diff --git a/README.md b/README.md
index 5b01d48..7986c9a 100644
--- a/README.md
+++ b/README.md
@@ -11,6 +11,7 @@ deliberate post detail link, or manually open a valid
 - Popular (`Top`) and chronological (`Latest`) search
 - Optional `filter:follows` search
 - Authenticated account list selection and list timelines from a deliberate URL or ID
+- Relay profile switching from the profiles exposed by the relay
 - Infinite cursor pagination with explicit retry
 - Deliberate post detail pages with the visible conversation
 - Infinite conversation loading with explicit continuation retry
@@ -45,9 +46,11 @@ cp .env.example .env.local
 The Nix development shell installs Hallmark's agent skill for the supported
 local agent targets.
 
-Set `TWITTER_RELAY_BASE_URL` in `.env.local` for Vite development, and set
-`BIRD_PROFILE_NAME` when the relay has multiple profiles. Export the same
-values in the process environment before `start` or `test:live`:
+Set `TWITTER_RELAY_BASE_URL` in `.env.local` for Vite development.
+`BIRD_PROFILE_NAME` optionally selects the initial relay profile; the header
+selector can switch between the profiles returned by the relay's `/profiles`
+endpoint. Export the same values in the process environment before `start` or
+`test:live`:
 
 ```bash
 export TWITTER_RELAY_BASE_URL=http://127.0.0.1:6900
diff --git a/e2e/integrations/reader.test.ts b/e2e/integrations/reader.test.ts
index 2917333..fd013fa 100644
--- a/e2e/integrations/reader.test.ts
+++ b/e2e/integrations/reader.test.ts
@@ -43,6 +43,39 @@ test('requires intent and infinitely loads a user timeline', async ({
   })
 })
 
+test('switches the relay profile and reloads visible content', async ({
+  homePage,
+  page,
+}) => {
+  await homePage.goTo()
+
+  const profile = page.getByLabel('プロファイル')
+  await expect(profile).toHaveValue('e2e')
+  await homePage.displayUser('@yuta')
+  await expect(page.getByText('user page 1')).toBeVisible()
+
+  await profile.selectOption('alternate')
+  await expect(page.getByRole('status')).toHaveText('切り替えました。')
+
+  await expect(page.getByText('alternate profile page 1')).toBeVisible()
+})
+
+test('keeps the profile selector within narrow viewports', async ({
+  homePage,
+  page,
+}) => {
+  for (const width of [320, 375, 414, 768]) {
+    await page.setViewportSize({ width, height: 900 })
+    await homePage.goTo()
+    await expect(page.getByLabel('プロファイル')).toBeVisible()
+    expect(
+      await page.evaluate(
+        () => document.documentElement.scrollWidth > window.innerWidth,
+      ),
+    ).toBe(false)
+  }
+})
+
 test('selects an authenticated list and infinitely loads its timeline', async ({
   listsPage,
   page,
@@ -152,6 +185,9 @@ test('keeps keyboard focus intentional and discovery or mutation controls absent
 }) => {
   await searchPage.goTo()
 
+  await searchPage.tabTo(
+    searchPage.page.getByRole('combobox', { name: 'プロファイル' }),
+  )
   await searchPage.tabTo(searchPage.page.getByRole('link', { name: '検索' }))
   await searchPage.tabTo(
     searchPage.page.getByRole('link', { name: 'ユーザー' }),
diff --git a/e2e/integrations/reader.test.ts-snapshots/mist-thread-desktop-linux.png b/e2e/integrations/reader.test.ts-snapshots/mist-thread-desktop-linux.png
index 9f685a2..12c918d 100644
Binary files a/e2e/integrations/reader.test.ts-snapshots/mist-thread-desktop-linux.png and b/e2e/integrations/reader.test.ts-snapshots/mist-thread-desktop-linux.png differ
diff --git a/e2e/integrations/reader.test.ts-snapshots/mist-thread-mobile-linux.png b/e2e/integrations/reader.test.ts-snapshots/mist-thread-mobile-linux.png
