diff --git a/packages/app/package.json b/packages/app/package.json
index 99eb13b26..dd78c4602 100644
--- a/packages/app/package.json
+++ b/packages/app/package.json
@@ -20,6 +20,7 @@
     "test:session-switch": "node scripts/session-switch.mjs",
     "test:fs-engine": "node scripts/fs-engine.mjs",
     "test:local-file-path": "node scripts/local-file-path.mjs",
+    "test:sidebar-sessions-guard": "node scripts/sidebar-sessions-guard.mjs",
     "test:browser-entry": "node scripts/browser-entry.mjs",
     "test:e2e": "pnpm test:local-file-path && node scripts/e2e.mjs && node scripts/session-switch.mjs && node scripts/fs-engine.mjs && node scripts/browser-entry.mjs",
     "bump:patch": "node scripts/bump-version.mjs patch",
diff --git a/packages/app/scripts/sidebar-sessions-guard.mjs b/packages/app/scripts/sidebar-sessions-guard.mjs
new file mode 100644
index 000000000..d74133b3d
--- /dev/null
+++ b/packages/app/scripts/sidebar-sessions-guard.mjs
@@ -0,0 +1,105 @@
+/**
+ * Test script for sidebar session guard logic.
+ *
+ * Verifies the fix for Issue #750: Session loss when switching workspaces.
+ *
+ * Run with: node packages/app/scripts/sidebar-sessions-guard.mjs
+ */
+
+import assert from "node:assert/strict";
+
+/**
+ * Determines whether the sidebar sessions should be updated.
+ * This is the pure function extracted from app.tsx for testability.
+ *
+ * @param {Array<{id: string}>} currentSessions - Current sidebar sessions
+ * @param {Array<{id: string}>} newSessions - New sessions from server
+ * @returns {boolean} true if should update, false to preserve existing
+ */
+function shouldUpdateSidebarSessions(currentSessions, newSessions) {
+  // Don't clear existing sidebar sessions if the session store is empty.
+  // This can happen during worker restart/reconnect when the server hasn't fully loaded
+  // its session database yet. Preserving existing sessions prevents the sidebar from
+  // flickering empty during the reconnection window.
+  if (currentSessions.length > 0 && newSessions.length === 0) {
+    return false;
+  }
+  return true;
+}
+
+const results = {
+  ok: true,
+  tests: [],
+};
+
+function test(name, fn) {
+  results.tests.push({ name, status: "running" });
+  const idx = results.tests.length - 1;
+
+  try {
+    fn();
+    results.tests[idx] = { name, status: "ok" };
+  } catch (e) {
+    results.ok = false;
+    results.tests[idx] = {
+      name,
+      status: "error",
+      error: e instanceof Error ? e.message : String(e),
+    };
+  }
+}
+
+// Test 1: Empty current, empty new -> should update (no-op, but allowed)
+test("empty current, empty new -> should update", () => {
+  const current = [];
+  const newSessions = [];
+  assert.equal(shouldUpdateSidebarSessions(current, newSessions), true);
+});
+
+// Test 2: Empty current, non-empty new -> should update (normal load)
+test("empty current, non-empty new -> should update", () => {
+  const current = [];
+  const newSessions = [{ id: "session-1" }];
+  assert.equal(shouldUpdateSidebarSessions(current, newSessions), true);
+});
+
+// Test 3: Non-empty current, empty new -> should NOT update (guard triggers)
+test("non-empty current, empty new -> should NOT update", () => {
+  const current = [{ id: "session-1" }];
+  const newSessions = [];
+  assert.equal(shouldUpdateSidebarSessions(current, newSessions), false);
+});
+
+// Test 4: Non-empty current, non-empty new -> should update (normal refresh)
+test("non-empty current, non-empty new -> should update", () => {
+  const current = [{ id: "session-1" }];
+  const newSessions = [{ id: "session-1" }, { id: "session-2" }];
+  assert.equal(shouldUpdateSidebarSessions(current, newSessions), true);
+});
+
+// Test 5: Multiple current, empty new -> should NOT update (guard triggers)
+test("multiple current, empty new -> should NOT update", () => {
+  const current = [{ id: "session-1" }, { id: "session-2" }, { id: "session-3" }];
+  const newSessions = [];
+  assert.equal(shouldUpdateSidebarSessions(current, newSessions), false);
+});
+
+// Test 6: Multiple current, fewer new -> should update (deletion allowed)
