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+# Device Selector for New Session Dialog — Implementation Plan
+
+> **Execution:** Use the subagent-driven-development workflow to implement this plan.
+
+**Goal:** When multi-device is enabled with remote instances configured, the "new session" dialog shows a device dropdown so users can create sessions on any device — local or remote.
+
+**Architecture:** Add a `POST /api/federation/{remote_id}/sessions` proxy endpoint on the backend (same pattern as existing `federation_connect` and `federation_bell_clear`). On the frontend, inject a `<select>` element into both `showNewSessionInput()` (desktop/sidebar) and `showFabSessionInput()` (mobile FAB), route `createNewSession()` through the federation proxy when a remote device is selected, and fix the auto-open polling to match remote sessions by `sessionKey`.
+
+**Tech Stack:** Python/FastAPI (backend), vanilla JS (frontend), pytest + monkeypatch (backend tests), node:test + source inspection (frontend tests)
+
+---
+
+### Task 1: Backend — Add `POST /api/federation/{remote_id}/sessions` proxy endpoint
+
+**Files:**
+- Modify: `muxplex/main.py:1085` (insert before the static file mount)
+- Test: `muxplex/tests/test_api.py` (append)
+
+**Step 1: Write the failing tests**
+
+Append these tests to the end of `muxplex/tests/test_api.py`:
+
+```python
+
+
+# --- Device selector: federation session create proxy ---
+
+
+def test_federation_create_session_proxies_to_remote(client, monkeypatch, tmp_path):
+    """POST /api/federation/{remote_id}/sessions proxies POST to remote's create-session endpoint.
+
+    Looks up remote by integer index, sends POST {remote_url}/api/sessions
+    with Bearer auth header and JSON body {name: ...}, and returns the remote's JSON response.
+    """
+    import json
+    from unittest.mock import MagicMock
+
+    import muxplex.settings as settings_mod
+
+    settings_path = tmp_path / "settings.json"
+    monkeypatch.setattr(settings_mod, "SETTINGS_PATH", settings_path)
+
+    settings_path.write_text(
+        json.dumps(
+            {
+                "remote_instances": [
+                    {
+                        "url": "http://remote-host:8088",
+                        "key": "secret-key-123",
+                        "name": "remote-host",
+                        "id": "remote-0",
+                    }
+                ],
+            }
+        )
+    )
+
+    # Track what POST was called with
+    post_calls = []
+
+    async def mock_post(url, **kwargs):
+        post_calls.append({"url": url, "kwargs": kwargs})
+        mock_resp = MagicMock()
+        mock_resp.status_code = 200
+        mock_resp.json.return_value = {"name": "my-project"}
+        return mock_resp
+
+    mock_fed_client = MagicMock()
+    mock_fed_client.post = mock_post
+    monkeypatch.setattr(client.app.state, "federation_client", mock_fed_client)
+
+    response = client.post("/api/federation/0/sessions", json={"name": "my-project"})
+    assert response.status_code == 200
+
+    # Verify the POST was made to the correct URL
+    assert len(post_calls) == 1, f"Expected exactly 1 POST call, got {len(post_calls)}"
+    call = post_calls[0]
+    assert (
+        call["url"] == "http://remote-host:8088/api/sessions"
+    ), f"Expected POST to remote sessions URL, got: {call['url']}"
+
+    # Verify Bearer auth was included
+    headers = call["kwargs"].get("headers", {})
+    assert headers.get("Authorization") == "Bearer secret-key-123", (
+        f"Expected Bearer auth header, got: {headers}"
+    )
+
+    # Verify JSON body was forwarded
+    assert call["kwargs"].get("json") == {"name": "my-project"}, (
+        f"Expected JSON body with name, got: {call['kwargs'].get('json')}"
+    )
+
+    # Verify the response is the remote's JSON
+    data = response.json()
+    assert data["name"] == "my-project"
+
+
+def test_federation_create_session_returns_404_for_invalid_remote(
+    client, monkeypatch, tmp_path
+):
+    """POST /api/federation/{remote_id}/sessions returns 404 when remote_id is out of range."""
