diff --git a/package-lock.json b/package-lock.json
index 6aad70d75..1652b764a 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -35400,6 +35400,7 @@
         "dotenv": "^17.2.3",
         "express": "^4.18.2",
         "express-basic-auth": "^1.2.1",
+        "fast-deep-equal": "^3.1.3",
         "fast-uri": "^3.1.0",
         "mnemonic-id": "^3.2.7",
         "node-pty": "1.2.0-beta.11",
diff --git a/packages/app/src/app/h/[serverId]/index.tsx b/packages/app/src/app/h/[serverId]/index.tsx
index 121a61fa9..75d37bcf6 100644
--- a/packages/app/src/app/h/[serverId]/index.tsx
+++ b/packages/app/src/app/h/[serverId]/index.tsx
@@ -57,11 +57,6 @@ function HostIndexRouteContent() {
       );
 
       const visibleWorkspaces = sessionWorkspaces ? Array.from(sessionWorkspaces.values()) : [];
-      visibleWorkspaces.sort((left, right) => {
-        const leftTime = left.activityAt?.getTime() ?? Number.NEGATIVE_INFINITY;
-        const rightTime = right.activityAt?.getTime() ?? Number.NEGATIVE_INFINITY;
-        return rightTime - leftTime;
-      });
 
       const primaryAgent = visibleAgents[0];
       if (primaryAgent?.cwd?.trim()) {
diff --git a/packages/app/src/hooks/use-sidebar-workspaces-list.test.ts b/packages/app/src/hooks/use-sidebar-workspaces-list.test.ts
index 88f4925c6..dd0f01ac4 100644
--- a/packages/app/src/hooks/use-sidebar-workspaces-list.test.ts
+++ b/packages/app/src/hooks/use-sidebar-workspaces-list.test.ts
@@ -15,7 +15,7 @@ function item(key: string): OrderedItem {
 }
 
 function workspace(
-  input: Pick<WorkspaceDescriptor, "id" | "projectId" | "name" | "status" | "activityAt"> &
+  input: Pick<WorkspaceDescriptor, "id" | "projectId" | "name" | "status"> &
     Partial<
       Pick<
         WorkspaceDescriptor,
@@ -32,7 +32,6 @@ function workspace(
     workspaceKind: input.workspaceKind ?? "local_checkout",
     name: input.name,
     status: input.status,
-    activityAt: input.activityAt,
     diffStat: null,
   };
 }
@@ -100,7 +99,6 @@ describe("buildSidebarProjectsFromWorkspaces", () => {
         projectId: "project-1",
         name: "feat/hard-cut",
         status: "failed",
-        activityAt: new Date("2026-01-01T00:00:00.000Z"),
       }),
     ];
 
@@ -117,21 +115,19 @@ describe("buildSidebarProjectsFromWorkspaces", () => {
     expect(projects[0]?.workspaces[0]?.statusBucket).toBe("failed");
   });
 
-  it("preserves stored project order even when activity changes", () => {
+  it("preserves stored project order even when input order differs", () => {
     const initialWorkspaces: WorkspaceDescriptor[] = [
       workspace({
         id: "/repo/b",
         projectId: "project-b",
         name: "feat/b",
         status: "running",
-        activityAt: new Date("2026-01-02T00:00:00.000Z"),
       }),
       workspace({
         id: "/repo/a",
         projectId: "project-a",
         name: "feat/a",
         status: "running",
-        activityAt: new Date("2026-01-01T00:00:00.000Z"),
       }),
     ];
 
@@ -148,19 +144,17 @@ describe("buildSidebarProjectsFromWorkspaces", () => {
     const updatedProjects = buildSidebarProjectsFromWorkspaces({
       serverId: "srv",
       workspaces: [
-        workspace({
-          id: "/repo/b",
-          projectId: "project-b",
-          name: "feat/b",
-          status: "running",
-          activityAt: new Date("2026-01-02T00:00:00.000Z"),
-        }),
         workspace({
           id: "/repo/a",
           projectId: "project-a",
           name: "feat/a",
           status: "running",
-          activityAt: new Date("2026-01-03T00:00:00.000Z"),
+        }),
+        workspace({
+          id: "/repo/b",
+          projectId: "project-b",
+          name: "feat/b",
+          status: "running",
         }),
       ],
       projectOrder: seededOrder,
@@ -168,8 +162,8 @@ describe("buildSidebarProjectsFromWorkspaces", () => {
     });
 
     expect(updatedProjects.map((project) => project.projectKey)).toEqual([
-      "project-b",
       "project-a",
+      "project-b",
     ]);
   });
 