+test("multiple current, fewer new -> should update", () => {
+  const current = [{ id: "session-1" }, { id: "session-2" }];
+  const newSessions = [{ id: "session-1" }];
+  assert.equal(shouldUpdateSidebarSessions(current, newSessions), true);
+});
+
+// Test 7: Single current, empty new -> should NOT update
+test("single current, empty new -> should NOT update", () => {
+  const current = [{ id: "only-session" }];
+  const newSessions = [];
+  assert.equal(shouldUpdateSidebarSessions(current, newSessions), false);
+});
+
+console.log(JSON.stringify(results, null, 2));
+
+if (!results.ok) {
+  process.exitCode = 1;
+}
diff --git a/packages/app/src/app/app.tsx b/packages/app/src/app/app.tsx
index b08430f4a..af20492b6 100644
--- a/packages/app/src/app/app.tsx
+++ b/packages/app/src/app/app.tsx
@@ -50,6 +50,7 @@ import {
   listCommands as listCommandsTyped,
 } from "./lib/opencode-session";
 import { clearPerfLogs, finishPerf, perfNow, recordPerfLog } from "./lib/perf-log";
+import { shouldUpdateSidebarSessions } from "./lib/sidebar-sessions-guard";
 import {
   AUTO_COMPACT_CONTEXT_PREF_KEY,
   DEFAULT_MODEL,
@@ -2676,6 +2677,16 @@ export default function App() {
         ? allSessions.filter((session) => normalizeDirectoryPath(session.directory) === activeWorkspaceRoot)
         : allSessions;
       const sorted = sortSessionsByActivity(scopedSessions);
+
+      // Don't clear existing sidebar sessions if the session store is empty for this workspace.
+      // This can happen during worker restart/reconnect when the server hasn't fully loaded
+      // its session database yet. Preserving existing sessions prevents the sidebar from
+      // flickering empty during the reconnection window.
+      const currentSidebarSessions = sidebarSessionsByWorkspaceId()[wsId] || [];
+      if (!shouldUpdateSidebarSessions(currentSidebarSessions, sorted)) {
+        return;
+      }
+
       setSidebarSessionsByWorkspaceId((prev) => ({
         ...prev,
         [wsId]: sorted.map((s) => ({
diff --git a/packages/app/src/app/lib/sidebar-sessions-guard.ts b/packages/app/src/app/lib/sidebar-sessions-guard.ts
new file mode 100644
index 000000000..31d41befd
--- /dev/null
+++ b/packages/app/src/app/lib/sidebar-sessions-guard.ts
@@ -0,0 +1,47 @@
+/**
+ * Determines whether the sidebar sessions should be updated based on the current
+ * and new session lists.
+ *
+ * This guard prevents the sidebar from being cleared when the server temporarily
+ * returns an empty list (e.g., during worker restart/reconnect before the session
+ * database is fully loaded).
+ *
+ * @param currentSessions - The sessions currently displayed in the sidebar
+ * @param newSessions - The new sessions from the server
+ * @returns true if the sidebar should be updated, false to preserve existing sessions
+ */
+export function shouldUpdateSidebarSessions(
+  currentSessions: Array<{ id: string }>,
+  newSessions: Array<{ id: string }>,
+): boolean {
+  // Don't clear existing sidebar sessions if the session store is empty.
+  // This can happen during worker restart/reconnect when the server hasn't fully loaded
+  // its session database yet. Preserving existing sessions prevents the sidebar from
+  // flickering empty during the reconnection window.
+  if (currentSessions.length > 0 && newSessions.length === 0) {
+    return false;
+  }
+
+  return true;
+}
+
+/**
+ * Filters sessions to only those belonging to a specific workspace root directory.
+ *
+ * @param sessions - All sessions
+ * @param workspaceRoot - The normalized workspace root directory
+ * @param normalizeDir - Function to normalize directory paths for comparison
+ * @returns Sessions scoped to the workspace
+ */
+export function scopeSessionsToWorkspace<T extends { directory: string }>(
+  sessions: T[],
+  workspaceRoot: string | null,
+  normalizeDir: (dir: string) => string,
+): T[] {
+  if (!workspaceRoot) {
+    return sessions;
+  }
+  return sessions.filter(
+    (session) => normalizeDir(session.directory) === workspaceRoot,
+  );
+}