index f9efcde..de2a180 100644
Binary files a/e2e/integrations/reader.test.ts-snapshots/mist-thread-mobile-linux.png and b/e2e/integrations/reader.test.ts-snapshots/mist-thread-mobile-linux.png differ
diff --git a/e2e/integrations/reader.test.ts-snapshots/mist-user-desktop-linux.png b/e2e/integrations/reader.test.ts-snapshots/mist-user-desktop-linux.png
index dec2712..6e7ddb4 100644
Binary files a/e2e/integrations/reader.test.ts-snapshots/mist-user-desktop-linux.png and b/e2e/integrations/reader.test.ts-snapshots/mist-user-desktop-linux.png differ
diff --git a/e2e/integrations/reader.test.ts-snapshots/mist-user-mobile-linux.png b/e2e/integrations/reader.test.ts-snapshots/mist-user-mobile-linux.png
index b2dfab8..217f5a8 100644
Binary files a/e2e/integrations/reader.test.ts-snapshots/mist-user-mobile-linux.png and b/e2e/integrations/reader.test.ts-snapshots/mist-user-mobile-linux.png differ
diff --git a/e2e/mock-relay.mjs b/e2e/mock-relay.mjs
index cf4fc93..e5b6e62 100644
--- a/e2e/mock-relay.mjs
+++ b/e2e/mock-relay.mjs
@@ -99,6 +99,11 @@ const timeline = (entries) => ({ instructions: [{ entries }] })
 
 const server = createServer(async (request, response) => {
   const url = new URL(request.url ?? '/', `http://127.0.0.1:${port}`)
+  if (request.method === 'GET' && url.pathname === '/profiles') {
+    send(response, { profiles: ['e2e', 'alternate'] })
+    return
+  }
+
   const operation = [
     'UserByScreenName',
     'UserTweets',
@@ -120,7 +125,8 @@ const server = createServer(async (request, response) => {
     })
     return
   }
-  if (request.headers['x-profile-name'] !== 'e2e') {
+  const profileName = request.headers['x-profile-name']
+  if (profileName !== 'e2e' && profileName !== 'alternate') {
     fail(response, 403, 'invalid profile')
     return
   }
@@ -166,10 +172,12 @@ const server = createServer(async (request, response) => {
       return
     }
     const secondPage = variables.cursor === 'user-next'
+    const firstPageText =
+      profileName === 'alternate' ? 'alternate profile page 1' : 'user page 1'
     const entries = secondPage
       ? [tweet('1010', 'user page 2')]
       : [
-          tweet('1001', 'user page 1', 'yuta', {
+          tweet('1001', firstPageText, 'yuta', {
             conversationId: '1000',
           }),
           cursor('user-next'),
diff --git a/src/components/app-shell.tsx b/src/components/app-shell.tsx
index 8539ac6..e001633 100644
--- a/src/components/app-shell.tsx
+++ b/src/components/app-shell.tsx
@@ -1,4 +1,5 @@
 import { Link } from '@tanstack/react-router'
+import { ProfileSwitcher } from '#/features/profiles/profile-switcher'
 
 export function AppShell({
   active,
@@ -11,6 +12,7 @@ export function AppShell({
     <main className="app">
       <header className="app-header">
         <p className="brand">Twitter Lite</p>
+        <ProfileSwitcher />
         <nav aria-label="閲覧方法" className="tabs">
           <Link
             aria-current={active === 'search' ? 'page' : undefined}
diff --git a/src/features/posts/bird-client.server.ts b/src/features/posts/bird-client.server.ts
index 92dc594..4419bbd 100644
--- a/src/features/posts/bird-client.server.ts
+++ b/src/features/posts/bird-client.server.ts
@@ -1,13 +1,17 @@
 import { TwitterClient } from '@yuta/bird'
 import type { BirdReader } from './post-service'
 
-let client: TwitterClient | undefined
+const clients = new Map<string | undefined, TwitterClient>()
 
