diff --git a/e2e/_verify_collab.spec.ts b/e2e/_verify_collab.spec.ts
new file mode 100644
index 0000000..23e8816
--- /dev/null
+++ b/e2e/_verify_collab.spec.ts
@@ -0,0 +1,116 @@
+import { test, expect, type Page } from '@playwright/test'
+
+// Manual verification for the collab duplication + probe fixes. Underscore-
+// prefixed so it stays OUT of the default CI suite (it needs the real
+// collab.noteser.app worker reachable + NEXT_PUBLIC_YJS_WS_URL set on the dev
+// server). Run explicitly (rename off the `_` prefix or point the runner at
+// it directly):
+//   NEXT_PUBLIC_YJS_WS_URL=wss://collab.noteser.app/<token> npm run dev   (separate shell)
+//   npx playwright test e2e/<renamed>.spec.ts
+
+const SENTINEL = 'ZQX-UNIQUE-BODY-7731'
+const BODY = `# Collab heading\nfirst body line ${SENTINEL}\nsecond body line`
+
+// Shape of the testHooks bridge this spec touches. Cast inline (rather than
+// augmenting the global Window) so it never collides with the canonical
+// declaration in src/utils/testHooks.ts under tsc.
+type Hooks = {
+  stores: {
+    noteStore: { getState(): {
+      addNote: (i: { title: string; content: string }) => { id: string }
+      notes: Array<{ id: string; collabId?: string; content: string }>
+    } }
+    workspaceStore: { getState(): { openNote: (id: string, o: { preview: boolean }) => void } }
+  }
+}
+// NOTE: page.evaluate callbacks run in the BROWSER, so they cast `window`
+// inline rather than closing over any Node-side helper.
+
+async function waitForHooks(page: Page) {
+  await page.waitForFunction(
+    () => typeof (window as unknown as { __noteser_test?: unknown }).__noteser_test !== 'undefined',
+    undefined, { timeout: 20_000 },
+  )
+}
+
+function trackErrors(page: Page): string[] {
+  const errs: string[] = []
+  page.on('console', m => { if (m.type() === 'error') errs.push(m.text()) })
+  page.on('pageerror', e => errs.push(String(e)))
+  return errs
+}
+
+const countOccurrences = (haystack: string, needle: string) => haystack.split(needle).length - 1
+
+test('collab: pill connected, body once, live A→B sync, single saved content', async ({ browser }) => {
+  // ── Context A — seeder ────────────────────────────────────────────────────
+  const ctxA = await browser.newContext()
+  const pageA = await ctxA.newPage()
+  const errA = trackErrors(pageA)
+  await pageA.goto('/')
+  await waitForHooks(pageA)
+
+  const noteId = await pageA.evaluate((content) => {
+    const h = (window as unknown as { __noteser_test: Hooks }).__noteser_test
+    const n = h.stores.noteStore.getState().addNote({ title: 'Collab Note', content })
+    h.stores.workspaceStore.getState().openNote(n.id, { preview: false })
+    return n.id
+  }, BODY)
+
+  // Editor must show the body (seeded into the fresh room over the wire) and
+  // exactly once — the doubling bug rendered it twice.
+  await expect(pageA.locator('.cm-content').first()).toContainText(SENTINEL, { timeout: 20_000 })
+  const aText = await pageA.locator('.cm-content').first().innerText()
+  expect(countOccurrences(aText, SENTINEL)).toBe(1)
+
+  // Pill must report connected ("Live: on") — the probe now dials /<token>/<room>.
+  await expect(pageA.getByTestId('status-bar-collab')).toHaveText(/Live: on/, { timeout: 20_000 })
+
+  // The note's stable collabId (room) is minted when the binding attaches.
+  await pageA.waitForFunction(
+    (id) => !!(window as unknown as { __noteser_test: Hooks }).__noteser_test
+      .stores.noteStore.getState().notes.find(n => n.id === id)?.collabId,
+    noteId, { timeout: 20_000 },
+  )
+  const collabId = await pageA.evaluate(
+    (id) => (window as unknown as { __noteser_test: Hooks }).__noteser_test
+      .stores.noteStore.getState().notes.find(n => n.id === id)!.collabId!,
+    noteId,
+  )
+
+  // Saved content must be the SINGLE body, never doubled.
+  const storedA = await pageA.evaluate(
+    (id) => (window as unknown as { __noteser_test: Hooks }).__noteser_test
+      .stores.noteStore.getState().notes.find(n => n.id === id)!.content,
+    noteId,
+  )
+  expect(countOccurrences(storedA, SENTINEL)).toBe(1)
+
+  // ── Context B — joiner via share link ─────────────────────────────────────
+  const ctxB = await browser.newContext()
+  const pageB = await ctxB.newPage()
+  const errB = trackErrors(pageB)
+  await pageB.goto(`/?collab=${collabId}&title=Collab%20Note`)
+  await waitForHooks(pageB)
+
+  // B materialises an EMPTY local note bound to the room and pulls A's content
+  // over the wire — exactly once.
+  await expect(pageB.locator('.cm-content').first()).toContainText(SENTINEL, { timeout: 20_000 })
+  const bText = await pageB.locator('.cm-content').first().innerText()
+  expect(countOccurrences(bText, SENTINEL)).toBe(1)
+  await expect(pageB.getByTestId('status-bar-collab')).toHaveText(/Live: on/, { timeout: 20_000 })
+
+  // ── Live sync A → B ───────────────────────────────────────────────────────
+  const liveToken = 'LIVE-EDIT-FROM-A-4477'
+  await pageA.locator('.cm-content').first().click()