+    import json
+
+    import muxplex.settings as settings_mod
+
+    settings_path = tmp_path / "settings.json"
+    monkeypatch.setattr(settings_mod, "SETTINGS_PATH", settings_path)
+
+    # No remote instances configured
+    settings_path.write_text(json.dumps({"remote_instances": []}))
+
+    response = client.post("/api/federation/0/sessions", json={"name": "test"})
+    assert response.status_code == 404
+
+
+def test_federation_create_session_returns_503_when_remote_unreachable(
+    client, monkeypatch, tmp_path
+):
+    """POST /api/federation/{remote_id}/sessions returns 503 when remote is unreachable."""
+    import json
+    from unittest.mock import MagicMock
+
+    import httpx
+
+    import muxplex.settings as settings_mod
+
+    settings_path = tmp_path / "settings.json"
+    monkeypatch.setattr(settings_mod, "SETTINGS_PATH", settings_path)
+
+    settings_path.write_text(
+        json.dumps(
+            {
+                "remote_instances": [
+                    {
+                        "url": "http://remote-host:8088",
+                        "key": "secret-key-123",
+                        "name": "remote-host",
+                    }
+                ],
+            }
+        )
+    )
+
+    async def mock_post(url, **kwargs):
+        raise httpx.ConnectError("Connection refused")
+
+    mock_fed_client = MagicMock()
+    mock_fed_client.post = mock_post
+    monkeypatch.setattr(client.app.state, "federation_client", mock_fed_client)
+
+    response = client.post("/api/federation/0/sessions", json={"name": "test"})
+    assert response.status_code == 503
+
+
+def test_federation_create_session_returns_502_when_remote_returns_error(
+    client, monkeypatch, tmp_path
+):
+    """POST /api/federation/{remote_id}/sessions returns 502 when remote returns HTTP error."""
+    import json
+    from unittest.mock import MagicMock
+
+    import httpx
+
+    import muxplex.settings as settings_mod
+
+    settings_path = tmp_path / "settings.json"
+    monkeypatch.setattr(settings_mod, "SETTINGS_PATH", settings_path)
+
+    settings_path.write_text(
+        json.dumps(
+            {
+                "remote_instances": [
+                    {
+                        "url": "http://remote-host:8088",
+                        "key": "wrong-key",
+                        "name": "remote-host",
+                    }
+                ],
+            }
+        )
+    )
+
+    async def mock_post(url, **kwargs):
+        mock_resp = MagicMock()
+        mock_resp.status_code = 422
+        mock_resp.raise_for_status.side_effect = httpx.HTTPStatusError(
+            "422", request=MagicMock(), response=mock_resp
+        )
+        return mock_resp
+
+    mock_fed_client = MagicMock()
+    mock_fed_client.post = mock_post
+    monkeypatch.setattr(client.app.state, "federation_client", mock_fed_client)
+
+    response = client.post("/api/federation/0/sessions", json={"name": "test"})
+    assert response.status_code == 502
+```
+
+**Step 2: Run tests to verify they fail**
+
+Run: `cd muxplex && python -m pytest muxplex/tests/test_api.py::test_federation_create_session_proxies_to_remote muxplex/tests/test_api.py::test_federation_create_session_returns_404_for_invalid_remote muxplex/tests/test_api.py::test_federation_create_session_returns_503_when_remote_unreachable muxplex/tests/test_api.py::test_federation_create_session_returns_502_when_remote_returns_error -v`
+
+Expected: All 4 FAIL (404/405 — no route exists yet)
+
+**Step 3: Implement the endpoint**
+
+In `muxplex/main.py`, insert the following **before** the comment block `# ---------------------------------------------------------------------------` / `# Static file serving` (line 1087). That is, insert it between the end of `federation_bell_clear` (line 1085) and the static mount comment (line 1087):
+
+```python
+
+
+@app.post("/api/federation/{remote_id}/sessions")
+async def federation_create_session(
+    remote_id: int, payload: CreateSessionPayload, request: Request
+) -> dict:
+    """Proxy a session-create POST to a remote instance.