-export function getBirdReader(): BirdReader {
-  client ??= new TwitterClient({
+export function getBirdReader(profileName?: string): BirdReader {
+  const existing = clients.get(profileName)
+  if (existing) return existing
+
+  const client = new TwitterClient({
     relayBaseUrl: process.env.TWITTER_RELAY_BASE_URL,
-    profileName: process.env.BIRD_PROFILE_NAME,
+    profileName,
     timeoutMs: 20_000,
   })
+  clients.set(profileName, client)
   return client
 }
diff --git a/src/features/posts/server-functions.ts b/src/features/posts/server-functions.ts
index f2922e1..1d6aca2 100644
--- a/src/features/posts/server-functions.ts
+++ b/src/features/posts/server-functions.ts
@@ -28,8 +28,11 @@ const configFailure = (diagnostic: unknown): LoadFailure => {
 }
 
 async function reader() {
-  const { getBirdReader } = await import('./bird-client.server')
-  return getBirdReader()
+  const [{ getBirdReader }, { getSelectedProfileName }] = await Promise.all([
+    import('./bird-client.server'),
+    import('../profiles/profile-service.server'),
+  ])
+  return getBirdReader(getSelectedProfileName())
 }
 
 export const loadUserPosts = createServerFn({ method: 'GET' })
diff --git a/src/features/profiles/profile-service.server.ts b/src/features/profiles/profile-service.server.ts
new file mode 100644
index 0000000..795ba07
--- /dev/null
+++ b/src/features/profiles/profile-service.server.ts
@@ -0,0 +1,69 @@
+import {
+  deleteCookie,
+  getCookie,
+  setCookie,
+} from '@tanstack/react-start/server'
+import { z } from 'zod'
+import type { ProfileSettings } from './types'
+
+const PROFILE_COOKIE = 'twitter-lite-profile'
+const profilesSchema = z.object({
+  profiles: z.array(z.string().min(1)),
+})
+
+function relayBaseUrl() {
+  const relayBaseUrl = process.env.TWITTER_RELAY_BASE_URL
+  if (!relayBaseUrl) {
+    throw new Error('TWITTER_RELAY_BASE_URL is required')
+  }
+  return relayBaseUrl
+}
+
+export async function fetchProfileNames(
+  baseUrl = relayBaseUrl(),
+  request: typeof fetch = fetch,
+): Promise<string[]> {
+  const response = await request(new URL('/profiles', baseUrl), {
+    signal: AbortSignal.timeout(20_000),
+  })
+  if (!response.ok) {
+    throw new Error(`Profiles request failed with HTTP ${response.status}`)
+  }
+  return profilesSchema.parse(await response.json()).profiles
+}
+
+export function getSelectedProfileName(): string | undefined {
+  return getCookie(PROFILE_COOKIE) || process.env.BIRD_PROFILE_NAME
+}
+
+export async function loadProfileSettings(): Promise<ProfileSettings> {
+  const profiles = await fetchProfileNames()
+  const selectedProfile = getSelectedProfileName()
+  return {
+    profiles,
+    ...(selectedProfile && profiles.includes(selectedProfile)
+      ? { selectedProfile }
+      : {}),
+  }
+}
+
+export async function saveProfileSelection(
+  profileName: string | null,
+): Promise<void> {
+  if (profileName === null) {
+    deleteCookie(PROFILE_COOKIE, { path: '/' })
+    return
+  }
+
+  const profiles = await fetchProfileNames()
+  if (!profiles.includes(profileName)) {
+    throw new Error(`Unknown relay profile: ${profileName}`)
+  }
+
+  setCookie(PROFILE_COOKIE, profileName, {
+    httpOnly: true,
+    maxAge: 60 * 60 * 24 * 365,
+    path: '/',
+    sameSite: 'lax',
+  })
+}
diff --git a/src/features/profiles/profile-service.test.ts b/src/features/profiles/profile-service.test.ts
new file mode 100644
index 0000000..9b03f8b
--- /dev/null
+++ b/src/features/profiles/profile-service.test.ts
@@ -0,0 +1,36 @@