+  await pageA.keyboard.press('Control+End')
+  await pageA.keyboard.type(`\n${liveToken}`)
+  await expect(pageB.locator('.cm-content').first()).toContainText(liveToken, { timeout: 20_000 })
+
+  // ── No CSP / WebSocket errors on either client ────────────────────────────
+  const offending = [...errA, ...errB].filter(e => /content security policy|csp|websocket|wss:/i.test(e))
+  expect(offending, `offending console errors:\n${offending.join('\n')}`).toEqual([])
+
+  await ctxA.close()
+  await ctxB.close()
+})
diff --git a/src/__tests__/collabNoDoubling.test.tsx b/src/__tests__/collabNoDoubling.test.tsx
new file mode 100644
index 0000000..ca9d4d8
--- /dev/null
+++ b/src/__tests__/collabNoDoubling.test.tsx
@@ -0,0 +1,103 @@
+/**
+ * @jest-environment jsdom
+ *
+ * Regression test for the live-collaboration content-DOUBLING bug.
+ *
+ * We mount a REAL CodeMirror EditorView with the REAL yCollab binding (only
+ * the websocket transport is faked — the provider's awareness is a real
+ * y-protocols Awareness over the binding's own Y.Doc, and we drive the
+ * provider's 'sync' event by hand). This exercises the exact path that
+ * doubled content in production:
+ *
+ *   editor doc + yCollab observer replaying the seeded Y.Text on top of it.
+ *
+ * The fix builds the editor EMPTY when collab is enabled and lets the Y.Text
+ * be the single content source. These tests pin that: after binding + sync the
+ * editor holds the note body EXACTLY ONCE — for the seeder (fresh room) and
+ * for a joiner (room already populated over the wire).
+ */
+
+import * as Y from 'yjs'
+import { Awareness } from 'y-protocols/awareness'
+import { EditorState } from '@codemirror/state'
+import { EditorView } from '@codemirror/view'
+import {
+  createCollabBinding,
+  type ProviderLike,
+  type ProviderFactory,
+} from '../components/editor/collabExtension'
+
+// Fake transport: real Awareness (yRemoteSelections needs it), manual 'sync'.
+class FakeProvider implements ProviderLike {
+  awareness: Awareness
+  private handlers: Array<(s: boolean) => void> = []
+  constructor(doc: Y.Doc) {
+    this.awareness = new Awareness(doc)
+  }
+  on(_event: 'sync', cb: (s: boolean) => void) { this.handlers.push(cb) }
+  off(_event: 'sync', cb: (s: boolean) => void) {
+    this.handlers = this.handlers.filter(h => h !== cb)
+  }
+  destroy() { this.awareness.destroy() }
+  fireSync() { this.handlers.forEach(h => h(true)) }
+}
+
+let lastProvider: FakeProvider | null = null
+const factory: ProviderFactory = (_url, _room, doc) => {
+  lastProvider = new FakeProvider(doc)
+  return lastProvider
+}
+
+// Mount a real editor with the binding spliced in, mirroring how the editor
+// renders: `editorDoc` is the value CodeMirror starts with ('' when collab is
+// enabled — the fix — or the body, which reproduces the old doubling bug).
+function mount(editorDoc: string, body: string) {
+  const binding = createCollabBinding({
+    url: 'wss://collab.example.com/token',
+    room: 'room-1',
+    initialContent: body,
+    user: null,
+    providerFactory: factory,
+  })
+  const view = new EditorView({
+    state: EditorState.create({ doc: editorDoc, extensions: [binding.extension] }),
+  })
+  return { binding, view }
+}
+
+describe('collab binding does not double content', () => {
+  test('SEEDER: empty editor + seed-on-sync → body appears exactly once', () => {
+    const body = '# Title\nthe body'
+    const { binding, view } = mount('', body)
+    expect(view.state.doc.toString()).toBe('') // nothing until sync
+    lastProvider!.fireSync()
+    expect(view.state.doc.toString()).toBe(body) // once, not doubled
+    binding.destroy()
+    view.destroy()
+  })
+
+  test('JOINER: empty editor + content arriving over the wire → body once, no re-seed', () => {
+    const body = 'remote body'
+    const { binding, view } = mount('', body)
+    // Simulate the shared doc being populated by another client before sync.
+    binding.ytext.insert(0, body)
+    // The yCollab observer should have already replayed it into the editor.
+    expect(view.state.doc.toString()).toBe(body)
+    // Sync now fires: seed-on-empty must NOT append our local copy.
+    lastProvider!.fireSync()
+    expect(view.state.doc.toString()).toBe(body)
+    binding.destroy()
+    view.destroy()
+  })
+
+  test('REGRESSION GUARD: initializing the editor WITH the body reproduces the doubling', () => {
+    // This is the pre-fix shape (value={initialContent}); it must double, which
+    // is exactly why the editor is built EMPTY when collab is enabled.
+    const body = 'hello'
+    const { binding, view } = mount(body, body)
+    lastProvider!.fireSync()
+    expect(view.state.doc.toString()).toBe(body + body)
+    binding.destroy()
+    view.destroy()
+  })
+})
diff --git a/src/__tests__/useCollaboration.test.ts b/src/__tests__/useCollaboration.test.ts
index fb7d4b2..775f886 100644
--- a/src/__tests__/useCollaboration.test.ts
+++ b/src/__tests__/useCollaboration.test.ts
@@ -10,7 +10,7 @@
  */
 