+
+    Looks up the remote by integer index into ``remote_instances`` in settings,
+    sends ``POST {remote_url}/api/sessions`` with a Bearer auth header and
+    the JSON body ``{name: ...}``, and returns the remote's JSON response.
+
+    Raises HTTP 404 if ``remote_id`` is not a valid integer index.
+    """
+    settings = load_settings()
+    remotes = settings.get("remote_instances", [])
+    if remote_id < 0 or remote_id >= len(remotes):
+        raise HTTPException(
+            status_code=404,
+            detail=f"Remote instance '{remote_id}' not found",
+        )
+    remote = remotes[remote_id]
+
+    remote_url: str = remote.get("url", "").rstrip("/")
+    remote_key: str = remote.get("key", "")
+    url = f"{remote_url}/api/sessions"
+
+    http_client: httpx.AsyncClient = request.app.state.federation_client
+    try:
+        resp = await http_client.post(
+            url,
+            headers={"Authorization": f"Bearer {remote_key}"},
+            json={"name": payload.name},
+        )
+        resp.raise_for_status()
+        return resp.json()
+    except httpx.HTTPStatusError as exc:
+        raise HTTPException(
+            status_code=502,
+            detail=f"Remote returned {exc.response.status_code}",
+        )
+    except Exception as exc:
+        _log.warning(
+            "federation_create_session: remote %s unreachable: %s", remote_url, exc
+        )
+        raise HTTPException(
+            status_code=503,
+            detail=f"Remote unreachable: {remote_url}",
+        )
+```
+
+**Step 4: Run tests to verify they pass**
+
+Run: `cd muxplex && python -m pytest muxplex/tests/test_api.py::test_federation_create_session_proxies_to_remote muxplex/tests/test_api.py::test_federation_create_session_returns_404_for_invalid_remote muxplex/tests/test_api.py::test_federation_create_session_returns_503_when_remote_unreachable muxplex/tests/test_api.py::test_federation_create_session_returns_502_when_remote_returns_error -v`
+
+Expected: All 4 PASS
+
+**Step 5: Commit**
+
+`cd muxplex && git add muxplex/main.py muxplex/tests/test_api.py && git commit -m "feat: add POST /api/federation/{remote_id}/sessions proxy endpoint"`
+
+---
+
+### Task 2: Frontend — Add device `<select>` helper and wire into `showNewSessionInput()`
+
+**Files:**
+- Modify: `muxplex/frontend/app.js:1902-1945` (the `_createSessionInput` and `showNewSessionInput` functions)
+- Test: `muxplex/frontend/tests/test_app.mjs` (append)
+
+**Step 1: Write the failing tests**
+
+Append these tests to the end of `muxplex/frontend/tests/test_app.mjs`:
+
+```javascript
+
+// --- Device selector in new session dialog ---
+
+test('_createDeviceSelect builds a <select> with Local + remote options', () => {
+  const source = fs.readFileSync(new URL('../app.js', import.meta.url), 'utf8');
+  assert.ok(source.includes('function _createDeviceSelect('), '_createDeviceSelect helper must exist');
+  const start = source.indexOf('function _createDeviceSelect(');
+  const snippet = source.slice(start, start + 800);
+  assert.ok(snippet.includes("createElement('select')"), '_createDeviceSelect must create a <select> element');
+  assert.ok(snippet.includes('remote_instances'), '_createDeviceSelect must read remote_instances from settings');
+  assert.ok(snippet.includes('device_name'), '_createDeviceSelect must use device_name for local option label');
+});
+
+test('showNewSessionInput creates device select when multi_device_enabled with remotes', () => {
+  const source = fs.readFileSync(new URL('../app.js', import.meta.url), 'utf8');
+  const start = source.indexOf('function showNewSessionInput(');
+  assert.ok(start !== -1, 'showNewSessionInput must exist');
+  const snippet = source.slice(start, start + 1200);
+  assert.ok(snippet.includes('_createDeviceSelect'), 'showNewSessionInput must call _createDeviceSelect');
+});
+
+test('showNewSessionInput passes remoteId from device select to createNewSession', () => {
+  const source = fs.readFileSync(new URL('../app.js', import.meta.url), 'utf8');
+  const start = source.indexOf('function showNewSessionInput(');
+  const snippet = source.slice(start, start + 1200);
+  // The Enter handler must read select.value and pass it to createNewSession
+  assert.ok(
+    snippet.includes('createNewSession(name,') || snippet.includes('createNewSession(name ,'),
+    'showNewSessionInput must pass a second argument (remoteId) to createNewSession'
+  );
+});
+```
+
+**Step 2: Run tests to verify they fail**
+
+Run: `cd muxplex && node --test muxplex/frontend/tests/test_app.mjs 2>&1 | tail -20`
+
+Expected: The 3 new tests FAIL
+
+**Step 3: Implement the device select helper and update `showNewSessionInput`**
+
+In `muxplex/frontend/app.js`, insert a new `_createDeviceSelect()` function immediately **after** the `_createSessionInput()` function (after line 1910, before line 1912):
+
+```javascript
+
+/**
+ * Create a device <select> dropdown for choosing where to create a new session.