+// @vitest-environment node
+
+import { describe, expect, it, vi } from 'vitest'
+import { fetchProfileNames } from './profile-service.server'
+
+describe('fetchProfileNames', () => {
+  it('loads profile names from the relay endpoint', async () => {
+    const request = vi.fn(async () =>
+      Response.json({ profiles: ['main', 'work'] }),
+    )
+
+    await expect(
+      fetchProfileNames('https://relay.invalid/api', request),
+    ).resolves.toEqual(['main', 'work'])
+    expect(request).toHaveBeenCalledWith(
+      new URL('https://relay.invalid/profiles'),
+      expect.objectContaining({ signal: expect.any(AbortSignal) }),
+    )
+  })
+
+  it('rejects malformed relay responses', async () => {
+    const request = vi.fn(async () => Response.json({ profiles: [42] }))
+
+    await expect(
+      fetchProfileNames('https://relay.invalid', request),
+    ).rejects.toThrow()
+  })
+
+  it('rejects unsuccessful relay responses', async () => {
+    const request = vi.fn(async () => new Response(null, { status: 503 }))
+
+    await expect(
+      fetchProfileNames('https://relay.invalid', request),
+    ).rejects.toThrow('Profiles request failed with HTTP 503')
+  })
+})
diff --git a/src/features/profiles/profile-switcher.test.tsx b/src/features/profiles/profile-switcher.test.tsx
new file mode 100644
index 0000000..f531bdf
--- /dev/null
+++ b/src/features/profiles/profile-switcher.test.tsx
@@ -0,0 +1,60 @@
+import { QueryClient, QueryClientProvider } from '@tanstack/react-query'
+import { fireEvent, render, screen, waitFor } from '@testing-library/react'
+import { describe, expect, it, vi } from 'vitest'
+import { ProfileSwitcher } from './profile-switcher'
+
+const serverFns = vi.hoisted(() => ({
+  loadProfiles: vi.fn(),
+  selectProfile: vi.fn(),
+}))
+
+vi.mock('./server-functions', () => serverFns)
+vi.mock('@tanstack/react-start', () => ({
+  useServerFn: (serverFn: unknown) => serverFn,
+}))
+
+function renderSwitcher() {
+  const queryClient = new QueryClient({
+    defaultOptions: { queries: { retry: false } },
+  })
+  render(
+    <QueryClientProvider client={queryClient}>
+      <ProfileSwitcher />
+    </QueryClientProvider>,
+  )
+}
+
+describe('ProfileSwitcher', () => {
+  it('shows relay profiles and switches the active profile', async () => {
+    serverFns.loadProfiles.mockResolvedValue({
+      profiles: ['main', 'work'],
+      selectedProfile: 'main',
+    })
+    serverFns.selectProfile.mockResolvedValue({ profileName: 'work' })
+    renderSwitcher()
+
+    const select = await screen.findByLabelText('プロファイル')
+    await waitFor(() => expect(select).toHaveValue('main'))
+
+    fireEvent.change(select, { target: { value: 'work' } })
+
+    await waitFor(() =>
+      expect(serverFns.selectProfile).toHaveBeenCalledWith({
+        data: { profileName: 'work' },
+      }),
+    )
+    expect(await screen.findByRole('status')).toHaveTextContent(
+      '切り替えました。',
+    )
+  })
+
+  it('reports a relay profiles failure without offering stale choices', async () => {
+    serverFns.loadProfiles.mockRejectedValue(new Error('relay unavailable'))
+    renderSwitcher()
+
+    const select = await screen.findByLabelText('プロファイル')
+    await waitFor(() => expect(select).toBeDisabled())
+    expect(select).toHaveTextContent('取得失敗')
+    expect(select).toHaveAttribute('aria-invalid', 'true')
+  })
+})
diff --git a/src/features/profiles/profile-switcher.tsx b/src/features/profiles/profile-switcher.tsx
new file mode 100644
index 0000000..5fa7703