 import { renderHook, act } from '@testing-library/react'
-import { useCollaboration } from '../hooks/useCollaboration'
+import { useCollaboration, buildProbeUrl } from '../hooks/useCollaboration'
 
 // Test double for window.WebSocket. Captures whichever instance the
 // hook constructs so tests can fire open/close events synchronously.
@@ -56,6 +56,23 @@ afterEach(() => {
   else process.env.NEXT_PUBLIC_YJS_WS_URL = ORIGINAL_URL
 })
 
+describe('buildProbeUrl', () => {
+  test('appends a /<room> segment so the worker sees /<token>/<room>', () => {
+    // Configured URL is the bare base + token, with NO room. The worker reads
+    // the LAST segment as the room and the one before it as the token, so the
+    // probe must dial `<base>/<token>/<room>` — never the bare URL (which the
+    // worker would read as a single-segment room with no token → 403).
+    expect(buildProbeUrl('wss://collab.noteser.app/deadbeefToken')).toBe(
+      'wss://collab.noteser.app/deadbeefToken/__probe__',
+    )
+  })
+  test('does not double the slash when the URL has a trailing slash', () => {
+    expect(buildProbeUrl('wss://collab.noteser.app/tok/')).toBe(
+      'wss://collab.noteser.app/tok/__probe__',
+    )
+  })
+})
+
 describe('useCollaboration', () => {
   test('without NEXT_PUBLIC_YJS_WS_URL: status is "off" and no WS is opened', () => {
     delete process.env.NEXT_PUBLIC_YJS_WS_URL
@@ -84,6 +101,20 @@ describe('useCollaboration', () => {
     expect(result.current.attempts).toBe(0)
   })
 