+ * Only returns an element when multi_device_enabled is true and remote_instances
+ * has at least one entry. Returns null otherwise (single-device setups).
+ * @returns {HTMLSelectElement|null}
+ */
+function _createDeviceSelect() {
+  var ss = _serverSettings || {};
+  if (!ss.multi_device_enabled) return null;
+  var remotes = ss.remote_instances || [];
+  if (remotes.length === 0) return null;
+
+  var select = document.createElement('select');
+  select.className = 'new-session-device-select';
+
+  // Local option — use device_name from settings, fallback to 'Local'
+  var localOpt = document.createElement('option');
+  localOpt.value = '';
+  localOpt.textContent = ss.device_name || 'Local';
+  select.appendChild(localOpt);
+
+  // Remote options — one per configured remote instance
+  for (var i = 0; i < remotes.length; i++) {
+    var opt = document.createElement('option');
+    opt.value = String(i);
+    opt.textContent = remotes[i].name || remotes[i].url || ('Remote ' + i);
+    select.appendChild(opt);
+  }
+
+  // Pre-select based on current device filter (if user is viewing a specific device)
+  if (_activeFilterDevice !== 'all') {
+    // Find the remote whose name matches the active filter
+    for (var j = 0; j < remotes.length; j++) {
+      if (remotes[j].name === _activeFilterDevice) {
+        select.value = String(j);
+        break;
+      }
+    }
+    // If active filter matches local device name, leave default (empty string)
+  }
+
+  return select;
+}
+```
+
+Then update `showNewSessionInput()` to create and insert the device select. Replace the entire `showNewSessionInput` function (lines 1920–1945) with:
+
+```javascript
+function showNewSessionInput(btn) {
+  var select = _createDeviceSelect();
+  var input = _createSessionInput();
+
+  function cleanup() {
+    if (select && select.parentNode) select.parentNode.removeChild(select);
+    if (input.parentNode) input.parentNode.removeChild(input);
+    btn.style.display = '';
+  }
+
+  input.addEventListener('keydown', function (e) {
+    if (e.key === 'Enter') {
+      var name = input.value.trim();
+      var remoteId = select ? select.value : '';
+      cleanup();
+      if (name) createNewSession(name, remoteId);
+    } else if (e.key === 'Escape') {
+      cleanup();
+    }
+  });
+
+  input.addEventListener('blur', function () {
+    setTimeout(cleanup, 150);
+  });
+
+  btn.style.display = 'none';
+  if (select) btn.parentNode.insertBefore(select, btn);
+  btn.parentNode.insertBefore(input, btn);
+  input.focus();
+}
+```
+
+**Step 4: Run tests to verify they pass**
+
+Run: `cd muxplex && node --test muxplex/frontend/tests/test_app.mjs 2>&1 | tail -20`
+
+Expected: All new tests PASS, no regressions
+
+**Step 5: Commit**
+
+`cd muxplex && git add muxplex/frontend/app.js muxplex/frontend/tests/test_app.mjs && git commit -m "feat: add device select dropdown to showNewSessionInput"`
+
+---
+
+### Task 3: Frontend — Add device `<select>` to `showFabSessionInput()` (mobile)
+
+**Files:**
+- Modify: `muxplex/frontend/app.js:1953-1987` (the `showFabSessionInput` function)
+- Test: `muxplex/frontend/tests/test_app.mjs` (append)
+
+**Step 1: Write the failing test**
+
+Append to `muxplex/frontend/tests/test_app.mjs`:
+
+```javascript
+
+test('showFabSessionInput creates device select when multi_device_enabled with remotes', () => {
+  const source = fs.readFileSync(new URL('../app.js', import.meta.url), 'utf8');
+  const start = source.indexOf('function showFabSessionInput(');
