    act(() => {
+      result.current.requireAuth(vi.fn())
+    })
+
+    expect(mockToast.mock.calls[0]).toBeDefined()
+    const toastArg = mockToast.mock.calls[0]![0] as {
+      action?: { label: string; altText: string; onClick: () => void }
+    }
+    expect(toastArg.action).toBeDefined()
+    expect(toastArg.action?.label).toBe('Log in')
+  })
+
+  it('executes action when authenticated', () => {
+    mockedUseAuth.mockReturnValue({
+      user: { did: 'did:plc:test', handle: 'test.bsky.social', displayName: null, avatarUrl: null },
+      isAuthenticated: true,
+      isLoading: false,
+      crossPostScopesGranted: false,
+      getAccessToken: vi.fn(() => 'mock-token'),
+      login: vi.fn(),
+      logout: vi.fn(),
+      setSessionFromCallback: vi.fn(),
+      requestCrossPostAuth: vi.fn(),
+      authFetch: vi.fn(),
+    })
+
+    const { result } = renderHook(() => useRequireAuth())
+    const action = vi.fn()
+
+    act(() => {
+      result.current.requireAuth(action)
+    })
+
+    expect(action).toHaveBeenCalled()
+    expect(mockToast).not.toHaveBeenCalled()
+  })
+
+  it('exposes isAuthenticated from auth context', () => {
+    const { result } = renderHook(() => useRequireAuth())
+    expect(result.current.isAuthenticated).toBe(false)
+  })
+})
diff --git a/src/hooks/use-require-auth.ts b/src/hooks/use-require-auth.ts
new file mode 100644
index 0000000..7a3ee9f
--- /dev/null
+++ b/src/hooks/use-require-auth.ts
@@ -0,0 +1,38 @@
+'use client'
+
+import { useCallback } from 'react'
+import { useAuth } from '@/hooks/use-auth'
+import { useToast } from '@/hooks/use-toast'
+
+/**
+ * Returns a guard function that checks auth state before performing an action.
+ * If the user is not authenticated, shows a toast prompting them to log in.
+ * If authenticated, executes the callback.
+ */
+export function useRequireAuth() {
+  const { isAuthenticated } = useAuth()
+  const { toast } = useToast()
+
+  const requireAuth = useCallback(
+    (action: () => void | Promise<void>) => {
+      if (!isAuthenticated) {
+        toast({
+          title: 'Login required',
+          description: 'You need to log in before you can perform this action.',
+          action: {
+            label: 'Log in',
+            altText: 'Go to login page',
+            onClick: () => {
+              window.location.href = `/login?returnTo=${encodeURIComponent(window.location.pathname)}`
+            },
+          },
+        })
+        return
+      }
+      void action()
+    },
+    [isAuthenticated, toast]
+  )
+
+  return { requireAuth, isAuthenticated }
+}
diff --git a/src/hooks/use-toast.ts b/src/hooks/use-toast.ts
new file mode 100644
index 0000000..6455761
--- /dev/null
+++ b/src/hooks/use-toast.ts
@@ -0,0 +1,13 @@
+'use client'
+
+import { useContext } from 'react'
+import { ToastContext } from '@/context/toast-context'
+import type { ToastContextValue } from '@/context/toast-context'
+
+export function useToast(): ToastContextValue {
+  const context = useContext(ToastContext)
+  if (!context) {
+    throw new Error('useToast must be used within a ToastProvider')
+  }
+  return context
+}