@@ -182,21 +176,18 @@ describe("buildSidebarProjectsFromWorkspaces", () => {
           projectId: "project-c",
           name: "feat/c",
           status: "running",
-          activityAt: new Date("2026-01-04T00:00:00.000Z"),
         }),
         workspace({
           id: "/repo/b",
           projectId: "project-b",
           name: "feat/b",
           status: "running",
-          activityAt: new Date("2026-01-02T00:00:00.000Z"),
         }),
         workspace({
           id: "/repo/a",
           projectId: "project-a",
           name: "feat/a",
           status: "running",
-          activityAt: new Date("2026-01-01T00:00:00.000Z"),
         }),
       ],
       projectOrder: ["project-b", "project-a", "project-c"],
@@ -219,14 +210,12 @@ describe("buildSidebarProjectsFromWorkspaces", () => {
           projectId: "project-1",
           name: "main",
           status: "running",
-          activityAt: new Date("2026-01-02T00:00:00.000Z"),
         }),
         workspace({
           id: "/repo/feature",
           projectId: "project-1",
           name: "feature",
           status: "running",
-          activityAt: new Date("2026-01-01T00:00:00.000Z"),
         }),
       ],
       projectOrder: ["project-1"],
@@ -246,14 +235,12 @@ describe("buildSidebarProjectsFromWorkspaces", () => {
           projectId: "project-1",
           name: "main",
           status: "running",
-          activityAt: new Date("2026-01-02T00:00:00.000Z"),
         }),
         workspace({
           id: "/repo/feature",
           projectId: "project-1",
           name: "feature",
           status: "running",
-          activityAt: new Date("2026-01-03T00:00:00.000Z"),
         }),
       ],
       projectOrder: ["project-1"],
@@ -263,8 +250,8 @@ describe("buildSidebarProjectsFromWorkspaces", () => {
     });
 
     expect(projects[0]?.workspaces.map((workspace) => workspace.workspaceId)).toEqual([
-      "/repo/main",
       "/repo/feature",
+      "/repo/main",
     ]);
   });
 });
diff --git a/packages/app/src/hooks/use-sidebar-workspaces-list.ts b/packages/app/src/hooks/use-sidebar-workspaces-list.ts
index 947c83423..34049979a 100644
--- a/packages/app/src/hooks/use-sidebar-workspaces-list.ts
+++ b/packages/app/src/hooks/use-sidebar-workspaces-list.ts
@@ -7,6 +7,7 @@ import {
 import { getHostRuntimeStore } from "@/runtime/host-runtime";
 import { useSidebarOrderStore } from "@/stores/sidebar-order-store";
 import type { WorkspaceDescriptor } from "@/stores/session-store";
+import type { WorkspaceDescriptorPayload } from "@server/shared/messages";
 import { projectDisplayNameFromProjectId } from "@/utils/project-display-name";
 
 const EMPTY_ORDER: string[] = [];
@@ -21,7 +22,6 @@ export interface SidebarWorkspaceEntry {
   workspaceId: string;
   workspaceKind: WorkspaceDescriptor["workspaceKind"];
   name: string;
-  activityAt: Date | null;
   statusBucket: SidebarStateBucket;
   diffStat: { additions: number; deletions: number } | null;
 }
@@ -34,7 +34,6 @@ export interface SidebarProjectEntry {
   statusBucket: SidebarStateBucket;
   activeCount: number;
   totalWorkspaces: number;
-  latestActivityAt: Date | null;
   workspaces: SidebarWorkspaceEntry[];
 }
 