+  assert.ok(start !== -1, 'showFabSessionInput must exist');
+  const snippet = source.slice(start, start + 1200);
+  assert.ok(snippet.includes('_createDeviceSelect'), 'showFabSessionInput must call _createDeviceSelect');
+  assert.ok(
+    snippet.includes('createNewSession(name,') || snippet.includes('createNewSession(name ,'),
+    'showFabSessionInput must pass remoteId to createNewSession'
+  );
+});
+```
+
+**Step 2: Run test to verify it fails**
+
+Run: `cd muxplex && node --test muxplex/frontend/tests/test_app.mjs 2>&1 | tail -10`
+
+Expected: New test FAILS
+
+**Step 3: Update `showFabSessionInput` to include device select**
+
+Replace the entire `showFabSessionInput` function (lines 1953–1987, note: line numbers may have shifted after Task 2 edits) with:
+
+```javascript
+function showFabSessionInput() {
+  if (document.querySelector('.fab-input-overlay')) return;
+
+  var fab = $('new-session-fab');
+
+  var overlay = document.createElement('div');
+  overlay.className = 'fab-input-overlay';
+
+  var select = _createDeviceSelect();
+  var input = _createSessionInput();
+
+  if (select) overlay.appendChild(select);
+  overlay.appendChild(input);
+
+  function cleanup() {
+    if (overlay.parentNode) overlay.parentNode.removeChild(overlay);
+    if (fab) fab.style.display = '';
+  }
+
+  input.addEventListener('keydown', function(e) {
+    if (e.key === 'Enter') {
+      var name = input.value.trim();
+      var remoteId = select ? select.value : '';
+      cleanup();
+      if (name) createNewSession(name, remoteId);
+    } else if (e.key === 'Escape') {
+      cleanup();
+    }
+  });
+
+  input.addEventListener('blur', function() {
+    setTimeout(cleanup, 150);
+  });
+
+  if (fab) fab.style.display = 'none';
+  document.body.appendChild(overlay);
+  input.focus();
+}
+```
+
+**Step 4: Run tests to verify they pass**
+
+Run: `cd muxplex && node --test muxplex/frontend/tests/test_app.mjs 2>&1 | tail -10`
+
+Expected: New test PASSES, no regressions
+
+**Step 5: Commit**
+
+`cd muxplex && git add muxplex/frontend/app.js muxplex/frontend/tests/test_app.mjs && git commit -m "feat: add device select to mobile FAB session input"`
+
+---
+
+### Task 4: Frontend — Update `createNewSession()` to route through federation proxy
+
+**Files:**
+- Modify: `muxplex/frontend/app.js:1998-2054` (the `createNewSession` function)
+- Test: `muxplex/frontend/tests/test_app.mjs` (append)
+
+**Step 1: Write the failing tests**
+
+Append to `muxplex/frontend/tests/test_app.mjs`:
+
+```javascript
+
+test('createNewSession accepts remoteId parameter and routes to federation endpoint', () => {
+  const source = fs.readFileSync(new URL('../app.js', import.meta.url), 'utf8');
+  const start = source.indexOf('async function createNewSession(');
+  assert.ok(start !== -1, 'createNewSession must exist');
+  const snippet = source.slice(start, start + 3000);
+
+  // Function signature must accept remoteId
+  assert.ok(
+    snippet.startsWith("async function createNewSession(name, remoteId") ||
+    snippet.startsWith("async function createNewSession(name,remoteId"),
+    'createNewSession must accept a remoteId parameter'
+  );
+
+  // Must route to federation endpoint when remoteId is set
+  assert.ok(
+    snippet.includes('/api/federation/'),
+    'createNewSession must POST to /api/federation/ endpoint when remoteId is set'
+  );
+
+  // Must still support local endpoint
+  assert.ok(
+    snippet.includes("'/api/sessions'"),
+    'createNewSession must still POST to /api/sessions for local sessions'