+  test('dials the probe room (/__probe__), not the bare configured URL', () => {
+    // Regression for the false "Live: unreachable" pill: the configured URL is
+    // the bare `<base>/<token>` with no room. The probe must append a room so
+    // the worker's `/<token>/<room>` auth check passes — the bare URL is read
+    // as a single-segment room with no token and rejected (403).
+    process.env.NEXT_PUBLIC_YJS_WS_URL = 'wss://collab.example.com/sometoken'
+    const { result } = renderHook(() => useCollaboration())
+    expect(MockWebSocket.lastConstructorArg).toBe(
+      'wss://collab.example.com/sometoken/__probe__',
+    )
+    // The exposed url stays the bare configured value (used by the pill tooltip).
+    expect(result.current.url).toBe('wss://collab.example.com/sometoken')
+  })
+
   test('close before open: status flips to disconnected and attempt counter ticks', () => {
     process.env.NEXT_PUBLIC_YJS_WS_URL = 'wss://collab.example.com/room'
     const { result } = renderHook(() => useCollaboration())
diff --git a/src/components/editor/CodeMirrorEditor.tsx b/src/components/editor/CodeMirrorEditor.tsx
index 67f8157..af5bf60 100644
--- a/src/components/editor/CodeMirrorEditor.tsx
+++ b/src/components/editor/CodeMirrorEditor.tsx
@@ -536,6 +536,21 @@ export function CodeMirrorEditor({
   const [wikilinkState, setWikilinkState] = useState<WikilinkState | null>(null)
   const [tagState, setTagState] = useState<TagState | null>(null)
 
+  // When collaboration is enabled the shared Y.Text is the SINGLE source of
+  // truth for the document content. The collab binding (yCollab) seeds the
+  // Y.Text with the note body after the first sync and replays it into the
+  // editor via its observer. If the editor ALSO started life holding the same
+  // body (value={initialContent}), that replayed insert lands on top of the
+  // existing text and the body is DOUBLED on screen — and the doubled text is
+  // what gets saved back to the note (= corruption that would sync to GitHub).
+  // So when collab is on we build the editor EMPTY and let the Y.Text populate
+  // it: the seeder sees its body exactly once, a joiner receives the shared
+  // body over the wire exactly once. When collab is off this is a no-op and
+  // the editor is byte-for-byte identical to before. getConfiguredUrl() is the
+  // exact gate the collab effect below uses, so this branches together with it.
+  const collabEnabled = getConfiguredUrl() != null
+  const editorInitialValue = collabEnabled ? '' : initialContent
+
   // Live-collaboration (Phase B). A Compartment lets us swap the yCollab
   // binding in/out per note without rebuilding the whole (memoized)
   // extension list. The compartment is created ONCE and stays empty unless
@@ -1247,7 +1262,7 @@ export function CodeMirrorEditor({
       <CodeMirror
         key={noteId}
         ref={cmRef}
-        value={initialContent}
+        value={editorInitialValue}
         extensions={extensions}
         onChange={handleChange}
         onCreateEditor={(view) => {
diff --git a/src/hooks/useCollaboration.ts b/src/hooks/useCollaboration.ts
index 364f4e3..88984a7 100644
--- a/src/hooks/useCollaboration.ts
+++ b/src/hooks/useCollaboration.ts
@@ -36,6 +36,23 @@ export interface CollabState {
 
 const MAX_ATTEMPTS = 5
 
+// Dedicated room the connectivity probe dials. The collab worker requires a
+// `/<AUTH_TOKEN>/<room>` path (token = second-to-last segment, room = last);
+// NEXT_PUBLIC_YJS_WS_URL is configured as the BARE `<base>/<token>` with NO
+// room. Opening a socket straight to that bare URL gives the worker a single
+// path segment, which it reads as the ROOM with no token → 403 → the probe
+// false-reports "unreachable" even though the real document binding (which
+// connects to `<url>/<room>`) authenticates fine. Appending a probe room
+// makes the probe hit the same `/<token>/<room>` shape the worker accepts.
+const PROBE_ROOM = '__probe__'
+
+// Build the URL the connectivity probe actually dials: the configured base
+// URL plus a dedicated probe room, mirroring how WebsocketProvider(url, room)
+// connects to `<url>/<room>`. Exported for unit testing.
+export function buildProbeUrl(url: string): string {
+  return `${url.replace(/\/+$/, '')}/${PROBE_ROOM}`
+}
+
 // Exported so the Phase-B collaboration binding (collabExtension) shares the
 // exact same gate as the Phase-A connectivity probe: a single source of
 // truth for "is collab enabled, and at what URL". Returns null unless
@@ -89,7 +106,9 @@ export function useCollaboration(): CollabState {
     setStatus('connecting')
     let ws: WebSocket
     try {
-      ws = new WebSocket(url)
+      // Dial the probe room (not the bare configured URL) so the worker sees
+      // the `/<token>/<room>` path it requires and accepts the socket.
+      ws = new WebSocket(buildProbeUrl(url))
     } catch {
       setStatus('error')
       return