diff --git a/src/components/ProjectSidebar.tsx b/src/components/ProjectSidebar.tsx
index 3c59a0a21..9f69d8b80 100644
--- a/src/components/ProjectSidebar.tsx
+++ b/src/components/ProjectSidebar.tsx
@@ -12,6 +12,7 @@ import { useDrag, useDrop, useDragLayer } from "react-dnd";
 import { sortProjectsByOrder, reorderProjects, normalizeOrder } from "@/utils/projectOrdering";
 import { matchesKeybind, formatKeybind, KEYBINDS } from "@/utils/ui/keybinds";
 import { abbreviatePath } from "@/utils/ui/pathAbbreviation";
+import { formatRelativeTime } from "@/utils/ui/dateTime";
 import { TooltipWrapper, Tooltip } from "./Tooltip";
 import { StatusIndicator } from "./StatusIndicator";
 // Removed: import { getModelName } from "@/utils/ai/models";
@@ -1070,6 +1071,8 @@ const ProjectSidebar: React.FC<ProjectSidebarProps> = ({
                                           "Assistant is responding"
                                         ) : isUnread ? (
                                           "Unread messages"
+                                        ) : workspaceState.recencyTimestamp ? (
+                                          `Idle • Last used ${formatRelativeTime(workspaceState.recencyTimestamp)}`
                                         ) : (
                                           "Idle"
                                         )
diff --git a/src/utils/ui/dateTime.test.ts b/src/utils/ui/dateTime.test.ts
new file mode 100644
index 000000000..be9b28be7
--- /dev/null
+++ b/src/utils/ui/dateTime.test.ts
@@ -0,0 +1,56 @@
+import { describe, test, expect } from "bun:test";
+import { formatRelativeTime } from "./dateTime";
+
+describe("formatRelativeTime", () => {
+  test("should return 'just now' for very recent timestamps", () => {
+    const now = Date.now();
+    expect(formatRelativeTime(now)).toBe("just now");
+    expect(formatRelativeTime(now - 30 * 1000)).toBe("just now"); // 30 seconds ago
+  });
+
+  test("should return 'just now' for future timestamps", () => {
+    const future = Date.now() + 5000;
+    expect(formatRelativeTime(future)).toBe("just now");
+  });
+
+  test("should format minutes correctly", () => {
+    const now = Date.now();
+    expect(formatRelativeTime(now - 1 * 60 * 1000)).toBe("1 minute ago");
+    expect(formatRelativeTime(now - 5 * 60 * 1000)).toBe("5 minutes ago");
+    expect(formatRelativeTime(now - 59 * 60 * 1000)).toBe("59 minutes ago");
+  });
+
+  test("should format hours correctly", () => {
+    const now = Date.now();
+    expect(formatRelativeTime(now - 1 * 60 * 60 * 1000)).toBe("1 hour ago");
+    expect(formatRelativeTime(now - 3 * 60 * 60 * 1000)).toBe("3 hours ago");
+    expect(formatRelativeTime(now - 23 * 60 * 60 * 1000)).toBe("23 hours ago");
+  });
+
+  test("should format days correctly", () => {
+    const now = Date.now();
+    expect(formatRelativeTime(now - 1 * 24 * 60 * 60 * 1000)).toBe("1 day ago");
+    expect(formatRelativeTime(now - 3 * 24 * 60 * 60 * 1000)).toBe("3 days ago");
+    expect(formatRelativeTime(now - 6 * 24 * 60 * 60 * 1000)).toBe("6 days ago");
+  });
+
+  test("should format weeks correctly", () => {
+    const now = Date.now();
+    expect(formatRelativeTime(now - 7 * 24 * 60 * 60 * 1000)).toBe("1 week ago");
+    expect(formatRelativeTime(now - 14 * 24 * 60 * 60 * 1000)).toBe("2 weeks ago");
+    expect(formatRelativeTime(now - 27 * 24 * 60 * 60 * 1000)).toBe("3 weeks ago");
+  });
+
+  test("should format months correctly", () => {
+    const now = Date.now();
+    expect(formatRelativeTime(now - 30 * 24 * 60 * 60 * 1000)).toBe("1 month ago");
+    expect(formatRelativeTime(now - 60 * 24 * 60 * 60 * 1000)).toBe("2 months ago");
+    expect(formatRelativeTime(now - 180 * 24 * 60 * 60 * 1000)).toBe("6 months ago");
+  });
+
+  test("should format years correctly", () => {
+    const now = Date.now();
+    expect(formatRelativeTime(now - 365 * 24 * 60 * 60 * 1000)).toBe("1 year ago");
+    expect(formatRelativeTime(now - 730 * 24 * 60 * 60 * 1000)).toBe("2 years ago");
+  });
+});
diff --git a/src/utils/ui/dateTime.ts b/src/utils/ui/dateTime.ts
index 5efa5bf9b..4502a4e7d 100644
--- a/src/utils/ui/dateTime.ts
+++ b/src/utils/ui/dateTime.ts
@@ -54,3 +54,44 @@ export function formatFullTimestamp(timestamp: number): string {
     hour12: true,
   });
 }
+
+/**
+ * Formats a Unix timestamp (milliseconds) into a human-readable relative time string.
+ * Examples: "2 minutes ago", "3 hours ago", "2 days ago", "3 weeks ago"
+ *
+ * @param timestamp Unix timestamp in milliseconds
+ * @returns Humanized relative time string
+ */
+export function formatRelativeTime(timestamp: number): string {
+  const now = Date.now();
+  const diffMs = now - timestamp;
+
+  // Handle future timestamps
+  if (diffMs < 0) {
+    return "just now";
+  }
+
+  const seconds = Math.floor(diffMs / 1000);
+  const minutes = Math.floor(seconds / 60);
+  const hours = Math.floor(minutes / 60);
+  const days = Math.floor(hours / 24);
+  const weeks = Math.floor(days / 7);
+  const months = Math.floor(days / 30);
+  const years = Math.floor(days / 365);
+
+  if (seconds < 60) {
+    return "just now";
+  } else if (minutes < 60) {
+    return minutes === 1 ? "1 minute ago" : `${minutes} minutes ago`;
+  } else if (hours < 24) {
+    return hours === 1 ? "1 hour ago" : `${hours} hours ago`;
+  } else if (days < 7) {
+    return days === 1 ? "1 day ago" : `${days} days ago`;
+  } else if (weeks < 4) {
+    return weeks === 1 ? "1 week ago" : `${weeks} weeks ago`;
+  } else if (months < 12) {
+    return months === 1 ? "1 month ago" : `${months} months ago`;
+  } else {
+    return years === 1 ? "1 year ago" : `${years} years ago`;
+  }
+}