+  );
+});
+
+test('createNewSession passes remoteId through to openSession for auto-open', () => {
+  const source = fs.readFileSync(new URL('../app.js', import.meta.url), 'utf8');
+  const start = source.indexOf('async function createNewSession(');
+  const snippet = source.slice(start, start + 3000);
+
+  // Must pass remoteId to openSession in the poll callback
+  assert.ok(
+    snippet.includes('openSession(sessionName, { remoteId') ||
+    snippet.includes('openSession(sessionName, {remoteId'),
+    'createNewSession must pass { remoteId } opts to openSession when auto-opening'
+  );
+});
+
+test('createNewSession matches remote sessions by sessionKey in poll loop', () => {
+  const source = fs.readFileSync(new URL('../app.js', import.meta.url), 'utf8');
+  const start = source.indexOf('async function createNewSession(');
+  const snippet = source.slice(start, start + 3000);
+
+  // When remoteId is set, must match by sessionKey (remoteId:name) not just name
+  assert.ok(
+    snippet.includes('sessionKey'),
+    'createNewSession must match remote sessions by sessionKey in the polling loop'
+  );
+});
+```
+
+**Step 2: Run tests to verify they fail**
+
+Run: `cd muxplex && node --test muxplex/frontend/tests/test_app.mjs 2>&1 | tail -20`
+
+Expected: The 3 new tests FAIL
+
+**Step 3: Update `createNewSession` to accept and use `remoteId`**
+
+Replace the entire `createNewSession` function (from `async function createNewSession(name)` through its closing `}`) with:
+
+```javascript
+async function createNewSession(name, remoteId) {
+  remoteId = remoteId || '';
+  try {
+    var endpoint = remoteId
+      ? '/api/federation/' + encodeURIComponent(remoteId) + '/sessions'
+      : '/api/sessions';
+    const res = await api('POST', endpoint, { name });
+    const data = await res.json();
+    const sessionName = data.name || name;
+    showToast('Creating session \'' + sessionName + '\'\u2026');
+
+    // Inject a loading placeholder tile so the user sees feedback immediately
+    var loadingTile = null;
+    var grid = document.getElementById('session-grid');
+    if (grid) {
+      loadingTile = document.createElement('div');
+      loadingTile.className = 'session-tile tile--loading';
+      loadingTile.id = 'loading-tile-' + sessionName;
+      loadingTile.innerHTML =
+        '<div class="tile-header"><span class="tile-name">' + escapeHtml(sessionName) + '</span>' +
+        '<span class="tile-meta">Creating...</span></div>' +
+        '<div class="tile-body"><pre class="loading-pulse"></pre></div>';
+      grid.appendChild(loadingTile);
+    }
+
+    function removeLoadingTile() {
+      var tile = document.getElementById('loading-tile-' + sessionName);
+      if (tile) tile.remove();
+    }
+
+    const ss = _serverSettings || {};
+    if (ss.auto_open_created === false) {
+      // Auto-open disabled — just do one refresh
+      await pollSessions();
+      removeLoadingTile();
+      return;
+    }
+
+    // For remote sessions, the sessionKey is "remoteId:name"
+    var expectedKey = remoteId ? (remoteId + ':' + sessionName) : sessionName;
+
+    // Poll until the session appears in _currentSessions (max 30s, every 2s)
+    var attempts = 0;
+    var maxAttempts = 15;
+    var pollForSession = setInterval(async function() {
+      attempts++;
+      await pollSessions();
+      var found = _currentSessions && _currentSessions.find(function(s) {
+        return (s.sessionKey || s.name) === expectedKey;
+      });