@@ -67,15 +66,6 @@ function compareWorkspaceBaseline(
   left: SidebarWorkspaceEntry,
   right: SidebarWorkspaceEntry,
 ): number {
-  if (left.activityAt && right.activityAt) {
-    const dateDelta = right.activityAt.getTime() - left.activityAt.getTime();
-    if (dateDelta !== 0) {
-      return dateDelta;
-    }
-  } else if (left.activityAt || right.activityAt) {
-    return left.activityAt ? -1 : 1;
-  }
-
   const nameDelta = left.name.localeCompare(right.name, undefined, {
     numeric: true,
     sensitivity: "base",
@@ -91,15 +81,6 @@ function compareWorkspaceBaseline(
 }
 
 function compareProjectBaseline(left: SidebarProjectEntry, right: SidebarProjectEntry): number {
-  if (left.latestActivityAt && right.latestActivityAt) {
-    const dateDelta = right.latestActivityAt.getTime() - left.latestActivityAt.getTime();
-    if (dateDelta !== 0) {
-      return dateDelta;
-    }
-  } else if (left.latestActivityAt || right.latestActivityAt) {
-    return left.latestActivityAt ? -1 : 1;
-  }
-
   return left.projectName.localeCompare(right.projectName, undefined, {
     numeric: true,
     sensitivity: "base",
@@ -126,7 +107,6 @@ export function buildSidebarProjectsFromWorkspaces(input: {
         statusBucket: "done",
         activeCount: 0,
         totalWorkspaces: 0,
-        latestActivityAt: null,
         workspaces: [],
       } satisfies SidebarProjectEntry);
 
@@ -136,7 +116,6 @@ export function buildSidebarProjectsFromWorkspaces(input: {
       workspaceId: workspace.id,
       workspaceKind: workspace.workspaceKind,
       name: workspace.name,
-      activityAt: workspace.activityAt,
       statusBucket: workspace.status,
       diffStat: workspace.diffStat,
     };
@@ -147,12 +126,6 @@ export function buildSidebarProjectsFromWorkspaces(input: {
       project.activeCount += 1;
     }
     project.statusBucket = aggregateBucket(project.statusBucket, workspace.status);
-    if (
-      !project.latestActivityAt ||
-      (workspace.activityAt && workspace.activityAt.getTime() > project.latestActivityAt.getTime())
-    ) {
-      project.latestActivityAt = workspace.activityAt;
-    }
 
     byProject.set(workspace.projectId, project);
   }
@@ -251,17 +224,7 @@ function getWorkspaceOrderScopeKey(serverId: string, projectKey: string): string
   return `${serverId.trim()}::${projectKey.trim()}`;
 }
 
-function toWorkspaceDescriptor(payload: {
-  id: string;
-  projectId: string;
-  projectDisplayName: string;
-  projectRootPath: string;
-  projectKind: WorkspaceDescriptor["projectKind"];
-  workspaceKind: WorkspaceDescriptor["workspaceKind"];
-  name: string;
-  status: WorkspaceDescriptor["status"];
-  activityAt: string | null;
-}): WorkspaceDescriptor {
+function toWorkspaceDescriptor(payload: WorkspaceDescriptorPayload): WorkspaceDescriptor {
   return normalizeWorkspaceDescriptor(payload);
 }
 
diff --git a/packages/app/src/screens/workspace/workspace-source-of-truth.test.ts b/packages/app/src/screens/workspace/workspace-source-of-truth.test.ts
index 84562c96d..b43130fc5 100644
--- a/packages/app/src/screens/workspace/workspace-source-of-truth.test.ts
+++ b/packages/app/src/screens/workspace/workspace-source-of-truth.test.ts
@@ -17,7 +17,6 @@ describe("workspace source of truth consumption", () => {
       workspaceKind: "local_checkout",
       name: "feat/workspace-sot",
       status: "running",
-      activityAt: new Date("2026-03-01T00:00:00.000Z"),
       diffStat: null,
     };
 
