Stop re-rendering every chat message on each agent update (#8916)

* fix(app): keep unchanged agent fields when an agent update is accepted

* fix(app): share unchanged agent fields at the store write, not in the update policy

packages/app/src/runtime/directory-sync/agent-replica.test.ts

@@ -390,4 +390,39 @@ describe("AgentDirectoryReplica", () => {
     expect(useSessionStore.getState().sessions[serverId]?.agents.has("agent")).toBe(true);
     store.clearSession(serverId);
   });
+  it("keeps unchanged agent fields when an update changes only the status", () => {
+    const serverId = "agent-replica-unchanged-fields";
+    const store = useSessionStore.getState();
+    store.initializeSession(serverId, null as unknown as DaemonClient);
+    const replica = new AgentDirectoryReplica(
+      serverId,
+      () => undefined,
+      () => undefined,
+    );
+    const agentInStore = () => useSessionStore.getState().sessions[serverId]?.agents.get("agent");
+    replica.commitSnapshot([entry(payload("agent"))], []);
+    const before = agentInStore();
+
+    replica.applyDelta({
+      kind: "upsert",
+      agent: { ...payload("agent"), status: "running", updatedAt: "2026-07-17T00:02:00.000Z" },
+      project: entry(payload("agent")).project,
+    });
+    const afterDelta = agentInStore();
+    replica.commitSnapshot(
+      [entry({ ...payload("agent"), status: "idle", updatedAt: "2026-07-17T00:03:00.000Z" })],
+      [],
+    );
+    const afterSnapshot = agentInStore();
+
+    expect(afterDelta?.status).toBe("running");
+    expect(afterSnapshot?.status).toBe("idle");
+    for (const agent of [afterDelta, afterSnapshot]) {
+      expect(agent?.capabilities).toBe(before?.capabilities);
+      expect(agent?.availableModes).toBe(before?.availableModes);
+      expect(agent?.labels).toBe(before?.labels);
+      expect(agent?.projectPlacement).toBe(before?.projectPlacement);
+    }
+    store.clearSession(serverId);
+  });
 });

packages/app/src/runtime/directory-sync/internal/agent-store.ts

@@ -21,6 +21,26 @@ function withoutAgent(agentId: string) {
   };
 }
 
+/**
+ * Every update arrives as a whole new agent, so each nested value (capabilities, modes, labels) is
+ * a new object even when it did not change. Keep the stored value for every field that is equal,
+ * and the stored agent when nothing changed, so a reader of one field sees a new value only when
+ * that field changes.
+ */
+function keepUnchangedFields(stored: Agent, incoming: Agent): Agent {
+  const next = { ...incoming };
+  let changed = Object.keys(stored).length !== Object.keys(next).length;
+  for (const key of Object.keys(next) as (keyof Agent)[]) {
+    if (next[key] === stored[key]) continue;
+    if (key in stored && equal(next[key], stored[key])) {
+      Object.assign(next, { [key]: stored[key] });
+    } else {
+      changed = true;
+    }
+  }
+  return changed ? next : stored;
+}
+
 function mergeSnapshotTurn(previous: Agent | undefined, incoming: Agent): Agent {
   if (!previous) return incoming;
   const activeTurn =
@@ -59,7 +79,8 @@ export class AgentStoreProjection {
     let accepted = agent;
     useSessionStore.getState().setAgents(this.serverId, (current) => {
       const previous = current.get(agent.id);
-      accepted = acceptAgentDirectoryUpdate(previous, mergeSnapshotTurn(previous, agent));
+      const winner = acceptAgentDirectoryUpdate(previous, mergeSnapshotTurn(previous, agent));
+      accepted = previous ? keepUnchangedFields(previous, winner) : winner;
       if (accepted === previous) return current;
       const next = new Map(current);
       next.set(agent.id, accepted);
@@ -117,7 +138,7 @@ export class AgentStoreProjection {
     for (const [agentId, fetchedAgent] of fetched) {
       const existing = current.get(agentId);
       const merged = mergeSnapshotTurn(existing, fetchedAgent);
-      agents.set(agentId, existing && equal(existing, merged) ? existing : merged);
+      agents.set(agentId, existing ? keepUnchangedFields(existing, merged) : merged);
       if (fetchedAgent.archivedAt) {
         clearArchiveAgentPending({ queryClient, serverId: this.serverId, agentId });
       }

packages/app/src/utils/agent-directory-reconciliation.test.ts

@@ -126,6 +126,23 @@ describe("agent directory reconciliation", () => {
     }).toEqual({ title: "newer page", status: "running", projectName: "repo" });
   });
 
+  it("takes the project of a newer buffered upsert", () => {
+    const snapshotEntry = entry("agent", "idle");
+    const newerProject = { ...snapshotEntry.project, projectName: "renamed project" };
+    const result = reconcileAgentDirectory({
+      snapshot: [snapshotEntry],
+      deltas: [
+        {
+          kind: "upsert",
+          agent: { ...snapshot("agent", "idle"), updatedAt: "2026-07-12T11:00:00.000Z" },
+          project: newerProject,
+        },
+      ],
+    });
+
+    expect(result[0]?.project.projectName).toBe("renamed project");
+  });
+
   it("clears a snapshot stop when a newer buffered upsert is running", () => {
     const result = reconcileAgentDirectory({
       snapshot: [entry("agent", "idle")],