+      if (found) {
+        clearInterval(pollForSession);
+        removeLoadingTile();
+        showToast('Session \'' + sessionName + '\' ready');
+        openSession(sessionName, { remoteId: remoteId });
+      } else if (attempts >= maxAttempts) {
+        clearInterval(pollForSession);
+        removeLoadingTile();
+        showToast('Session \'' + sessionName + '\' is taking longer than expected');
+      }
+    }, 2000);
+  } catch (err) {
+    showToast(err.message || 'Failed to create session');
+  }
+}
+```
+
+**Step 4: Run tests to verify they pass**
+
+Run: `cd muxplex && node --test muxplex/frontend/tests/test_app.mjs 2>&1 | tail -20`
+
+Expected: All new tests PASS, no regressions
+
+**Step 5: Commit**
+
+`cd muxplex && git add muxplex/frontend/app.js muxplex/frontend/tests/test_app.mjs && git commit -m "feat: route createNewSession through federation proxy for remote devices"`
+
+---
+
+### Task 5: CSS — Style the device select dropdown
+
+**Files:**
+- Modify: `muxplex/frontend/style.css` (after `.new-session-input::placeholder` rule, ~line 1523)
+- Test: `muxplex/frontend/tests/test_app.mjs` (append)
+
+**Step 1: Write the failing test**
+
+Append to `muxplex/frontend/tests/test_app.mjs`:
+
+```javascript
+
+test('CSS has new-session-device-select styling', () => {
+  const source = fs.readFileSync(new URL('../style.css', import.meta.url), 'utf8');
+  assert.ok(source.includes('.new-session-device-select'), 'style.css must have .new-session-device-select rule');
+});
+```
+
+**Step 2: Run test to verify it fails**
+
+Run: `cd muxplex && node --test muxplex/frontend/tests/test_app.mjs 2>&1 | tail -5`
+
+Expected: FAIL — CSS rule doesn't exist yet
+
+**Step 3: Add the CSS rule**
+
+In `muxplex/frontend/style.css`, insert immediately after the `.new-session-input::placeholder` rule (after line 1523):
+
+```css
+
+.new-session-device-select {
+  background: var(--bg);
+  border: 1px solid var(--border);
+  border-radius: 4px;
+  font-size: 12px;
+  font-family: var(--font-ui);
+  padding: 4px 6px;
+  color: var(--text);
+  outline: none;
+  margin-right: 6px;
+  vertical-align: middle;
+}
+
+.fab-input-overlay .new-session-device-select {
+  display: block;
+  width: 100%;
+  margin-bottom: 6px;
+  margin-right: 0;
+}
+```
+
+**Step 4: Run tests to verify they pass**
+
+Run: `cd muxplex && node --test muxplex/frontend/tests/test_app.mjs 2>&1 | tail -5`
+
+Expected: PASS
+
+**Step 5: Commit**
+
+`cd muxplex && git add muxplex/frontend/style.css muxplex/frontend/tests/test_app.mjs && git commit -m "feat: style device select dropdown for new session dialog"`
+
+---
+
+### Task 6: Full test suite verification
+
+**Files:**
+- No new files — verification only
+
+**Step 1: Run the full backend test suite**
+
+Run: `cd muxplex && python -m pytest muxplex/tests/test_api.py -v --tb=short 2>&1 | tail -30`
+
+Expected: All tests PASS (including the 4 new federation_create_session tests)
+
+**Step 2: Run the full frontend test suite**
+
+Run: `cd muxplex && node --test muxplex/frontend/tests/test_app.mjs 2>&1 | tail -30`
+
+Expected: All tests PASS (including the 5 new device-selector tests)
+
+**Step 3: Run Python quality checks**
+
+Run: `cd muxplex && python -m ruff check muxplex/main.py muxplex/tests/test_api.py && python -m ruff format --check muxplex/main.py muxplex/tests/test_api.py`
+
+Expected: No issues
+
+**Step 4: Commit (if any lint/format fixes were needed)**
+
+`cd muxplex && git add -A && git commit -m "chore: lint/format fixes" --allow-empty`