diff --git a/packages/app/src/stores/session-store.test.ts b/packages/app/src/stores/session-store.test.ts
index ee6d649bb..c724bdc15 100644
--- a/packages/app/src/stores/session-store.test.ts
+++ b/packages/app/src/stores/session-store.test.ts
@@ -13,7 +13,6 @@ function createWorkspace(
     workspaceKind: input.workspaceKind ?? "local_checkout",
     name: input.name ?? "main",
     status: input.status ?? "done",
-    activityAt: input.activityAt ?? null,
     diffStat: input.diffStat ?? null,
   };
 }
diff --git a/packages/app/src/stores/session-store.ts b/packages/app/src/stores/session-store.ts
index cc31a8a9f..086f11a75 100644
--- a/packages/app/src/stores/session-store.ts
+++ b/packages/app/src/stores/session-store.ts
@@ -120,14 +120,12 @@ export interface WorkspaceDescriptor {
   workspaceKind: WorkspaceDescriptorPayload["workspaceKind"];
   name: string;
   status: WorkspaceDescriptorPayload["status"];
-  activityAt: Date | null;
   diffStat: { additions: number; deletions: number } | null;
 }
 
 export function normalizeWorkspaceDescriptor(
   payload: WorkspaceDescriptorPayload,
 ): WorkspaceDescriptor {
-  const activityAt = payload.activityAt ? new Date(payload.activityAt) : null;
   return {
     id: normalizeWorkspaceIdentity(payload.id) ?? payload.id,
     projectId: payload.projectId,
@@ -137,7 +135,6 @@ export function normalizeWorkspaceDescriptor(
     workspaceKind: payload.workspaceKind,
     name: payload.name,
     status: payload.status,
-    activityAt: activityAt && !Number.isNaN(activityAt.getTime()) ? activityAt : null,
     diffStat: payload.diffStat ?? null,
   };
 }
diff --git a/packages/app/src/utils/sidebar-project-row-model.test.ts b/packages/app/src/utils/sidebar-project-row-model.test.ts
index 9bc7538d3..b2c2a0055 100644
--- a/packages/app/src/utils/sidebar-project-row-model.test.ts
+++ b/packages/app/src/utils/sidebar-project-row-model.test.ts
@@ -15,7 +15,6 @@ function workspace(overrides: Partial<SidebarWorkspaceEntry> = {}): SidebarWorks
     workspaceId: "/repo",
     workspaceKind: "directory",
     name: "paseo",
-    activityAt: null,
     statusBucket: "done",
     diffStat: null,
     ...overrides,
@@ -31,7 +30,6 @@ function project(overrides: Partial<SidebarProjectEntry> = {}): SidebarProjectEn
     statusBucket: "done",
     activeCount: 0,
     totalWorkspaces: 1,
-    latestActivityAt: null,
     workspaces: [workspace()],
     ...overrides,
   };
diff --git a/packages/app/src/utils/sidebar-shortcuts.test.ts b/packages/app/src/utils/sidebar-shortcuts.test.ts
index 355cfa509..ac092fd9a 100644
--- a/packages/app/src/utils/sidebar-shortcuts.test.ts
+++ b/packages/app/src/utils/sidebar-shortcuts.test.ts
@@ -13,7 +13,6 @@ function workspace(serverId: string, cwd: string): SidebarWorkspaceEntry {
     workspaceId: cwd,
     workspaceKind: "local_checkout",
     name: cwd,
-    activityAt: null,
     statusBucket: "done",
     diffStat: null,
   };
@@ -28,7 +27,6 @@ function project(projectKey: string, workspaces: SidebarWorkspaceEntry[]): Sideb
     statusBucket: "done",
     activeCount: 0,
     totalWorkspaces: workspaces.length,
-    latestActivityAt: null,
     workspaces,
   };
 }
diff --git a/packages/app/src/utils/workspace-archive-navigation.test.ts b/packages/app/src/utils/workspace-archive-navigation.test.ts
index eeaba5bca..e171d79cb 100644
--- a/packages/app/src/utils/workspace-archive-navigation.test.ts
+++ b/packages/app/src/utils/workspace-archive-navigation.test.ts
@@ -17,7 +17,6 @@ function workspace(
     workspaceKind: input.workspaceKind ?? "worktree",
     name: input.name ?? input.id,
     status: input.status ?? "done",
-    activityAt: input.activityAt ?? null,
     diffStat: input.diffStat ?? null,
   };
 }
diff --git a/packages/server/package.json b/packages/server/package.json
index 6ab7895e4..473acbf4d 100644
--- a/packages/server/package.json
+++ b/packages/server/package.json
@@ -76,6 +76,7 @@
     "dotenv": "^17.2.3",
     "express": "^4.18.2",
     "express-basic-auth": "^1.2.1",
+    "fast-deep-equal": "^3.1.3",
     "fast-uri": "^3.1.0",
     "mnemonic-id": "^3.2.7",
     "node-pty": "1.2.0-beta.11",
diff --git a/packages/server/src/server/session.ts b/packages/server/src/server/session.ts
index f73051c65..c5da9e0a2 100644
--- a/packages/server/src/server/session.ts
+++ b/packages/server/src/server/session.ts
@@ -1,3 +1,4 @@
+import equal from "fast-deep-equal";
 import { v4 as uuidv4 } from "uuid";
 import { stat } from "fs/promises";
 import { exec, execFile } from "node:child_process";
@@ -323,6 +324,7 @@ type WorkspaceUpdatesSubscriptionState = {
   filter?: WorkspaceUpdatesFilter;
   isBootstrapping: boolean;
   pendingUpdatesByWorkspaceId: Map<string, WorkspaceUpdatePayload>;
+  lastEmittedByWorkspaceId: Map<string, WorkspaceUpdatePayload>;
 };
 type FetchWorkspacesCursor = {
   sort: FetchWorkspacesRequestSort[];
@@ -5301,25 +5303,6 @@ export class Session {
     return "done";
   }
 