--- /dev/null
+++ b/src/features/profiles/profile-switcher.tsx
@@ -0,0 +1,72 @@
+import { useMutation, useQuery, useQueryClient } from '@tanstack/react-query'
+import { useServerFn } from '@tanstack/react-start'
+import { loadProfiles, selectProfile } from './server-functions'
+
+export function ProfileSwitcher() {
+  const queryClient = useQueryClient()
+  const load = useServerFn(loadProfiles)
+  const select = useServerFn(selectProfile)
+  const profiles = useQuery({
+    queryKey: ['profiles'],
+    queryFn: () => load(),
+    retry: false,
+    refetchOnWindowFocus: false,
+    refetchOnReconnect: false,
+  })
+  const selection = useMutation({
+    mutationFn: (profileName: string | null) =>
+      select({ data: { profileName } }),
+    onSuccess: () => queryClient.invalidateQueries(),
+  })
+
+  const state = profiles.isError
+    ? 'error'
+    : selection.isError
+      ? 'error'
+      : selection.isSuccess
+        ? 'success'
+        : profiles.isPending || selection.isPending
+          ? 'loading'
+          : 'default'
+
+  return (
+    <div className="profile-switcher" data-state={state}>
+      <label htmlFor="relay-profile">プロファイル</label>
+      <select
+        aria-invalid={state === 'error' ? 'true' : undefined}
+        disabled={profiles.isPending || profiles.isError || selection.isPending}
+        id="relay-profile"
+        onChange={(event) =>
+          selection.mutate(
+            event.target.value.length ? event.target.value : null,
+          )
+        }
+        value={profiles.data?.selectedProfile ?? ''}
+      >
+        <option value="">
+          {profiles.isPending
+            ? '読み込み中'
+            : profiles.isError
+              ? '取得失敗'
+              : '自動'}
+        </option>
+        {profiles.data?.profiles.map((profile) => (
+          <option key={profile} value={profile}>
+            {profile}
+          </option>
+        ))}
+      </select>
+      <span
+        aria-live="polite"
+        className="profile-switcher-status"
+        role={selection.isError ? 'alert' : 'status'}
+      >
+        {selection.isError
+          ? '切り替えに失敗しました。'
+          : selection.isSuccess
+            ? '切り替えました。'
+            : ''}
+      </span>
+    </div>
+  )
+}
diff --git a/src/features/profiles/server-functions.ts b/src/features/profiles/server-functions.ts
new file mode 100644
index 0000000..8635d15
--- /dev/null
+++ b/src/features/profiles/server-functions.ts
@@ -0,0 +1,22 @@
+import { createServerFn } from '@tanstack/react-start'
+import { z } from 'zod'
+import type { ProfileSelection } from './types'
+
+const profileSelectionSchema = z.object({
+  profileName: z.string().min(1).nullable(),
+})
+
+export const loadProfiles = createServerFn({ method: 'GET' }).handler(
+  async () => {
+    const { loadProfileSettings } = await import('./profile-service.server')
+    return loadProfileSettings()
+  },
+)
+
+export const selectProfile = createServerFn({ method: 'POST' })
+  .validator(profileSelectionSchema)
+  .handler(async ({ data }): Promise<ProfileSelection> => {
+    const { saveProfileSelection } = await import('./profile-service.server')
+    await saveProfileSelection(data.profileName)
+    return data
+  })
diff --git a/src/features/profiles/types.ts b/src/features/profiles/types.ts
new file mode 100644
index 0000000..a135edc
--- /dev/null
+++ b/src/features/profiles/types.ts
@@ -0,0 +1,8 @@
+export type ProfileSettings = {
+  profiles: string[]
+  selectedProfile?: string
+}
+