-  private accumulateLatestActivityAt(
-    current: string | null,
-    agent: AgentSnapshotPayload,
-  ): string | null {
-    const candidateRaw = agent.lastUserMessageAt ?? agent.updatedAt;
-    const candidateMs = Date.parse(candidateRaw);
-    if (Number.isNaN(candidateMs)) {
-      return current;
-    }
-    if (!current) {
-      return new Date(candidateMs).toISOString();
-    }
-    const currentMs = Date.parse(current);
-    if (Number.isNaN(currentMs) || candidateMs > currentMs) {
-      return new Date(candidateMs).toISOString();
-    }
-    return current;
-  }
-
   private async describeWorkspaceRecord(
     workspace: PersistedWorkspaceRecord,
     projectRecord?: PersistedProjectRecord | null,
@@ -5464,7 +5447,6 @@ export class Session {
       if (this.workspaceStatePriority[bucket] < this.workspaceStatePriority[existing.status]) {
         existing.status = bucket;
       }
-      existing.activityAt = this.accumulateLatestActivityAt(existing.activityAt, agent);
     }
 
     return descriptorsByWorkspaceId;
@@ -5746,6 +5728,8 @@ export class Session {
       subscription.pendingUpdatesByWorkspaceId.set(workspaceId, payload);
       return;
     }
+    const workspaceId = payload.kind === "upsert" ? payload.workspace.id : payload.id;
+    subscription.lastEmittedByWorkspaceId.set(workspaceId, payload);
     this.emit({
       type: "workspace_update",
       payload,
@@ -5878,6 +5862,7 @@ export class Session {
           : null;
 
       if (!nextWorkspace) {
+        subscription.lastEmittedByWorkspaceId.delete(workspaceId);
         this.bufferOrEmitWorkspaceUpdate(subscription, {
           kind: "remove",
           id: workspaceId,
@@ -5885,10 +5870,17 @@ export class Session {
         continue;
       }
 
-      this.bufferOrEmitWorkspaceUpdate(subscription, {
+      const nextPayload: WorkspaceUpdatePayload = {
         kind: "upsert",
         workspace: nextWorkspace,
-      });
+      };
+
+      const lastEmitted = subscription.lastEmittedByWorkspaceId.get(workspaceId);
+      if (lastEmitted && lastEmitted.kind === "upsert" && equal(lastEmitted.workspace, nextWorkspace)) {
+        continue;
+      }
+
+      this.bufferOrEmitWorkspaceUpdate(subscription, nextPayload);
     }
 
     if (!options?.skipReconcile) {
@@ -6001,6 +5993,7 @@ export class Session {
           filter: request.filter,
           isBootstrapping: true,
           pendingUpdatesByWorkspaceId: new Map(),
+          lastEmittedByWorkspaceId: new Map(),
         };
       }