+export type ProfileSelection = {
+  profileName: string | null
+}
diff --git a/src/routes/-feed-wiring.test.tsx b/src/routes/-feed-wiring.test.tsx
index 50274f9..a11184a 100644
--- a/src/routes/-feed-wiring.test.tsx
+++ b/src/routes/-feed-wiring.test.tsx
@@ -23,6 +23,10 @@ vi.mock('../features/posts/use-list-choices', async (importOriginal) => {
   return { ...original, useListChoices }
 })
 
+vi.mock('../features/profiles/profile-switcher', () => ({
+  ProfileSwitcher: () => null,
+}))
+
 vi.mock('./__root', async () => {
   const { createRootRouteWithContext, Outlet } = await import(
     '@tanstack/react-router'
diff --git a/src/styles.css b/src/styles.css
index 6c89a70..54f9469 100644
--- a/src/styles.css
+++ b/src/styles.css
@@ -639,8 +639,9 @@ video.media {
 
 .app-header {
   display: grid;
+  grid-template-columns: minmax(0, 1fr) minmax(8rem, 9.5rem);
   align-items: start;
-  gap: var(--space-lg);
+  gap: var(--space-lg) var(--space-md);
   padding: var(--space-lg) var(--space-md) 0;
   border-bottom: var(--rule-hairline) solid var(--border);
   background: var(--paper);
@@ -665,6 +666,7 @@ video.media {
 
 .tabs {
   display: grid;
+  grid-column: 1 / -1;
   grid-template-columns: repeat(3, minmax(0, 1fr));
   gap: 0;
 }
@@ -689,6 +691,72 @@ video.media {
   font-weight: 700;
 }
 
+/* Hallmark · component: select · genre: editorial · theme: Garden
+ * states: default · hover · focus · active · disabled · loading · error · success
+ * contrast: pass
+ */
+.profile-switcher {
+  position: relative;
+  display: grid;
+  width: 100%;
+  min-width: 0;
+  gap: var(--space-2xs);
+}
+
+.profile-switcher label {
+  color: var(--secondary);
+  font-size: var(--text-xs);
+  font-weight: 700;
+  line-height: 1;
+  white-space: nowrap;
+}
+
+.profile-switcher select {
+  width: 100%;
+  min-height: 2.75rem;
+  padding: var(--space-xs) 1.8rem var(--space-xs) var(--space-sm);
+  border: var(--rule-hairline) solid var(--control-border);
+  border-radius: var(--radius-sm);
+  outline: var(--rule-active) solid transparent;
+  outline-offset: var(--space-3xs);
+  background: var(--paper);
+  color: var(--ink);
+  cursor: pointer;
+  font: inherit;
+  font-size: var(--text-sm);
+  text-overflow: ellipsis;
+}
+
+.profile-switcher select:active {
+  background: var(--color-paper-3);
+}
+
+.profile-switcher select:disabled {
+  cursor: not-allowed;
+  opacity: 0.55;
+}
+
+.profile-switcher[data-state="error"] select {
+  border-color: var(--error);
+}
+
+.profile-switcher[data-state="success"] select {
+  border-color: var(--color-success);
+}
+
+.profile-switcher-status {
+  position: absolute;
+  inset-block-start: 100%;
+  inset-inline-end: 0;
+  color: var(--error);
+  font-size: var(--text-xs);
+  white-space: nowrap;
+}
+
+.profile-switcher[data-state="success"] .profile-switcher-status {
+  color: var(--color-success);
+}
+
 .reader {
   width: 100%;
   min-width: 0;
@@ -1055,6 +1123,10 @@ button[data-state="success"] {
     color: var(--accent-hover);
   }
 
+  .profile-switcher select:not(:disabled):hover {
+    background: var(--color-paper-2);
+  }
+
   .field input:hover {
     background: var(--surface);
   }
@@ -1146,6 +1218,7 @@ button[data-state="success"] {
   .app-header {
     position: sticky;
     top: var(--space-lg);
+    grid-template-columns: minmax(0, 1fr);
     grid-row: 1;
     grid-column: 1;
     align-self: start;
@@ -1157,6 +1230,7 @@ button[data-state="success"] {
   }
 
   .tabs {
+    grid-column: 1;
     grid-template-columns: minmax(0, 1fr);
   }